docs: update .all-contributorsrc [skip ci]

* #1317 Add Special Case Pattern

To focus on pattern itself, I implement DB and
maintenance lock by the singleton instance.

* #1317 Add special cases unit tests

Assert the logger output
(ref: https://stackoverflow.com/a/52229629)

* #1317 Add README.md

Add Special Case Pattern README

* #1317 Format: add a new line to end of file

Co-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com>

.all-contributorsrc

@@ -315,7 +315,8 @@
       "avatar_url": "https://avatars1.githubusercontent.com/u/6295975?v=4",
       "profile": "https://github.com/Anurag870",
       "contributions": [
-        "code"
+        "code",
+        "doc"
       ]
     },
     {
@@ -946,7 +947,8 @@
       "avatar_url": "https://avatars3.githubusercontent.com/u/23739158?v=4",
       "profile": "https://github.com/grzesiekkedzior",
       "contributions": [
-        "code"
+        "code",
+        "review"
       ]
     },
     {
@@ -1240,6 +1242,132 @@
       "contributions": [
         "doc"
       ]
+    },
+    {
+      "login": "akashchandwani",
+      "name": "Akash Chandwani",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/3483277?v=4",
+      "profile": "https://github.com/akashchandwani",
+      "contributions": [
+        "review"
+      ]
+    },
+    {
+      "login": "manannikov",
+      "name": "Pavlo Manannikov",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/7019769?v=4",
+      "profile": "http://www.linkedin.com/in/manannikov",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "eimanip",
+      "name": "Eiman",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/20307301?v=4",
+      "profile": "https://github.com/eimanip",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "OrangePants-R",
+      "name": "Rocky",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/42976136?v=4",
+      "profile": "https://github.com/OrangePants-R",
+      "contributions": [
+        "doc"
+      ]
+    },
+    {
+      "login": "ibrahimAlii",
+      "name": "Ibrahim ali abdelghany",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/21141301?v=4",
+      "profile": "https://ibrahimalii.github.io/",
+      "contributions": [
+        "review"
+      ]
+    },
+    {
+      "login": "gkulkarni2020",
+      "name": "Girish Kulkarni",
+      "avatar_url": "https://avatars3.githubusercontent.com/u/5161548?v=4",
+      "profile": "https://github.com/gkulkarni2020",
+      "contributions": [
+        "doc"
+      ]
+    },
+    {
+      "login": "omk13",
+      "name": "Omar Karazoun",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/59054172?v=4",
+      "profile": "https://github.com/omk13",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "jeff303",
+      "name": "Jeff Evans",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/3521562?v=4",
+      "profile": "https://github.com/jeff303",
+      "contributions": [
+        "code"
+      ]
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+    {
+      "login": "viveksb007",
+      "name": "Vivek Singh",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/12713808?v=4",
+      "profile": "https://viveksb007.github.io",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "siavashsoleymani",
+      "name": "siavash",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/18074419?v=4",
+      "profile": "https://github.com/siavashsoleymani",
+      "contributions": [
+        "code"
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+    {
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+      "name": "ruchpeanuts",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/29301900?v=4",
+      "profile": "https://github.com/ruchpeanuts",
+      "contributions": [
+        "doc"
+      ]
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+    {
+      "login": "warp125",
+      "name": "warp125",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/48073115?v=4",
+      "profile": "https://github.com/warp125",
+      "contributions": [
+        "translation"
+      ]
+    },
+    {
+      "login": "tkhadir",
+      "name": "KHADIR Tayeb",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/45130488?v=4",
+      "profile": "http://libkhadir.fr",
+      "contributions": [
+        "translation"
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+    {
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+      "name": "ignite1771",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/59446563?v=4",
+      "profile": "https://github.com/ignite1771",
+      "contributions": [
+        "code"
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   "contributorsPerLine": 4,

.github/workflows/maven-ci.yml

@@ -39,7 +39,8 @@ jobs:
 
     steps:
 
-    - uses: actions/checkout@v2
+    - name: Checkout Code
+      uses: actions/checkout@v2
       with:
         # Disabling shallow clone for improving relevancy of SonarQube reporting
         fetch-depth: 0
@@ -56,7 +57,8 @@ jobs:
         key: ${{ runner.os }}-sonar
         restore-keys: ${{ runner.os }}-sonar
 
-    - uses: actions/cache@v2
+    - name: Cache Maven dependencies
+      uses: actions/cache@v2
       with:
         path: ~/.m2/repository
         key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}

.github/workflows/maven-pr-builder.yml

@@ -29,6 +29,7 @@ name: Java PR Builder
 on:
   pull_request:
     branches: [ master ]
+    types: [ opened, reopened, synchronize, labeled, unlabeled ]
 
 jobs:
   build:
@@ -36,20 +37,26 @@ jobs:
     runs-on: ubuntu-20.04
 
     steps:
-    - uses: actions/checkout@v2
+    - name: Checkout Code
+      uses: actions/checkout@v2
+    
     - name: Set up JDK 11
       uses: actions/setup-java@v1
       with:
         java-version: 11
-    - uses: actions/cache@v2
+    
+    - name: Cache Maven Dependecies
+      uses: actions/cache@v2
       with:
         path: ~/.m2/repository
         key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
         restore-keys: |
           ${{ runner.os }}-maven-
+    
     # Some tests need screen access
     - name: Install xvfb
       run: sudo apt-get install -y xvfb
+    
     # This worflow is only for building Pull Requests, the master branch runs Sonar analysis on the main repository.
     # SonarQube scan does not work for forked repositories.
     # See https://jira.sonarsource.com/browse/MMF-1371

README.md

@@ -10,9 +10,15 @@
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=coverage)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns)
 [![Join the chat at https://gitter.im/iluwatar/java-design-patterns](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/iluwatar/java-design-patterns?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 <!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
-[![All Contributors](https://img.shields.io/badge/all_contributors-136-orange.svg?style=flat-square)](#contributors-)
+[![All Contributors](https://img.shields.io/badge/all_contributors-150-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
+<br/>
+
+Read in different language : [![CN](/assets/flags/CN.png)**CN**](/zh/README.md),[![KR](/assets/flags/KR.png)**KR**](/ko/README.md),[![FR](/assets/flags/FR.png)**FR**](/fr/README.md),
+
+<br/>
+
 # Introduction
 
 Design patterns are the best formalized practices a programmer can use to
@@ -70,211 +76,234 @@ This project is licensed under the terms of the MIT license.
 <!-- markdownlint-disable -->
 <table>
   <tr>
-    <td align="center"><a href="https://github.com/iluwatar"><img src="https://avatars1.githubusercontent.com/u/582346?v=4" width="100px;" alt=""/><br /><sub><b>Ilkka Seppälä</b></sub></a><br /><a href="#projectManagement-iluwatar" title="Project Management">📆</a> <a href="#maintenance-iluwatar" title="Maintenance">🚧</a> <a href="#content-iluwatar" title="Content">🖋</a></td>
-    <td align="center"><a href="https://github.com/amit1307"><img src="https://avatars0.githubusercontent.com/u/23420222?v=4" width="100px;" alt=""/><br /><sub><b>amit1307</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amit1307" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/npathai"><img src="https://avatars2.githubusercontent.com/u/1792515?v=4" width="100px;" alt=""/><br /><sub><b>Narendra Pathai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=npathai" title="Code">💻</a> <a href="#ideas-npathai" title="Ideas, Planning, & Feedback">🤔</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Anpathai" title="Reviewed Pull Requests">👀</a></td>
-    <td align="center"><a href="https://github.com/fluxw42"><img src="https://avatars1.githubusercontent.com/u/1545460?v=4" width="100px;" alt=""/><br /><sub><b>Jeroen Meulemeester</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fluxw42" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/iluwatar"><img src="https://avatars1.githubusercontent.com/u/582346?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ilkka Seppälä</b></sub></a><br /><a href="#projectManagement-iluwatar" title="Project Management">📆</a> <a href="#maintenance-iluwatar" title="Maintenance">🚧</a> <a href="#content-iluwatar" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/amit1307"><img src="https://avatars0.githubusercontent.com/u/23420222?v=4?s=100" width="100px;" alt=""/><br /><sub><b>amit1307</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amit1307" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/npathai"><img src="https://avatars2.githubusercontent.com/u/1792515?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Narendra Pathai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=npathai" title="Code">💻</a> <a href="#ideas-npathai" title="Ideas, Planning, & Feedback">🤔</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Anpathai" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://github.com/fluxw42"><img src="https://avatars1.githubusercontent.com/u/1545460?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Jeroen Meulemeester</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fluxw42" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="http://www.joemccarthy.co.uk"><img src="https://avatars0.githubusercontent.com/u/4526195?v=4" width="100px;" alt=""/><br /><sub><b>Joseph McCarthy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mikulucky" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/thomasoss"><img src="https://avatars1.githubusercontent.com/u/22516154?v=4" width="100px;" alt=""/><br /><sub><b>Thomas</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=thomasoss" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/anuragagarwal561994"><img src="https://avatars1.githubusercontent.com/u/6075379?v=4" width="100px;" alt=""/><br /><sub><b>Anurag Agarwal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=anuragagarwal561994" title="Code">💻</a></td>
-    <td align="center"><a href="https://markusmo3.github.io"><img src="https://avatars1.githubusercontent.com/u/3317416?v=4" width="100px;" alt=""/><br /><sub><b>Markus Moser</b></sub></a><br /><a href="#design-markusmo3" title="Design">🎨</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=markusmo3" title="Code">💻</a> <a href="#ideas-markusmo3" title="Ideas, Planning, & Feedback">🤔</a></td>
+    <td align="center"><a href="http://www.joemccarthy.co.uk"><img src="https://avatars0.githubusercontent.com/u/4526195?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Joseph McCarthy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mikulucky" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/thomasoss"><img src="https://avatars1.githubusercontent.com/u/22516154?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Thomas</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=thomasoss" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/anuragagarwal561994"><img src="https://avatars1.githubusercontent.com/u/6075379?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Anurag Agarwal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=anuragagarwal561994" title="Code">💻</a></td>
+    <td align="center"><a href="https://markusmo3.github.io"><img src="https://avatars1.githubusercontent.com/u/3317416?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Markus Moser</b></sub></a><br /><a href="#design-markusmo3" title="Design">🎨</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=markusmo3" title="Code">💻</a> <a href="#ideas-markusmo3" title="Ideas, Planning, & Feedback">🤔</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://twitter.com/i_sabiq"><img src="https://avatars1.githubusercontent.com/u/19510920?v=4" width="100px;" alt=""/><br /><sub><b>Sabiq Ihab</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=isabiq" title="Code">💻</a></td>
-    <td align="center"><a href="http://inbravo.github.io"><img src="https://avatars3.githubusercontent.com/u/5253764?v=4" width="100px;" alt=""/><br /><sub><b>Amit Dixit</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=inbravo" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/piyushchaudhari04"><img src="https://avatars3.githubusercontent.com/u/10268029?v=4" width="100px;" alt=""/><br /><sub><b>Piyush Kailash Chaudhari</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=piyushchaudhari04" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/joshzambales"><img src="https://avatars1.githubusercontent.com/u/8704552?v=4" width="100px;" alt=""/><br /><sub><b>joshzambales</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=joshzambales" title="Code">💻</a></td>
+    <td align="center"><a href="https://twitter.com/i_sabiq"><img src="https://avatars1.githubusercontent.com/u/19510920?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Sabiq Ihab</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=isabiq" title="Code">💻</a></td>
+    <td align="center"><a href="http://inbravo.github.io"><img src="https://avatars3.githubusercontent.com/u/5253764?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Amit Dixit</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=inbravo" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/piyushchaudhari04"><img src="https://avatars3.githubusercontent.com/u/10268029?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Piyush Kailash Chaudhari</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=piyushchaudhari04" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/joshzambales"><img src="https://avatars1.githubusercontent.com/u/8704552?v=4?s=100" width="100px;" alt=""/><br /><sub><b>joshzambales</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=joshzambales" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/Crossy147"><img src="https://avatars2.githubusercontent.com/u/7272996?v=4" width="100px;" alt=""/><br /><sub><b>Kamil Pietruszka</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Crossy147" title="Code">💻</a></td>
-    <td align="center"><a href="http://cs.joensuu.fi/~zkhayda"><img src="https://avatars2.githubusercontent.com/u/660742?v=4" width="100px;" alt=""/><br /><sub><b>Zafar Khaydarov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Documentation">📖</a></td>
-    <td align="center"><a href="https://kemitix.github.io/"><img src="https://avatars1.githubusercontent.com/u/1147749?v=4" width="100px;" alt=""/><br /><sub><b>Paul Campbell</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kemitix" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Argyro-Sioziou"><img src="https://avatars0.githubusercontent.com/u/22822639?v=4" width="100px;" alt=""/><br /><sub><b>Argyro Sioziou</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Argyro-Sioziou" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Crossy147"><img src="https://avatars2.githubusercontent.com/u/7272996?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Kamil Pietruszka</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Crossy147" title="Code">💻</a></td>
+    <td align="center"><a href="http://cs.joensuu.fi/~zkhayda"><img src="https://avatars2.githubusercontent.com/u/660742?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Zafar Khaydarov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://kemitix.github.io/"><img src="https://avatars1.githubusercontent.com/u/1147749?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Paul Campbell</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kemitix" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Argyro-Sioziou"><img src="https://avatars0.githubusercontent.com/u/22822639?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Argyro Sioziou</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Argyro-Sioziou" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/TylerMcConville"><img src="https://avatars0.githubusercontent.com/u/4946449?v=4" width="100px;" alt=""/><br /><sub><b>TylerMcConville</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=TylerMcConville" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/saksham93"><img src="https://avatars1.githubusercontent.com/u/37399540?v=4" width="100px;" alt=""/><br /><sub><b>saksham93</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=saksham93" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/nikhilbarar"><img src="https://avatars2.githubusercontent.com/u/37332144?v=4" width="100px;" alt=""/><br /><sub><b>nikhilbarar</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nikhilbarar" title="Code">💻</a></td>
-    <td align="center"><a href="http://colinbut.com"><img src="https://avatars2.githubusercontent.com/u/10725674?v=4" width="100px;" alt=""/><br /><sub><b>Colin But</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=colinbut" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/TylerMcConville"><img src="https://avatars0.githubusercontent.com/u/4946449?v=4?s=100" width="100px;" alt=""/><br /><sub><b>TylerMcConville</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=TylerMcConville" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/saksham93"><img src="https://avatars1.githubusercontent.com/u/37399540?v=4?s=100" width="100px;" alt=""/><br /><sub><b>saksham93</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=saksham93" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/nikhilbarar"><img src="https://avatars2.githubusercontent.com/u/37332144?v=4?s=100" width="100px;" alt=""/><br /><sub><b>nikhilbarar</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nikhilbarar" title="Code">💻</a></td>
+    <td align="center"><a href="http://colinbut.com"><img src="https://avatars2.githubusercontent.com/u/10725674?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Colin But</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=colinbut" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/ruslanpa"><img src="https://avatars2.githubusercontent.com/u/1503411?v=4" width="100px;" alt=""/><br /><sub><b>Ruslan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ruslanpa" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/JuhoKang"><img src="https://avatars1.githubusercontent.com/u/4745294?v=4" width="100px;" alt=""/><br /><sub><b>Juho Kang</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=JuhoKang" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/dheeraj-mummareddy"><img src="https://avatars2.githubusercontent.com/u/7002230?v=4" width="100px;" alt=""/><br /><sub><b>Dheeraj Mummareddy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dheeraj-mummareddy" title="Code">💻</a></td>
-    <td align="center"><a href="https://www.bernardosulzbach.com"><img src="https://avatars0.githubusercontent.com/u/8271090?v=4" width="100px;" alt=""/><br /><sub><b>Bernardo Sulzbach</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=bernardosulzbach" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ruslanpa"><img src="https://avatars2.githubusercontent.com/u/1503411?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ruslan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ruslanpa" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/JuhoKang"><img src="https://avatars1.githubusercontent.com/u/4745294?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Juho Kang</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=JuhoKang" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/dheeraj-mummareddy"><img src="https://avatars2.githubusercontent.com/u/7002230?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Dheeraj Mummareddy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dheeraj-mummareddy" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.bernardosulzbach.com"><img src="https://avatars0.githubusercontent.com/u/8271090?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Bernardo Sulzbach</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=bernardosulzbach" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/4lexis"><img src="https://avatars0.githubusercontent.com/u/19871727?v=4" width="100px;" alt=""/><br /><sub><b>Aleksandar Dudukovic</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=4lexis" title="Code">💻</a></td>
-    <td align="center"><a href="https://www.yusufaytas.com"><img src="https://avatars2.githubusercontent.com/u/1049483?v=4" width="100px;" alt=""/><br /><sub><b>Yusuf Aytaş</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yusufaytas" title="Code">💻</a></td>
-    <td align="center"><a href="http://futurehomes.hu"><img src="https://avatars2.githubusercontent.com/u/1001491?v=4" width="100px;" alt=""/><br /><sub><b>Mihály Kuprivecz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=qpi" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/kapinuss"><img src="https://avatars0.githubusercontent.com/u/17639945?v=4" width="100px;" alt=""/><br /><sub><b>Stanislav Kapinus</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kapinuss" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/4lexis"><img src="https://avatars0.githubusercontent.com/u/19871727?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Aleksandar Dudukovic</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=4lexis" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.yusufaytas.com"><img src="https://avatars2.githubusercontent.com/u/1049483?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Yusuf Aytaş</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yusufaytas" title="Code">💻</a></td>
+    <td align="center"><a href="http://futurehomes.hu"><img src="https://avatars2.githubusercontent.com/u/1001491?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Mihály Kuprivecz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=qpi" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/kapinuss"><img src="https://avatars0.githubusercontent.com/u/17639945?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Stanislav Kapinus</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kapinuss" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/gvsharma"><img src="https://avatars1.githubusercontent.com/u/6648152?v=4" width="100px;" alt=""/><br /><sub><b>GVSharma</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=gvsharma" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/SrdjanPaunovic"><img src="https://avatars1.githubusercontent.com/u/22815104?v=4" width="100px;" alt=""/><br /><sub><b>Srđan Paunović</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=SrdjanPaunovic" title="Code">💻</a></td>
-    <td align="center"><a href="https://sideris.xyz/"><img src="https://avatars3.githubusercontent.com/u/5484694?v=4" width="100px;" alt=""/><br /><sub><b>Petros G. Sideris</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=sideris" title="Code">💻</a></td>
-    <td align="center"><a href="https://www.linkedin.com/in/pramodgupta3/"><img src="https://avatars1.githubusercontent.com/u/2184241?v=4" width="100px;" alt=""/><br /><sub><b>Pramod Gupta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AIAmPramod" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://github.com/gvsharma"><img src="https://avatars1.githubusercontent.com/u/6648152?v=4?s=100" width="100px;" alt=""/><br /><sub><b>GVSharma</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=gvsharma" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/SrdjanPaunovic"><img src="https://avatars1.githubusercontent.com/u/22815104?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Srđan Paunović</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=SrdjanPaunovic" title="Code">💻</a></td>
+    <td align="center"><a href="https://sideris.xyz/"><img src="https://avatars3.githubusercontent.com/u/5484694?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Petros G. Sideris</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=sideris" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/pramodgupta3/"><img src="https://avatars1.githubusercontent.com/u/2184241?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Pramod Gupta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AIAmPramod" title="Reviewed Pull Requests">👀</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://amarnath510.github.io/portfolio"><img src="https://avatars0.githubusercontent.com/u/4599623?v=4" width="100px;" alt=""/><br /><sub><b>Amarnath Chandana</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Amarnath510" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Anurag870"><img src="https://avatars1.githubusercontent.com/u/6295975?v=4" width="100px;" alt=""/><br /><sub><b>Anurag870</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Anurag870" title="Code">💻</a></td>
-    <td align="center"><a href="http://theerroris.me"><img src="https://avatars0.githubusercontent.com/u/1685953?v=4" width="100px;" alt=""/><br /><sub><b>Wes Gilleland</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Deathnerd" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Harshrajsinh"><img src="https://avatars2.githubusercontent.com/u/22811531?v=4" width="100px;" alt=""/><br /><sub><b>Harshraj Thakor</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Harshrajsinh" title="Code">💻</a></td>
+    <td align="center"><a href="https://amarnath510.github.io/portfolio"><img src="https://avatars0.githubusercontent.com/u/4599623?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Amarnath Chandana</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Amarnath510" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Anurag870"><img src="https://avatars1.githubusercontent.com/u/6295975?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Anurag870</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Anurag870" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=Anurag870" title="Documentation">📖</a></td>
+    <td align="center"><a href="http://theerroris.me"><img src="https://avatars0.githubusercontent.com/u/1685953?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Wes Gilleland</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Deathnerd" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Harshrajsinh"><img src="https://avatars2.githubusercontent.com/u/22811531?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Harshraj Thakor</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Harshrajsinh" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/MaVdbussche"><img src="https://avatars1.githubusercontent.com/u/26136934?v=4" width="100px;" alt=""/><br /><sub><b>Martin Vandenbussche</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=MaVdbussche" title="Code">💻</a></td>
-    <td align="center"><a href="https://alexsomai.com"><img src="https://avatars1.githubusercontent.com/u/5720977?v=4" width="100px;" alt=""/><br /><sub><b>Alexandru Somai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=alexsomai" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/amogozov"><img src="https://avatars3.githubusercontent.com/u/7372215?v=4" width="100px;" alt=""/><br /><sub><b>Artur Mogozov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amogozov" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/anthonycampbell"><img src="https://avatars3.githubusercontent.com/u/10249255?v=4" width="100px;" alt=""/><br /><sub><b>anthony</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=anthonycampbell" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/MaVdbussche"><img src="https://avatars1.githubusercontent.com/u/26136934?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Martin Vandenbussche</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=MaVdbussche" title="Code">💻</a></td>
+    <td align="center"><a href="https://alexsomai.com"><img src="https://avatars1.githubusercontent.com/u/5720977?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Alexandru Somai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=alexsomai" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/amogozov"><img src="https://avatars3.githubusercontent.com/u/7372215?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Artur Mogozov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amogozov" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/anthonycampbell"><img src="https://avatars3.githubusercontent.com/u/10249255?v=4?s=100" width="100px;" alt=""/><br /><sub><b>anthony</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=anthonycampbell" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="http://ccygnus.com/"><img src="https://avatars1.githubusercontent.com/u/9342724?v=4" width="100px;" alt=""/><br /><sub><b>Christian Cygnus</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=christophercolumbusdog" title="Code">💻</a></td>
-    <td align="center"><a href="https://about.me/dzmitryh"><img src="https://avatars2.githubusercontent.com/u/5390492?v=4" width="100px;" alt=""/><br /><sub><b>Dima Gubin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dzmitryh" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/jjjimenez100"><img src="https://avatars3.githubusercontent.com/u/22243493?v=4" width="100px;" alt=""/><br /><sub><b>Joshua Jimenez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=jjjimenez100" title="Code">💻</a></td>
-    <td align="center"><a href="http://about.me/kaiwinter"><img src="https://avatars0.githubusercontent.com/u/110982?v=4" width="100px;" alt=""/><br /><sub><b>Kai Winter</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kaiwinter" title="Code">💻</a></td>
+    <td align="center"><a href="http://ccygnus.com/"><img src="https://avatars1.githubusercontent.com/u/9342724?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Christian Cygnus</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=christophercolumbusdog" title="Code">💻</a></td>
+    <td align="center"><a href="https://about.me/dzmitryh"><img src="https://avatars2.githubusercontent.com/u/5390492?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Dima Gubin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dzmitryh" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/jjjimenez100"><img src="https://avatars3.githubusercontent.com/u/22243493?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Joshua Jimenez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=jjjimenez100" title="Code">💻</a></td>
+    <td align="center"><a href="http://about.me/kaiwinter"><img src="https://avatars0.githubusercontent.com/u/110982?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Kai Winter</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kaiwinter" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/lbroman"><img src="https://avatars1.githubusercontent.com/u/86007?v=4" width="100px;" alt=""/><br /><sub><b>lbroman</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=lbroman" title="Code">💻</a></td>
-    <td align="center"><a href="https://przemeknowak.com"><img src="https://avatars1.githubusercontent.com/u/3254609?v=4" width="100px;" alt=""/><br /><sub><b>Przemek</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=pnowy" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/prafful1"><img src="https://avatars0.githubusercontent.com/u/14350274?v=4" width="100px;" alt=""/><br /><sub><b>Prafful Agarwal</b></sub></a><br /><a href="#content-prafful1" title="Content">🖋</a></td>
-    <td align="center"><a href="https://github.com/sankypanhale"><img src="https://avatars1.githubusercontent.com/u/6478783?v=4" width="100px;" alt=""/><br /><sub><b>Sanket Panhale</b></sub></a><br /><a href="#content-sankypanhale" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/lbroman"><img src="https://avatars1.githubusercontent.com/u/86007?v=4?s=100" width="100px;" alt=""/><br /><sub><b>lbroman</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=lbroman" title="Code">💻</a></td>
+    <td align="center"><a href="https://przemeknowak.com"><img src="https://avatars1.githubusercontent.com/u/3254609?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Przemek</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=pnowy" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/prafful1"><img src="https://avatars0.githubusercontent.com/u/14350274?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Prafful Agarwal</b></sub></a><br /><a href="#content-prafful1" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/sankypanhale"><img src="https://avatars1.githubusercontent.com/u/6478783?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Sanket Panhale</b></sub></a><br /><a href="#content-sankypanhale" title="Content">🖋</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/staillebois"><img src="https://avatars0.githubusercontent.com/u/23701200?v=4" width="100px;" alt=""/><br /><sub><b>staillebois</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=staillebois" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/valdar-hu"><img src="https://avatars3.githubusercontent.com/u/17962817?v=4" width="100px;" alt=""/><br /><sub><b>Krisztián Nagy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=valdar-hu" title="Code">💻</a></td>
-    <td align="center"><a href="https://www.vanogrid.com"><img src="https://avatars0.githubusercontent.com/u/4307918?v=4" width="100px;" alt=""/><br /><sub><b>Alexander Ivanov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vanogrid" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/yosfik"><img src="https://avatars3.githubusercontent.com/u/4850270?v=4" width="100px;" alt=""/><br /><sub><b>Yosfik Alqadri</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yosfik" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/staillebois"><img src="https://avatars0.githubusercontent.com/u/23701200?v=4?s=100" width="100px;" alt=""/><br /><sub><b>staillebois</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=staillebois" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/valdar-hu"><img src="https://avatars3.githubusercontent.com/u/17962817?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Krisztián Nagy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=valdar-hu" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.vanogrid.com"><img src="https://avatars0.githubusercontent.com/u/4307918?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Alexander Ivanov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vanogrid" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/yosfik"><img src="https://avatars3.githubusercontent.com/u/4850270?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Yosfik Alqadri</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yosfik" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/7agustibm"><img src="https://avatars0.githubusercontent.com/u/8149332?v=4" width="100px;" alt=""/><br /><sub><b>Agustí Becerra Milà</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=7agustibm" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Juaanma"><img src="https://avatars3.githubusercontent.com/u/7390500?v=4" width="100px;" alt=""/><br /><sub><b>Juan Manuel Suárez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Juaanma" title="Code">💻</a></td>
-    <td align="center"><a href="http://www.devsedge.net/"><img src="https://avatars0.githubusercontent.com/u/9956006?v=4" width="100px;" alt=""/><br /><sub><b>Luigi Cortese</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=LuigiCortese" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Rzeposlaw"><img src="https://avatars2.githubusercontent.com/u/18425745?v=4" width="100px;" alt=""/><br /><sub><b>Katarzyna Rzepecka</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Rzeposlaw" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/7agustibm"><img src="https://avatars0.githubusercontent.com/u/8149332?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Agustí Becerra Milà</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=7agustibm" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Juaanma"><img src="https://avatars3.githubusercontent.com/u/7390500?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Juan Manuel Suárez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Juaanma" title="Code">💻</a></td>
+    <td align="center"><a href="http://www.devsedge.net/"><img src="https://avatars0.githubusercontent.com/u/9956006?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Luigi Cortese</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=LuigiCortese" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Rzeposlaw"><img src="https://avatars2.githubusercontent.com/u/18425745?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Katarzyna Rzepecka</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Rzeposlaw" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="http://adamski.pro"><img src="https://avatars1.githubusercontent.com/u/6537430?v=4" width="100px;" alt=""/><br /><sub><b>adamski.pro</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=akrystian" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/baislsl"><img src="https://avatars0.githubusercontent.com/u/17060584?v=4" width="100px;" alt=""/><br /><sub><b>Shengli Bai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=baislsl" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/besok"><img src="https://avatars2.githubusercontent.com/u/29834592?v=4" width="100px;" alt=""/><br /><sub><b>Boris</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=besok" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/dmitraver"><img src="https://avatars3.githubusercontent.com/u/1798156?v=4" width="100px;" alt=""/><br /><sub><b>Dmitry Avershin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dmitraver" title="Code">💻</a></td>
+    <td align="center"><a href="http://adamski.pro"><img src="https://avatars1.githubusercontent.com/u/6537430?v=4?s=100" width="100px;" alt=""/><br /><sub><b>adamski.pro</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=akrystian" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/baislsl"><img src="https://avatars0.githubusercontent.com/u/17060584?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Shengli Bai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=baislsl" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/besok"><img src="https://avatars2.githubusercontent.com/u/29834592?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Boris</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=besok" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/dmitraver"><img src="https://avatars3.githubusercontent.com/u/1798156?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Dmitry Avershin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dmitraver" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/fanofxiaofeng"><img src="https://avatars0.githubusercontent.com/u/3983683?v=4" width="100px;" alt=""/><br /><sub><b>靳阳</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fanofxiaofeng" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/hoangnam2261"><img src="https://avatars2.githubusercontent.com/u/31692990?v=4" width="100px;" alt=""/><br /><sub><b>hoangnam2261</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hoangnam2261" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/jarpit96"><img src="https://avatars2.githubusercontent.com/u/10098713?v=4" width="100px;" alt=""/><br /><sub><b>Arpit Jain</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=jarpit96" title="Code">💻</a></td>
-    <td align="center"><a href="http://joningi.net"><img src="https://avatars2.githubusercontent.com/u/6115148?v=4" width="100px;" alt=""/><br /><sub><b>Jón Ingi Sveinbjörnsson</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=joningiwork" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/fanofxiaofeng"><img src="https://avatars0.githubusercontent.com/u/3983683?v=4?s=100" width="100px;" alt=""/><br /><sub><b>靳阳</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fanofxiaofeng" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/hoangnam2261"><img src="https://avatars2.githubusercontent.com/u/31692990?v=4?s=100" width="100px;" alt=""/><br /><sub><b>hoangnam2261</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hoangnam2261" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/jarpit96"><img src="https://avatars2.githubusercontent.com/u/10098713?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Arpit Jain</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=jarpit96" title="Code">💻</a></td>
+    <td align="center"><a href="http://joningi.net"><img src="https://avatars2.githubusercontent.com/u/6115148?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Jón Ingi Sveinbjörnsson</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=joningiwork" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/kirill-vlasov"><img src="https://avatars3.githubusercontent.com/u/16112495?v=4" width="100px;" alt=""/><br /><sub><b>Kirill Vlasov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kirill-vlasov" title="Code">💻</a></td>
-    <td align="center"><a href="http://mitchell-irvin.com"><img src="https://avatars0.githubusercontent.com/u/16233245?v=4" width="100px;" alt=""/><br /><sub><b>Mitchell Irvin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mitchellirvin" title="Code">💻</a></td>
-    <td align="center"><a href="https://ranjeet-floyd.github.io"><img src="https://avatars0.githubusercontent.com/u/1992972?v=4" width="100px;" alt=""/><br /><sub><b>Ranjeet</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ranjeet-floyd" title="Code">💻</a></td>
-    <td align="center"><a href="https://alwayswithme.github.io"><img src="https://avatars3.githubusercontent.com/u/3234786?v=4" width="100px;" alt=""/><br /><sub><b>PhoenixYip</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Alwayswithme" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/kirill-vlasov"><img src="https://avatars3.githubusercontent.com/u/16112495?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Kirill Vlasov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kirill-vlasov" title="Code">💻</a></td>
+    <td align="center"><a href="http://mitchell-irvin.com"><img src="https://avatars0.githubusercontent.com/u/16233245?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Mitchell Irvin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mitchellirvin" title="Code">💻</a></td>
+    <td align="center"><a href="https://ranjeet-floyd.github.io"><img src="https://avatars0.githubusercontent.com/u/1992972?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ranjeet</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ranjeet-floyd" title="Code">💻</a></td>
+    <td align="center"><a href="https://alwayswithme.github.io"><img src="https://avatars3.githubusercontent.com/u/3234786?v=4?s=100" width="100px;" alt=""/><br /><sub><b>PhoenixYip</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Alwayswithme" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/MSaifAsif"><img src="https://avatars1.githubusercontent.com/u/6280554?v=4" width="100px;" alt=""/><br /><sub><b>M Saif Asif</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=MSaifAsif" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/kanwarpreet25"><img src="https://avatars0.githubusercontent.com/u/39183641?v=4" width="100px;" alt=""/><br /><sub><b>kanwarpreet25</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kanwarpreet25" title="Code">💻</a></td>
-    <td align="center"><a href="http://leonmak.me"><img src="https://avatars3.githubusercontent.com/u/13071508?v=4" width="100px;" alt=""/><br /><sub><b>Leon Mak</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=leonmak" title="Code">💻</a></td>
-    <td align="center"><a href="http://www.wramdemark.se"><img src="https://avatars2.githubusercontent.com/u/7052193?v=4" width="100px;" alt=""/><br /><sub><b>Per Wramdemark</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=perwramdemark" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/MSaifAsif"><img src="https://avatars1.githubusercontent.com/u/6280554?v=4?s=100" width="100px;" alt=""/><br /><sub><b>M Saif Asif</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=MSaifAsif" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/kanwarpreet25"><img src="https://avatars0.githubusercontent.com/u/39183641?v=4?s=100" width="100px;" alt=""/><br /><sub><b>kanwarpreet25</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kanwarpreet25" title="Code">💻</a></td>
+    <td align="center"><a href="http://leonmak.me"><img src="https://avatars3.githubusercontent.com/u/13071508?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Leon Mak</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=leonmak" title="Code">💻</a></td>
+    <td align="center"><a href="http://www.wramdemark.se"><img src="https://avatars2.githubusercontent.com/u/7052193?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Per Wramdemark</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=perwramdemark" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/waisuan"><img src="https://avatars2.githubusercontent.com/u/10975700?v=4" width="100px;" alt=""/><br /><sub><b>Evan Sia Wai Suan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=waisuan" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/AnaghaSasikumar"><img src="https://avatars2.githubusercontent.com/u/42939261?v=4" width="100px;" alt=""/><br /><sub><b>AnaghaSasikumar</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=AnaghaSasikumar" title="Code">💻</a></td>
-    <td align="center"><a href="https://christofferh.com"><img src="https://avatars1.githubusercontent.com/u/767643?v=4" width="100px;" alt=""/><br /><sub><b>Christoffer Hamberg</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=christofferh" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/dgruntz"><img src="https://avatars0.githubusercontent.com/u/1516800?v=4" width="100px;" alt=""/><br /><sub><b>Dominik Gruntz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dgruntz" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/waisuan"><img src="https://avatars2.githubusercontent.com/u/10975700?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Evan Sia Wai Suan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=waisuan" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/AnaghaSasikumar"><img src="https://avatars2.githubusercontent.com/u/42939261?v=4?s=100" width="100px;" alt=""/><br /><sub><b>AnaghaSasikumar</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=AnaghaSasikumar" title="Code">💻</a></td>
+    <td align="center"><a href="https://christofferh.com"><img src="https://avatars1.githubusercontent.com/u/767643?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Christoffer Hamberg</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=christofferh" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/dgruntz"><img src="https://avatars0.githubusercontent.com/u/1516800?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Dominik Gruntz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dgruntz" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://bitbucket.org/hannespernpeintner/"><img src="https://avatars3.githubusercontent.com/u/1679437?v=4" width="100px;" alt=""/><br /><sub><b>Hannes</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hannespernpeintner" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/leogtzr"><img src="https://avatars0.githubusercontent.com/u/1211969?v=4" width="100px;" alt=""/><br /><sub><b>Leo Gutiérrez Ramírez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=leogtzr" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/npczwh"><img src="https://avatars0.githubusercontent.com/u/14066422?v=4" width="100px;" alt=""/><br /><sub><b>Zhang WH</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=npczwh" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/oconnelc"><img src="https://avatars0.githubusercontent.com/u/1112973?v=4" width="100px;" alt=""/><br /><sub><b>Christopher O'Connell</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=oconnelc" title="Code">💻</a></td>
+    <td align="center"><a href="https://bitbucket.org/hannespernpeintner/"><img src="https://avatars3.githubusercontent.com/u/1679437?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Hannes</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hannespernpeintner" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/leogtzr"><img src="https://avatars0.githubusercontent.com/u/1211969?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Leo Gutiérrez Ramírez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=leogtzr" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/npczwh"><img src="https://avatars0.githubusercontent.com/u/14066422?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Zhang WH</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=npczwh" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/oconnelc"><img src="https://avatars0.githubusercontent.com/u/1112973?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Christopher O'Connell</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=oconnelc" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/giorgosmav21"><img src="https://avatars2.githubusercontent.com/u/22855493?v=4" width="100px;" alt=""/><br /><sub><b>George Mavroeidis</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=giorgosmav21" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/hbothra15"><img src="https://avatars1.githubusercontent.com/u/7418012?v=4" width="100px;" alt=""/><br /><sub><b>Hemant Bothra</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hbothra15" title="Code">💻</a> <a href="#design-hbothra15" title="Design">🎨</a></td>
-    <td align="center"><a href="https://www.kevinpeters.net/about/"><img src="https://avatars1.githubusercontent.com/u/12736734?v=4" width="100px;" alt=""/><br /><sub><b>Kevin Peters</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=igeligel" title="Code">💻</a></td>
-    <td align="center"><a href="https://llorllale.github.io/"><img src="https://avatars1.githubusercontent.com/u/2019896?v=4" width="100px;" alt=""/><br /><sub><b>George Aristy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=llorllale" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/giorgosmav21"><img src="https://avatars2.githubusercontent.com/u/22855493?v=4?s=100" width="100px;" alt=""/><br /><sub><b>George Mavroeidis</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=giorgosmav21" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/hbothra15"><img src="https://avatars1.githubusercontent.com/u/7418012?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Hemant Bothra</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hbothra15" title="Code">💻</a> <a href="#design-hbothra15" title="Design">🎨</a></td>
+    <td align="center"><a href="https://www.kevinpeters.net/about/"><img src="https://avatars1.githubusercontent.com/u/12736734?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Kevin Peters</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=igeligel" title="Code">💻</a></td>
+    <td align="center"><a href="https://llorllale.github.io/"><img src="https://avatars1.githubusercontent.com/u/2019896?v=4?s=100" width="100px;" alt=""/><br /><sub><b>George Aristy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=llorllale" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/mookkiah"><img src="https://avatars1.githubusercontent.com/u/8975264?v=4" width="100px;" alt=""/><br /><sub><b>Mahendran Mookkiah</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mookkiah" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Azureyjt"><img src="https://avatars2.githubusercontent.com/u/18476317?v=4" width="100px;" alt=""/><br /><sub><b>Azureyjt</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Azureyjt" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/vehpsr"><img src="https://avatars2.githubusercontent.com/u/3133265?v=4" width="100px;" alt=""/><br /><sub><b>gans</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vehpsr" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/ThatGuyWithTheHat"><img src="https://avatars0.githubusercontent.com/u/24470582?v=4" width="100px;" alt=""/><br /><sub><b>Matt</b></sub></a><br /><a href="#content-ThatGuyWithTheHat" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/mookkiah"><img src="https://avatars1.githubusercontent.com/u/8975264?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Mahendran Mookkiah</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mookkiah" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Azureyjt"><img src="https://avatars2.githubusercontent.com/u/18476317?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Azureyjt</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Azureyjt" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/vehpsr"><img src="https://avatars2.githubusercontent.com/u/3133265?v=4?s=100" width="100px;" alt=""/><br /><sub><b>gans</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vehpsr" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ThatGuyWithTheHat"><img src="https://avatars0.githubusercontent.com/u/24470582?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Matt</b></sub></a><br /><a href="#content-ThatGuyWithTheHat" title="Content">🖋</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://www.linkedin.com/in/gopinathlangote/"><img src="https://avatars2.githubusercontent.com/u/10210778?v=4" width="100px;" alt=""/><br /><sub><b>Gopinath Langote</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=gopinath-langote" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/hoswey"><img src="https://avatars3.githubusercontent.com/u/3689445?v=4" width="100px;" alt=""/><br /><sub><b>Hoswey</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hoswey" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/amit2103"><img src="https://avatars3.githubusercontent.com/u/7566692?v=4" width="100px;" alt=""/><br /><sub><b>Amit Pandey</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amit2103" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/gwildor28"><img src="https://avatars0.githubusercontent.com/u/16000365?v=4" width="100px;" alt=""/><br /><sub><b>gwildor28</b></sub></a><br /><a href="#content-gwildor28" title="Content">🖋</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/gopinathlangote/"><img src="https://avatars2.githubusercontent.com/u/10210778?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Gopinath Langote</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=gopinath-langote" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/hoswey"><img src="https://avatars3.githubusercontent.com/u/3689445?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Hoswey</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hoswey" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/amit2103"><img src="https://avatars3.githubusercontent.com/u/7566692?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Amit Pandey</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amit2103" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/gwildor28"><img src="https://avatars0.githubusercontent.com/u/16000365?v=4?s=100" width="100px;" alt=""/><br /><sub><b>gwildor28</b></sub></a><br /><a href="#content-gwildor28" title="Content">🖋</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://t.me/paul_docker"><img src="https://avatars1.githubusercontent.com/u/2404785?v=4" width="100px;" alt=""/><br /><sub><b>田浩</b></sub></a><br /><a href="#content-llitfkitfk" title="Content">🖋</a></td>
-    <td align="center"><a href="https://twitter.com/StPitsios"><img src="https://avatars1.githubusercontent.com/u/6773603?v=4" width="100px;" alt=""/><br /><sub><b>Stamatis Pitsios</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=pitsios-s" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/qza"><img src="https://avatars3.githubusercontent.com/u/233149?v=4" width="100px;" alt=""/><br /><sub><b>qza</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=qza" title="Code">💻</a></td>
-    <td align="center"><a href="http://tschis.github.io"><img src="https://avatars1.githubusercontent.com/u/20662669?v=4" width="100px;" alt=""/><br /><sub><b>Rodolfo Forte</b></sub></a><br /><a href="#content-Tschis" title="Content">🖋</a></td>
+    <td align="center"><a href="https://t.me/paul_docker"><img src="https://avatars1.githubusercontent.com/u/2404785?v=4?s=100" width="100px;" alt=""/><br /><sub><b>田浩</b></sub></a><br /><a href="#content-llitfkitfk" title="Content">🖋</a></td>
+    <td align="center"><a href="https://twitter.com/StPitsios"><img src="https://avatars1.githubusercontent.com/u/6773603?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Stamatis Pitsios</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=pitsios-s" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/qza"><img src="https://avatars3.githubusercontent.com/u/233149?v=4?s=100" width="100px;" alt=""/><br /><sub><b>qza</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=qza" title="Code">💻</a></td>
+    <td align="center"><a href="http://tschis.github.io"><img src="https://avatars1.githubusercontent.com/u/20662669?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Rodolfo Forte</b></sub></a><br /><a href="#content-Tschis" title="Content">🖋</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/ankurkaushal"><img src="https://avatars2.githubusercontent.com/u/2236616?v=4" width="100px;" alt=""/><br /><sub><b>Ankur Kaushal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ankurkaushal" title="Code">💻</a></td>
-    <td align="center"><a href="https://www.linkedin.com/in/ovidijus-okinskas/"><img src="https://avatars0.githubusercontent.com/u/20372387?v=4" width="100px;" alt=""/><br /><sub><b>Ovidijus Okinskas</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=okinskas" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/robertt240"><img src="https://avatars1.githubusercontent.com/u/9137432?v=4" width="100px;" alt=""/><br /><sub><b>Robert Kasperczyk</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=robertt240" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/trautonen"><img src="https://avatars3.githubusercontent.com/u/1641063?v=4" width="100px;" alt=""/><br /><sub><b>Tapio Rautonen</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=trautonen" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ankurkaushal"><img src="https://avatars2.githubusercontent.com/u/2236616?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ankur Kaushal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ankurkaushal" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/ovidijus-okinskas/"><img src="https://avatars0.githubusercontent.com/u/20372387?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ovidijus Okinskas</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=okinskas" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/robertt240"><img src="https://avatars1.githubusercontent.com/u/9137432?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Robert Kasperczyk</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=robertt240" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/trautonen"><img src="https://avatars3.githubusercontent.com/u/1641063?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Tapio Rautonen</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=trautonen" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="http://vk.com/yuri.orlov"><img src="https://avatars0.githubusercontent.com/u/1595733?v=4" width="100px;" alt=""/><br /><sub><b>Yuri Orlov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yorlov" title="Code">💻</a></td>
-    <td align="center"><a href="https://www.linkedin.com/in/varunu28/"><img src="https://avatars0.githubusercontent.com/u/7676016?v=4" width="100px;" alt=""/><br /><sub><b>Varun Upadhyay</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=varunu28" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/PalAditya"><img src="https://avatars2.githubusercontent.com/u/25523604?v=4" width="100px;" alt=""/><br /><sub><b>Aditya Pal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=PalAditya" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/grzesiekkedzior"><img src="https://avatars3.githubusercontent.com/u/23739158?v=4" width="100px;" alt=""/><br /><sub><b>grzesiekkedzior</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=grzesiekkedzior" title="Code">💻</a></td>
+    <td align="center"><a href="http://vk.com/yuri.orlov"><img src="https://avatars0.githubusercontent.com/u/1595733?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Yuri Orlov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yorlov" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/varunu28/"><img src="https://avatars0.githubusercontent.com/u/7676016?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Varun Upadhyay</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=varunu28" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/PalAditya"><img src="https://avatars2.githubusercontent.com/u/25523604?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Aditya Pal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=PalAditya" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/grzesiekkedzior"><img src="https://avatars3.githubusercontent.com/u/23739158?v=4?s=100" width="100px;" alt=""/><br /><sub><b>grzesiekkedzior</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=grzesiekkedzior" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Agrzesiekkedzior" title="Reviewed Pull Requests">👀</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/sivasubramanim"><img src="https://avatars2.githubusercontent.com/u/51107434?v=4" width="100px;" alt=""/><br /><sub><b>Sivasubramani M</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=sivasubramanim" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/d4gg4d"><img src="https://avatars2.githubusercontent.com/u/99457?v=4" width="100px;" alt=""/><br /><sub><b>Sami Airaksinen</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=d4gg4d" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/vertti"><img src="https://avatars0.githubusercontent.com/u/557751?v=4" width="100px;" alt=""/><br /><sub><b>Janne Sinivirta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vertti" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Bobo1239"><img src="https://avatars1.githubusercontent.com/u/2302947?v=4" width="100px;" alt=""/><br /><sub><b>Boris-Chengbiao Zhou</b></sub></a><br /><a href="#content-Bobo1239" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/sivasubramanim"><img src="https://avatars2.githubusercontent.com/u/51107434?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Sivasubramani M</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=sivasubramanim" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/d4gg4d"><img src="https://avatars2.githubusercontent.com/u/99457?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Sami Airaksinen</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=d4gg4d" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/vertti"><img src="https://avatars0.githubusercontent.com/u/557751?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Janne Sinivirta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vertti" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Bobo1239"><img src="https://avatars1.githubusercontent.com/u/2302947?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Boris-Chengbiao Zhou</b></sub></a><br /><a href="#content-Bobo1239" title="Content">🖋</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://jahhein.github.io"><img src="https://avatars2.githubusercontent.com/u/10779515?v=4" width="100px;" alt=""/><br /><sub><b>Jacob Hein</b></sub></a><br /><a href="#content-Jahhein" title="Content">🖋</a></td>
-    <td align="center"><a href="https://github.com/iamrichardjones"><img src="https://avatars3.githubusercontent.com/u/14842151?v=4" width="100px;" alt=""/><br /><sub><b>Richard Jones</b></sub></a><br /><a href="#content-iamrichardjones" title="Content">🖋</a></td>
-    <td align="center"><a href="https://rachelcarmena.github.io"><img src="https://avatars0.githubusercontent.com/u/22792183?v=4" width="100px;" alt=""/><br /><sub><b>Rachel M. Carmena</b></sub></a><br /><a href="#content-rachelcarmena" title="Content">🖋</a></td>
-    <td align="center"><a href="https://zd-zero.github.io"><img src="https://avatars0.githubusercontent.com/u/21978370?v=4" width="100px;" alt=""/><br /><sub><b>Zaerald Denze Lungos</b></sub></a><br /><a href="#content-zd-zero" title="Content">🖋</a></td>
+    <td align="center"><a href="https://jahhein.github.io"><img src="https://avatars2.githubusercontent.com/u/10779515?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Jacob Hein</b></sub></a><br /><a href="#content-Jahhein" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/iamrichardjones"><img src="https://avatars3.githubusercontent.com/u/14842151?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Richard Jones</b></sub></a><br /><a href="#content-iamrichardjones" title="Content">🖋</a></td>
+    <td align="center"><a href="https://rachelcarmena.github.io"><img src="https://avatars0.githubusercontent.com/u/22792183?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Rachel M. Carmena</b></sub></a><br /><a href="#content-rachelcarmena" title="Content">🖋</a></td>
+    <td align="center"><a href="https://zd-zero.github.io"><img src="https://avatars0.githubusercontent.com/u/21978370?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Zaerald Denze Lungos</b></sub></a><br /><a href="#content-zd-zero" title="Content">🖋</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://webpro.nl"><img src="https://avatars1.githubusercontent.com/u/456426?v=4" width="100px;" alt=""/><br /><sub><b>Lars Kappert</b></sub></a><br /><a href="#content-webpro" title="Content">🖋</a></td>
-    <td align="center"><a href="https://xiaod.info"><img src="https://avatars2.githubusercontent.com/u/21277644?v=4" width="100px;" alt=""/><br /><sub><b>Mike Liu</b></sub></a><br /><a href="#translation-xiaod-dev" title="Translation">🌍</a></td>
-    <td align="center"><a href="https://github.com/charlesfinley"><img src="https://avatars1.githubusercontent.com/u/6307904?v=4" width="100px;" alt=""/><br /><sub><b>Matt Dolan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=charlesfinley" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Acharlesfinley" title="Reviewed Pull Requests">👀</a></td>
-    <td align="center"><a href="https://github.com/MananS77"><img src="https://avatars3.githubusercontent.com/u/21033516?v=4" width="100px;" alt=""/><br /><sub><b>Manan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AMananS77" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://webpro.nl"><img src="https://avatars1.githubusercontent.com/u/456426?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Lars Kappert</b></sub></a><br /><a href="#content-webpro" title="Content">🖋</a></td>
+    <td align="center"><a href="https://xiaod.info"><img src="https://avatars2.githubusercontent.com/u/21277644?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Mike Liu</b></sub></a><br /><a href="#translation-xiaod-dev" title="Translation">🌍</a></td>
+    <td align="center"><a href="https://github.com/charlesfinley"><img src="https://avatars1.githubusercontent.com/u/6307904?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Matt Dolan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=charlesfinley" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Acharlesfinley" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://github.com/MananS77"><img src="https://avatars3.githubusercontent.com/u/21033516?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Manan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AMananS77" title="Reviewed Pull Requests">👀</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/nishant"><img src="https://avatars2.githubusercontent.com/u/15331971?v=4" width="100px;" alt=""/><br /><sub><b>Nishant Arora</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nishant" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/raja-peeyush-kumar-singh"><img src="https://avatars0.githubusercontent.com/u/5496024?v=4" width="100px;" alt=""/><br /><sub><b>Peeyush</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=raja-peeyush-kumar-singh" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/ravening"><img src="https://avatars1.githubusercontent.com/u/10645273?v=4" width="100px;" alt=""/><br /><sub><b>Rakesh</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ravening" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Aravening" title="Reviewed Pull Requests">👀</a></td>
-    <td align="center"><a href="https://github.com/vINCENT8888801"><img src="https://avatars0.githubusercontent.com/u/8037883?v=4" width="100px;" alt=""/><br /><sub><b>Wei Seng</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vINCENT8888801" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/nishant"><img src="https://avatars2.githubusercontent.com/u/15331971?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Nishant Arora</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nishant" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/raja-peeyush-kumar-singh"><img src="https://avatars0.githubusercontent.com/u/5496024?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Peeyush</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=raja-peeyush-kumar-singh" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ravening"><img src="https://avatars1.githubusercontent.com/u/10645273?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Rakesh</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ravening" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Aravening" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://github.com/vINCENT8888801"><img src="https://avatars0.githubusercontent.com/u/8037883?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Wei Seng</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vINCENT8888801" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://www.linkedin.com/in/ashish-trivedi-218379135/"><img src="https://avatars3.githubusercontent.com/u/23194128?v=4" width="100px;" alt=""/><br /><sub><b>Ashish Trivedi</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ashishtrivedi16" title="Code">💻</a></td>
-    <td align="center"><a href="https://rayyounghong.com"><img src="https://avatars1.githubusercontent.com/u/41055099?v=4" width="100px;" alt=""/><br /><sub><b>洪月阳</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=RayYH" title="Code">💻</a></td>
-    <td align="center"><a href="https://xdvrx1.github.io/"><img src="https://avatars0.githubusercontent.com/u/47092464?v=4" width="100px;" alt=""/><br /><sub><b>xdvrx1</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Axdvrx1" title="Reviewed Pull Requests">👀</a> <a href="#ideas-xdvrx1" title="Ideas, Planning, & Feedback">🤔</a></td>
-    <td align="center"><a href="http://subho.xyz"><img src="https://avatars0.githubusercontent.com/u/13291222?v=4" width="100px;" alt=""/><br /><sub><b>Subhrodip Mohanta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ohbus" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Aohbus" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/ashish-trivedi-218379135/"><img src="https://avatars3.githubusercontent.com/u/23194128?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ashish Trivedi</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ashishtrivedi16" title="Code">💻</a></td>
+    <td align="center"><a href="https://rayyounghong.com"><img src="https://avatars1.githubusercontent.com/u/41055099?v=4?s=100" width="100px;" alt=""/><br /><sub><b>洪月阳</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=RayYH" title="Code">💻</a></td>
+    <td align="center"><a href="https://xdvrx1.github.io/"><img src="https://avatars0.githubusercontent.com/u/47092464?v=4?s=100" width="100px;" alt=""/><br /><sub><b>xdvrx1</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Axdvrx1" title="Reviewed Pull Requests">👀</a> <a href="#ideas-xdvrx1" title="Ideas, Planning, & Feedback">🤔</a></td>
+    <td align="center"><a href="http://subho.xyz"><img src="https://avatars0.githubusercontent.com/u/13291222?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Subhrodip Mohanta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ohbus" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Aohbus" title="Reviewed Pull Requests">👀</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/nahteb"><img src="https://avatars3.githubusercontent.com/u/13121570?v=4" width="100px;" alt=""/><br /><sub><b>Bethan Palmer</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nahteb" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/ToxicDreamz"><img src="https://avatars0.githubusercontent.com/u/45225562?v=4" width="100px;" alt=""/><br /><sub><b>Toxic Dreamz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ToxicDreamz" title="Code">💻</a></td>
-    <td align="center"><a href="http://www.edycutjong.com"><img src="https://avatars1.githubusercontent.com/u/1098102?v=4" width="100px;" alt=""/><br /><sub><b>Edy Cu Tjong</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=edycutjong" title="Documentation">📖</a></td>
-    <td align="center"><a href="https://github.com/mkrzywanski"><img src="https://avatars0.githubusercontent.com/u/15279585?v=4" width="100px;" alt=""/><br /><sub><b>Michał Krzywański</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mkrzywanski" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/nahteb"><img src="https://avatars3.githubusercontent.com/u/13121570?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Bethan Palmer</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nahteb" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ToxicDreamz"><img src="https://avatars0.githubusercontent.com/u/45225562?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Toxic Dreamz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ToxicDreamz" title="Code">💻</a></td>
+    <td align="center"><a href="http://www.edycutjong.com"><img src="https://avatars1.githubusercontent.com/u/1098102?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Edy Cu Tjong</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=edycutjong" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://github.com/mkrzywanski"><img src="https://avatars0.githubusercontent.com/u/15279585?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Michał Krzywański</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mkrzywanski" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://www.stefan-birkner.de"><img src="https://avatars1.githubusercontent.com/u/711349?v=4" width="100px;" alt=""/><br /><sub><b>Stefan Birkner</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=stefanbirkner" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/fedorskvorcov"><img src="https://avatars3.githubusercontent.com/u/43882212?v=4" width="100px;" alt=""/><br /><sub><b>Fedor Skvorcov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fedorskvorcov" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/samilAyoub"><img src="https://avatars0.githubusercontent.com/u/61546990?v=4" width="100px;" alt=""/><br /><sub><b>samilAyoub</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=samilAyoub" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/vdlald"><img src="https://avatars0.githubusercontent.com/u/29997701?v=4" width="100px;" alt=""/><br /><sub><b>Vladislav Golubinov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vdlald" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.stefan-birkner.de"><img src="https://avatars1.githubusercontent.com/u/711349?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Stefan Birkner</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=stefanbirkner" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/fedorskvorcov"><img src="https://avatars3.githubusercontent.com/u/43882212?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Fedor Skvorcov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fedorskvorcov" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/samilAyoub"><img src="https://avatars0.githubusercontent.com/u/61546990?v=4?s=100" width="100px;" alt=""/><br /><sub><b>samilAyoub</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=samilAyoub" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/vdlald"><img src="https://avatars0.githubusercontent.com/u/29997701?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Vladislav Golubinov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vdlald" title="Code">💻</a></td>
   </tr>
   <tr>
-    <td align="center"><a href="https://github.com/swarajsaaj"><img src="https://avatars2.githubusercontent.com/u/6285049?v=4" width="100px;" alt=""/><br /><sub><b>Swaraj</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=swarajsaaj" title="Code">💻</a></td>
-    <td align="center"><a href="http://christophflick.de"><img src="https://avatars0.githubusercontent.com/u/4465376?v=4" width="100px;" alt=""/><br /><sub><b>Christoph Flick</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ChFlick" title="Documentation">📖</a></td>
-    <td align="center"><a href="https://github.com/Ascenio"><img src="https://avatars1.githubusercontent.com/u/7662016?v=4" width="100px;" alt=""/><br /><sub><b>Ascênio</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AAscenio" title="Reviewed Pull Requests">👀</a></td>
-    <td align="center"><a href="https://www.linkedin.com/in/domenico-sibilio/"><img src="https://avatars2.githubusercontent.com/u/24280982?v=4" width="100px;" alt=""/><br /><sub><b>Domenico Sibilio</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dsibilio" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://github.com/swarajsaaj"><img src="https://avatars2.githubusercontent.com/u/6285049?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Swaraj</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=swarajsaaj" title="Code">💻</a></td>
+    <td align="center"><a href="http://christophflick.de"><img src="https://avatars0.githubusercontent.com/u/4465376?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Christoph Flick</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ChFlick" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://github.com/Ascenio"><img src="https://avatars1.githubusercontent.com/u/7662016?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ascênio</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AAscenio" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/domenico-sibilio/"><img src="https://avatars2.githubusercontent.com/u/24280982?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Domenico Sibilio</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dsibilio" title="Documentation">📖</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/akashchandwani"><img src="https://avatars2.githubusercontent.com/u/3483277?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Akash Chandwani</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Aakashchandwani" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="http://www.linkedin.com/in/manannikov"><img src="https://avatars2.githubusercontent.com/u/7019769?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Pavlo Manannikov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=manannikov" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/eimanip"><img src="https://avatars0.githubusercontent.com/u/20307301?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Eiman</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=eimanip" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/OrangePants-R"><img src="https://avatars0.githubusercontent.com/u/42976136?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Rocky</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=OrangePants-R" title="Documentation">📖</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://ibrahimalii.github.io/"><img src="https://avatars2.githubusercontent.com/u/21141301?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Ibrahim ali abdelghany</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AibrahimAlii" title="Reviewed Pull Requests">👀</a></td>
+    <td align="center"><a href="https://github.com/gkulkarni2020"><img src="https://avatars3.githubusercontent.com/u/5161548?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Girish Kulkarni</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=gkulkarni2020" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://github.com/omk13"><img src="https://avatars0.githubusercontent.com/u/59054172?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Omar Karazoun</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=omk13" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/jeff303"><img src="https://avatars0.githubusercontent.com/u/3521562?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Jeff Evans</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=jeff303" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://viveksb007.github.io"><img src="https://avatars1.githubusercontent.com/u/12713808?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Vivek Singh</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=viveksb007" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/siavashsoleymani"><img src="https://avatars2.githubusercontent.com/u/18074419?v=4?s=100" width="100px;" alt=""/><br /><sub><b>siavash</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=siavashsoleymani" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ruchpeanuts"><img src="https://avatars0.githubusercontent.com/u/29301900?v=4?s=100" width="100px;" alt=""/><br /><sub><b>ruchpeanuts</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ruchpeanuts" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://github.com/warp125"><img src="https://avatars1.githubusercontent.com/u/48073115?v=4?s=100" width="100px;" alt=""/><br /><sub><b>warp125</b></sub></a><br /><a href="#translation-warp125" title="Translation">🌍</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://libkhadir.fr"><img src="https://avatars1.githubusercontent.com/u/45130488?v=4?s=100" width="100px;" alt=""/><br /><sub><b>KHADIR Tayeb</b></sub></a><br /><a href="#translation-tkhadir" title="Translation">🌍</a></td>
+    <td align="center"><a href="https://github.com/ignite1771"><img src="https://avatars2.githubusercontent.com/u/59446563?v=4?s=100" width="100px;" alt=""/><br /><sub><b>ignite1771</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ignite1771" title="Code">💻</a></td>
   </tr>
 </table>
 
-<!-- markdownlint-enable -->
+<!-- markdownlint-restore -->
 <!-- prettier-ignore-end -->
+
 <!-- ALL-CONTRIBUTORS-LIST:END -->

circuit-breaker/README.md

@@ -19,12 +19,15 @@ cannot bring the whole application down, and we can reconnect to the service as
 
 Real world example
 
-> Imagine a web application that has both local files/images and remote database entries to serve. 
-> The database might not be responding due to a variety of reasons, so if the application keeps 
-> trying to read from the database using multiple threads/processes, soon all of them will hang 
-> causing our entire web application will crash. We should be able to detect this situation and show 
-> the user an appropriate message so that he/she can explore other parts of the app unaffected by 
-> the database failure. 
+> Imagine a web application that has both local files/images and remote services that are used for 
+> fetching data. These remote services may be either healthy and responsive at times, or may become 
+> slow and unresponsive at some point of time due to variety of reasons. So if one of the remote 
+> services is slow or not responding successfully, our application will try to fetch response from 
+> the remote service using multiple threads/processes, soon all of them will hang (also called 
+> [thread starvation](https://en.wikipedia.org/wiki/Starvation_(computer_science))) causing our entire web application to crash. We should be able to detect 
+> this situation and show the user an appropriate message so that he/she can explore other parts of 
+> the app unaffected by the remote service failure. Meanwhile, the other services that are working 
+> normally, should keep functioning unaffected by this failure.
 
 In plain words
 
@@ -52,40 +55,104 @@ In terms of code, the end user application is:
 
 ```java
 public class App {
-   
+
   private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
-    
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args
+   */
   public static void main(String[] args) {
-    var obj = new MonitoringService();
-    var circuitBreaker = new CircuitBreaker(3000, 1, 2000 * 1000 * 1000); 
+
     var serverStartTime = System.nanoTime();
-    while (true) {
-      LOGGER.info(obj.localResourceResponse());
-      LOGGER.info(obj.remoteResourceResponse(circuitBreaker, serverStartTime));
-      LOGGER.info(circuitBreaker.getState());
-      try {
-        Thread.sleep(5 * 1000); 
-      } catch (InterruptedException e) {
-        LOGGER.error(e.getMessage());
-      }
+
+    var delayedService = new DelayedRemoteService(serverStartTime, 5);
+    var delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000, 2,
+        2000 * 1000 * 1000);
+
+    var quickService = new QuickRemoteService();
+    var quickServiceCircuitBreaker = new DefaultCircuitBreaker(quickService, 3000, 2,
+        2000 * 1000 * 1000);
+
+    //Create an object of monitoring service which makes both local and remote calls
+    var monitoringService = new MonitoringService(delayedServiceCircuitBreaker,
+        quickServiceCircuitBreaker);
+
+    //Fetch response from local resource
+    LOGGER.info(monitoringService.localResourceResponse());
+
+    //Fetch response from delayed service 2 times, to meet the failure threshold
+    LOGGER.info(monitoringService.delayedServiceResponse());
+    LOGGER.info(monitoringService.delayedServiceResponse());
+
+    //Fetch current state of delayed service circuit breaker after crossing failure threshold limit
+    //which is OPEN now
+    LOGGER.info(delayedServiceCircuitBreaker.getState());
+
+    //Meanwhile, the delayed service is down, fetch response from the healthy quick service
+    LOGGER.info(monitoringService.quickServiceResponse());
+    LOGGER.info(quickServiceCircuitBreaker.getState());
+
+    //Wait for the delayed service to become responsive
+    try {
+      LOGGER.info("Waiting for delayed service to become responsive");
+      Thread.sleep(5000);
+    } catch (InterruptedException e) {
+      e.printStackTrace();
     }
+    //Check the state of delayed circuit breaker, should be HALF_OPEN
+    LOGGER.info(delayedServiceCircuitBreaker.getState());
+
+    //Fetch response from delayed service, which should be healthy by now
+    LOGGER.info(monitoringService.delayedServiceResponse());
+    //As successful response is fetched, it should be CLOSED again.
+    LOGGER.info(delayedServiceCircuitBreaker.getState());
   }
 }
 ```
 
 The monitoring service: 
 
-``` java
+```java
 public class MonitoringService {
 
+  private final CircuitBreaker delayedService;
+
+  private final CircuitBreaker quickService;
+
+  public MonitoringService(CircuitBreaker delayedService, CircuitBreaker quickService) {
+    this.delayedService = delayedService;
+    this.quickService = quickService;
+  }
+
+  //Assumption: Local service won't fail, no need to wrap it in a circuit breaker logic
   public String localResourceResponse() {
     return "Local Service is working";
   }
 
-  public String remoteResourceResponse(CircuitBreaker circuitBreaker, long serverStartTime) {
+  /**
+   * Fetch response from the delayed service (with some simulated startup time).
+   *
+   * @return response string
+   */
+  public String delayedServiceResponse() {
     try {
-      return circuitBreaker.call("delayedService", serverStartTime);
-    } catch (Exception e) {
+      return this.delayedService.attemptRequest();
+    } catch (RemoteServiceException e) {
+      return e.getMessage();
+    }
+  }
+
+  /**
+   * Fetches response from a healthy service without any failure.
+   *
+   * @return response string
+   */
+  public String quickServiceResponse() {
+    try {
+      return this.quickService.attemptRequest();
+    } catch (RemoteServiceException e) {
       return e.getMessage();
     }
   }
@@ -95,72 +162,127 @@ As it can be seen, it does the call to get local resources directly, but it wrap
 remote (costly) service in a circuit breaker object, which prevents faults as follows:
 
 ```java
-public class CircuitBreaker {
+public class DefaultCircuitBreaker implements CircuitBreaker {
+
   private final long timeout;
   private final long retryTimePeriod;
+  private final RemoteService service;
   long lastFailureTime;
+  private String lastFailureResponse;
   int failureCount;
   private final int failureThreshold;
   private State state;
   private final long futureTime = 1000 * 1000 * 1000 * 1000;
 
-  CircuitBreaker(long timeout, int failureThreshold, long retryTimePeriod) {
+  /**
+   * Constructor to create an instance of Circuit Breaker.
+   *
+   * @param timeout          Timeout for the API request. Not necessary for this simple example
+   * @param failureThreshold Number of failures we receive from the depended service before changing
+   *                         state to 'OPEN'
+   * @param retryTimePeriod  Time period after which a new request is made to remote service for
+   *                         status check.
+   */
+  DefaultCircuitBreaker(RemoteService serviceToCall, long timeout, int failureThreshold,
+      long retryTimePeriod) {
+    this.service = serviceToCall;
+    // We start in a closed state hoping that everything is fine
     this.state = State.CLOSED;
     this.failureThreshold = failureThreshold;
+    // Timeout for the API request.
+    // Used to break the calls made to remote resource if it exceeds the limit
     this.timeout = timeout;
     this.retryTimePeriod = retryTimePeriod;
+    //An absurd amount of time in future which basically indicates the last failure never happened
     this.lastFailureTime = System.nanoTime() + futureTime;
     this.failureCount = 0;
   }
-    
-  private void reset() {
+
+  // Reset everything to defaults
+  @Override
+  public void recordSuccess() {
     this.failureCount = 0;
-    this.lastFailureTime = System.nanoTime() + futureTime; 
+    this.lastFailureTime = System.nanoTime() + futureTime;
     this.state = State.CLOSED;
   }
 
-  private void recordFailure() {
+  @Override
+  public void recordFailure(String response) {
     failureCount = failureCount + 1;
     this.lastFailureTime = System.nanoTime();
+    // Cache the failure response for returning on open state
+    this.lastFailureResponse = response;
   }
-    
-  protected void setState() {
-    if (failureCount > failureThreshold) { 
+
+  // Evaluate the current state based on failureThreshold, failureCount and lastFailureTime.
+  protected void evaluateState() {
+    if (failureCount >= failureThreshold) { //Then something is wrong with remote service
       if ((System.nanoTime() - lastFailureTime) > retryTimePeriod) {
+        //We have waited long enough and should try checking if service is up
         state = State.HALF_OPEN;
       } else {
+        //Service would still probably be down
         state = State.OPEN;
       }
     } else {
+      //Everything is working fine
       state = State.CLOSED;
     }
   }
-    
+
+  @Override
   public String getState() {
+    evaluateState();
     return state.name();
   }
-  
-  public void setStateForBypass(State state) {
+
+  /**
+   * Break the circuit beforehand if it is known service is down Or connect the circuit manually if
+   * service comes online before expected.
+   *
+   * @param state State at which circuit is in
+   */
+  @Override
+  public void setState(State state) {
     this.state = state;
+    switch (state) {
+      case OPEN:
+        this.failureCount = failureThreshold;
+        this.lastFailureTime = System.nanoTime();
+        break;
+      case HALF_OPEN:
+        this.failureCount = failureThreshold;
+        this.lastFailureTime = System.nanoTime() - retryTimePeriod;
+        break;
+      default:
+        this.failureCount = 0;
+    }
   }
-   
-  public String call(String serviceToCall, long serverStartTime) throws Exception {
-    setState();
+
+  /**
+   * Executes service call.
+   *
+   * @return Value from the remote resource, stale response or a custom exception
+   */
+  @Override
+  public String attemptRequest() throws RemoteServiceException {
+    evaluateState();
     if (state == State.OPEN) {
-      return "This is stale response from API";
+      // return cached response if the circuit is in OPEN state
+      return this.lastFailureResponse;
     } else {
-      if (serviceToCall.equals("delayedService")) {
-        var delayedService = new DelayedService(20);
-        var response = delayedService.response(serverStartTime);
-        if (response.split(" ")[3].equals("working")) {
-          reset();
-          return response;
-        } else {
-          recordFailure();
-          throw new Exception("Remote service not responding");
-        }
-      } else {
-        throw new Exception("Unknown Service Name");
+      // Make the API request if the circuit is not OPEN
+      try {
+        //In a real application, this would be run in a thread and the timeout
+        //parameter of the circuit breaker would be utilized to know if service
+        //is working. Here, we simulate that based on server response itself
+        var response = service.call();
+        // Yay!! the API responded fine. Let's reset everything.
+        recordSuccess();
+        return response;
+      } catch (RemoteServiceException ex) {
+        recordFailure(ex.getMessage());
+        throw ex;
       }
     }
   }

circuit-breaker/etc/circuit-breaker.urm.puml

@@ -5,32 +5,52 @@ package com.iluwatar.circuitbreaker {
     + App()
     + main(args : String[]) {static}
   }
-  class CircuitBreaker {
+  interface CircuitBreaker {
+    + attemptRequest() : String {abstract}
+    + getState() : String {abstract}
+    + recordFailure(String) {abstract}
+    + recordSuccess() {abstract}
+    + setState(State) {abstract}
+  }
+  class DefaultCircuitBreaker {
     ~ failureCount : int
     - failureThreshold : int
     - futureTime : long
+    - lastFailureResponse : String
     ~ lastFailureTime : long
     - retryTimePeriod : long
+    - service : RemoteService
     - state : State
     - timeout : long
-    ~ CircuitBreaker(timeout : long, failureThreshold : int, retryTimePeriod : long)
-    + call(serviceToCall : String, serverStartTime : long) : String
+    ~ DefaultCircuitBreaker(serviceToCall : RemoteService, timeout : long, failureThreshold : int, retryTimePeriod : long)
+    + attemptRequest() : String
+    # evaluateState()
     + getState() : String
-    - recordFailure()
-    - reset()
-    # setState()
-    + setStateForBypass(state : State)
+    + recordFailure(response : String)
+    + recordSuccess()
+    + setState(state : State)
   }
-  class DelayedService {
+  class DelayedRemoteService {
     - delay : int
-    + DelayedService()
-    + DelayedService(delay : int)
-    + response(serverStartTime : long) : String
+    - serverStartTime : long
+    + DelayedRemoteService()
+    + DelayedRemoteService(serverStartTime : long, delay : int)
+    + call() : String
   }
   class MonitoringService {
-    + MonitoringService()
+    - delayedService : CircuitBreaker
+    - quickService : CircuitBreaker
+    + MonitoringService(delayedService : CircuitBreaker, quickService : CircuitBreaker)
+    + delayedServiceResponse() : String
     + localResourceResponse() : String
-    + remoteResourceResponse(circuitBreaker : CircuitBreaker, serverStartTime : long) : String
+    + quickServiceResponse() : String
+  }
+  class QuickRemoteService {
+    + QuickRemoteService()
+    + call() : String
+  }
+  interface RemoteService {
+    + call() : String {abstract}
   }
   enum State {
     + CLOSED {static}
@@ -40,5 +60,10 @@ package com.iluwatar.circuitbreaker {
     + values() : State[] {static}
   }
 }
-CircuitBreaker -->  "-state" State
+DefaultCircuitBreaker -->  "-state" State
+MonitoringService -->  "-delayedService" CircuitBreaker
+DefaultCircuitBreaker -->  "-service" RemoteService
+DefaultCircuitBreaker ..|> CircuitBreaker 
+DelayedRemoteService ..|> RemoteService 
+QuickRemoteService ..|> RemoteService 
 @enduml

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/App.java

@@ -36,17 +36,18 @@ import org.slf4j.LoggerFactory;
  * operational again, so that we can use it
  * </p>
  * <p>
- * In this example, the circuit breaker pattern is demonstrated by using two services: {@link
- * MonitoringService} and {@link DelayedService}. The monitoring service is responsible for calling
- * two services: a local service and a remote service {@link DelayedService} , and by using the
- * circuit breaker construction we ensure that if the call to remote service is going to fail, we
- * are going to save our resources and not make the function call at all, by wrapping our call to
- * the remote service in the circuit breaker object.
+ * In this example, the circuit breaker pattern is demonstrated by using three services: {@link
+ * DelayedRemoteService}, {@link QuickRemoteService} and {@link MonitoringService}. The monitoring
+ * service is responsible for calling three services: a local service, a quick remove service
+ * {@link QuickRemoteService} and a delayed remote service {@link DelayedRemoteService} , and by
+ * using the circuit breaker construction we ensure that if the call to remote service is going to
+ * fail, we are going to save our resources and not make the function call at all, by wrapping our
+ * call to the remote services in the {@link DefaultCircuitBreaker} implementation object.
  * </p>
  * <p>
- * This works as follows: The {@link CircuitBreaker} object can be in one of three states:
- * <b>Open</b>, <b>Closed</b> and <b>Half-Open</b>, which represents the real world circuits. If the
- * state is closed (initial), we assume everything is alright and perform the function call.
+ * This works as follows: The {@link DefaultCircuitBreaker} object can be in one of three states:
+ * <b>Open</b>, <b>Closed</b> and <b>Half-Open</b>, which represents the real world circuits. If
+ * the state is closed (initial), we assume everything is alright and perform the function call.
  * However, every time the call fails, we note it and once it crosses a threshold, we set the state
  * to Open, preventing any further calls to the remote server. Then, after a certain retry period
  * (during which we expect thee service to recover), we make another call to the remote server and
@@ -63,22 +64,50 @@ public class App {
    *
    * @param args command line args
    */
-  @SuppressWarnings("squid:S2189")
   public static void main(String[] args) {
-    //Create an object of monitoring service which makes both local and remote calls
-    var obj = new MonitoringService();
-    //Set the circuit Breaker parameters
-    var circuitBreaker = new CircuitBreaker(3000, 1, 2000 * 1000 * 1000);
+
     var serverStartTime = System.nanoTime();
-    while (true) {
-      LOGGER.info(obj.localResourceResponse());
-      LOGGER.info(obj.remoteResourceResponse(circuitBreaker, serverStartTime));
-      LOGGER.info(circuitBreaker.getState());
-      try {
-        Thread.sleep(5 * 1000);
-      } catch (InterruptedException e) {
-        LOGGER.error(e.getMessage());
-      }
+
+    var delayedService = new DelayedRemoteService(serverStartTime, 5);
+    var delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000, 2,
+        2000 * 1000 * 1000);
+
+    var quickService = new QuickRemoteService();
+    var quickServiceCircuitBreaker = new DefaultCircuitBreaker(quickService, 3000, 2,
+        2000 * 1000 * 1000);
+
+    //Create an object of monitoring service which makes both local and remote calls
+    var monitoringService = new MonitoringService(delayedServiceCircuitBreaker,
+        quickServiceCircuitBreaker);
+
+    //Fetch response from local resource
+    LOGGER.info(monitoringService.localResourceResponse());
+
+    //Fetch response from delayed service 2 times, to meet the failure threshold
+    LOGGER.info(monitoringService.delayedServiceResponse());
+    LOGGER.info(monitoringService.delayedServiceResponse());
+
+    //Fetch current state of delayed service circuit breaker after crossing failure threshold limit
+    //which is OPEN now
+    LOGGER.info(delayedServiceCircuitBreaker.getState());
+
+    //Meanwhile, the delayed service is down, fetch response from the healthy quick service
+    LOGGER.info(monitoringService.quickServiceResponse());
+    LOGGER.info(quickServiceCircuitBreaker.getState());
+
+    //Wait for the delayed service to become responsive
+    try {
+      LOGGER.info("Waiting for delayed service to become responsive");
+      Thread.sleep(5000);
+    } catch (InterruptedException e) {
+      e.printStackTrace();
     }
+    //Check the state of delayed circuit breaker, should be HALF_OPEN
+    LOGGER.info(delayedServiceCircuitBreaker.getState());
+
+    //Fetch response from delayed service, which should be healthy by now
+    LOGGER.info(monitoringService.delayedServiceResponse());
+    //As successful response is fetched, it should be CLOSED again.
+    LOGGER.info(delayedServiceCircuitBreaker.getState());
   }
 }

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/CircuitBreaker.java

@@ -24,114 +24,22 @@
 package com.iluwatar.circuitbreaker;
 
 /**
- * The circuit breaker class with all configurations.
+ * The Circuit breaker interface.
  */
-public class CircuitBreaker {
-  private final long timeout;
-  private final long retryTimePeriod;
-  long lastFailureTime;
-  int failureCount;
-  private final int failureThreshold;
-  private State state;
-  private final long futureTime = 1000 * 1000 * 1000 * 1000;
+public interface CircuitBreaker {
 
-  /**
-   * Constructor to create an instance of Circuit Breaker.
-   *
-   * @param timeout          Timeout for the API request. Not necessary for this simple example
-   * @param failureThreshold Number of failures we receive from the depended service before changing
-   *                         state to 'OPEN'
-   * @param retryTimePeriod  Time period after which a new request is made to remote service for
-   *                         status check.
-   */
-  CircuitBreaker(long timeout, int failureThreshold, long retryTimePeriod) {
-    // We start in a closed state hoping that everything is fine
-    this.state = State.CLOSED;
-    this.failureThreshold = failureThreshold;
-    // Timeout for the API request.
-    // Used to break the calls made to remote resource if it exceeds the limit
-    this.timeout = timeout;
-    this.retryTimePeriod = retryTimePeriod;
-    //An absurd amount of time in future which basically indicates the last failure never happened
-    this.lastFailureTime = System.nanoTime() + futureTime;
-    this.failureCount = 0;
-  }
+  // Success response. Reset everything to defaults
+  void recordSuccess();
 
-  //Reset everything to defaults
-  private void reset() {
-    this.failureCount = 0;
-    this.lastFailureTime = System.nanoTime() + futureTime;
-    this.state = State.CLOSED;
-  }
+  // Failure response. Handle accordingly with response and change state if required.
+  void recordFailure(String response);
 
-  private void recordFailure() {
-    failureCount = failureCount + 1;
-    this.lastFailureTime = System.nanoTime();
-  }
+  // Get the current state of circuit breaker
+  String getState();
 
-  protected void setState() {
-    if (failureCount > failureThreshold) { //Then something is wrong with remote service
-      if ((System.nanoTime() - lastFailureTime) > retryTimePeriod) {
-        //We have waited long enough and should try checking if service is up
-        state = State.HALF_OPEN;
-      } else {
-        //Service would still probably be down
-        state = State.OPEN;
-      }
-    } else {
-      //Everything is working fine
-      state = State.CLOSED;
-    }
-  }
+  // Set the specific state manually.
+  void setState(State state);
 
-  public String getState() {
-    return state.name();
-  }
-
-  /**
-   * Break the circuit beforehand if it is known service is down Or connect the circuit manually if
-   * service comes online before expected.
-   *
-   * @param state State at which circuit is in
-   */
-  public void setStateForBypass(State state) {
-    this.state = state;
-  }
-
-  /**
-   * Executes service call.
-   *
-   * @param serviceToCall   The name of the service in String. Can be changed to data URLs in case
-   *                        of web applications
-   * @param serverStartTime Time at which actual server was started which makes calls to this
-   *                        service
-   * @return Value from the remote resource, stale response or a custom exception
-   */
-  public String call(String serviceToCall, long serverStartTime) throws Exception {
-    setState();
-    if (state == State.OPEN) {
-      // return  cached response if no the circuit is in OPEN state
-      return "This is stale response from API";
-    } else {
-      // Make the API request if the circuit is not OPEN
-      if (serviceToCall.equals("delayedService")) {
-        var delayedService = new DelayedService(20);
-        var response = delayedService.response(serverStartTime);
-        //In a real application, this would be run in a thread and the timeout
-        //parameter of the circuit breaker would be utilized to know if service
-        //is working. Here, we simulate that based on server response itself
-        if (response.split(" ")[3].equals("working")) {
-          // Yay!! the API responded fine. Let's reset everything.
-          reset();
-          return response;
-        } else {
-          // Uh-oh!! the call still failed. Let's update that in our records.
-          recordFailure();
-          throw new Exception("Remote service not responding");
-        }
-      } else {
-        throw new Exception("Unknown Service Name");
-      }
-    }
-  }
-}
+  // Attempt to fetch response from the remote service.
+  String attemptRequest() throws RemoteServiceException;
+}

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/DefaultCircuitBreaker.java

@@ -0,0 +1,155 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.circuitbreaker;
+
+/**
+ * The delay based Circuit breaker implementation that works in a
+ * CLOSED->OPEN-(retry_time_period)->HALF_OPEN->CLOSED flow with some retry time period for failed
+ * services and a failure threshold for service to open circuit.
+ */
+public class DefaultCircuitBreaker implements CircuitBreaker {
+
+  private final long timeout;
+  private final long retryTimePeriod;
+  private final RemoteService service;
+  long lastFailureTime;
+  private String lastFailureResponse;
+  int failureCount;
+  private final int failureThreshold;
+  private State state;
+  private final long futureTime = 1000 * 1000 * 1000 * 1000;
+
+  /**
+   * Constructor to create an instance of Circuit Breaker.
+   *
+   * @param timeout          Timeout for the API request. Not necessary for this simple example
+   * @param failureThreshold Number of failures we receive from the depended service before changing
+   *                         state to 'OPEN'
+   * @param retryTimePeriod  Time period after which a new request is made to remote service for
+   *                         status check.
+   */
+  DefaultCircuitBreaker(RemoteService serviceToCall, long timeout, int failureThreshold,
+      long retryTimePeriod) {
+    this.service = serviceToCall;
+    // We start in a closed state hoping that everything is fine
+    this.state = State.CLOSED;
+    this.failureThreshold = failureThreshold;
+    // Timeout for the API request.
+    // Used to break the calls made to remote resource if it exceeds the limit
+    this.timeout = timeout;
+    this.retryTimePeriod = retryTimePeriod;
+    //An absurd amount of time in future which basically indicates the last failure never happened
+    this.lastFailureTime = System.nanoTime() + futureTime;
+    this.failureCount = 0;
+  }
+
+  // Reset everything to defaults
+  @Override
+  public void recordSuccess() {
+    this.failureCount = 0;
+    this.lastFailureTime = System.nanoTime() + futureTime;
+    this.state = State.CLOSED;
+  }
+
+  @Override
+  public void recordFailure(String response) {
+    failureCount = failureCount + 1;
+    this.lastFailureTime = System.nanoTime();
+    // Cache the failure response for returning on open state
+    this.lastFailureResponse = response;
+  }
+
+  // Evaluate the current state based on failureThreshold, failureCount and lastFailureTime.
+  protected void evaluateState() {
+    if (failureCount >= failureThreshold) { //Then something is wrong with remote service
+      if ((System.nanoTime() - lastFailureTime) > retryTimePeriod) {
+        //We have waited long enough and should try checking if service is up
+        state = State.HALF_OPEN;
+      } else {
+        //Service would still probably be down
+        state = State.OPEN;
+      }
+    } else {
+      //Everything is working fine
+      state = State.CLOSED;
+    }
+  }
+
+  @Override
+  public String getState() {
+    evaluateState();
+    return state.name();
+  }
+
+  /**
+   * Break the circuit beforehand if it is known service is down Or connect the circuit manually if
+   * service comes online before expected.
+   *
+   * @param state State at which circuit is in
+   */
+  @Override
+  public void setState(State state) {
+    this.state = state;
+    switch (state) {
+      case OPEN:
+        this.failureCount = failureThreshold;
+        this.lastFailureTime = System.nanoTime();
+        break;
+      case HALF_OPEN:
+        this.failureCount = failureThreshold;
+        this.lastFailureTime = System.nanoTime() - retryTimePeriod;
+        break;
+      default:
+        this.failureCount = 0;
+    }
+  }
+
+  /**
+   * Executes service call.
+   *
+   * @return Value from the remote resource, stale response or a custom exception
+   */
+  @Override
+  public String attemptRequest() throws RemoteServiceException {
+    evaluateState();
+    if (state == State.OPEN) {
+      // return cached response if the circuit is in OPEN state
+      return this.lastFailureResponse;
+    } else {
+      // Make the API request if the circuit is not OPEN
+      try {
+        //In a real application, this would be run in a thread and the timeout
+        //parameter of the circuit breaker would be utilized to know if service
+        //is working. Here, we simulate that based on server response itself
+        var response = service.call();
+        // Yay!! the API responded fine. Let's reset everything.
+        recordSuccess();
+        return response;
+      } catch (RemoteServiceException ex) {
+        recordFailure(ex.getMessage());
+        throw ex;
+      }
+    }
+  }
+}

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/DelayedRemoteService.java

@@ -27,7 +27,9 @@ package com.iluwatar.circuitbreaker;
  * This simulates the remote service It responds only after a certain timeout period (default set to
  * 20 seconds).
  */
-public class DelayedService {
+public class DelayedRemoteService implements RemoteService {
+
+  private final long serverStartTime;
   private final int delay;
 
   /**
@@ -35,22 +37,23 @@ public class DelayedService {
    *
    * @param delay the delay after which service would behave properly, in seconds
    */
-  public DelayedService(int delay) {
+  public DelayedRemoteService(long serverStartTime, int delay) {
+    this.serverStartTime = serverStartTime;
     this.delay = delay;
   }
 
-  public DelayedService() {
-    this.delay = 60;
+  public DelayedRemoteService() {
+    this.serverStartTime = System.nanoTime();
+    this.delay = 20;
   }
 
   /**
    * Responds based on delay, current time and server start time if the service is down / working.
    *
-   * @param serverStartTime Time at which actual server was started which makes calls to this
-   *                        service
    * @return The state of the service
    */
-  public String response(long serverStartTime) {
+  @Override
+  public String call() throws RemoteServiceException {
     var currentTime = System.nanoTime();
     //Since currentTime and serverStartTime are both in nanoseconds, we convert it to
     //seconds by diving by 10e9 and ensure floating point division by multiplying it
@@ -58,9 +61,8 @@ public class DelayedService {
     //send the reply
     if ((currentTime - serverStartTime) * 1.0 / (1000 * 1000 * 1000) < delay) {
       //Can use Thread.sleep() here to block and simulate a hung server
-      return "Delayed service is down";
-    } else {
-      return "Delayed service is working";
+      throw new RemoteServiceException("Delayed service is down");
     }
+    return "Delayed service is working";
   }
 }

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/MonitoringService.java

@@ -24,28 +24,47 @@
 package com.iluwatar.circuitbreaker;
 
 /**
- * The service class which makes local and remote calls Uses {@link CircuitBreaker} object to ensure
- * remote calls don't use up resources.
+ * The service class which makes local and remote calls Uses {@link DefaultCircuitBreaker} object to
+ * ensure remote calls don't use up resources.
  */
 public class MonitoringService {
 
+  private final CircuitBreaker delayedService;
+
+  private final CircuitBreaker quickService;
+
+  public MonitoringService(CircuitBreaker delayedService, CircuitBreaker quickService) {
+    this.delayedService = delayedService;
+    this.quickService = quickService;
+  }
+
   //Assumption: Local service won't fail, no need to wrap it in a circuit breaker logic
   public String localResourceResponse() {
     return "Local Service is working";
   }
 
   /**
-   * Try to get result from remote server.
+   * Fetch response from the delayed service (with some simulated startup time).
    *
-   * @param circuitBreaker  The circuitBreaker object with all parameters
-   * @param serverStartTime Time at which actual server was started which makes calls to this
-   *                        service
-   * @return result from the remote response or exception raised by it.
+   * @return response string
    */
-  public String remoteResourceResponse(CircuitBreaker circuitBreaker, long serverStartTime) {
+  public String delayedServiceResponse() {
     try {
-      return circuitBreaker.call("delayedService", serverStartTime);
-    } catch (Exception e) {
+      return this.delayedService.attemptRequest();
+    } catch (RemoteServiceException e) {
+      return e.getMessage();
+    }
+  }
+
+  /**
+   * Fetches response from a healthy service without any failure.
+   *
+   * @return response string
+   */
+  public String quickServiceResponse() {
+    try {
+      return this.quickService.attemptRequest();
+    } catch (RemoteServiceException e) {
       return e.getMessage();
     }
   }

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/QuickRemoteService.java

@@ -23,19 +23,13 @@
 
 package com.iluwatar.circuitbreaker;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
-
-import org.junit.jupiter.api.Test;
-
 /**
- * Monitoring Service test
+ * A quick response remote service, that responds healthy without any delay or failure.
  */
-public class DelayedServiceTest {
+public class QuickRemoteService implements RemoteService {
 
-  //Improves code coverage
-  @Test
-  public void testDefaultConstructor() {
-    var obj = new DelayedService();
-    assertEquals(obj.response(System.nanoTime()), "Delayed service is down");
+  @Override
+  public String call() throws RemoteServiceException {
+    return "Quick Service is working";
   }
 }

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/RemoteService.java

@@ -0,0 +1,34 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.circuitbreaker;
+
+/**
+ * The Remote service interface, used by {@link CircuitBreaker} for fetching response from remote
+ * services.
+ */
+public interface RemoteService {
+
+  //Fetch response from remote service.
+  String call() throws RemoteServiceException;
+}

circuit-breaker/src/main/java/com/iluwatar/circuitbreaker/RemoteServiceException.java

@@ -21,39 +21,14 @@
  * THE SOFTWARE.
  */
 
-package com.iluwatar.command;
+package com.iluwatar.circuitbreaker;
 
 /**
- * ShrinkSpell is a concrete command.
+ * Exception thrown when {@link RemoteService} does not respond successfully.
  */
-public class ShrinkSpell implements Command {
+public class RemoteServiceException extends Exception {
 
-  private Size oldSize;
-  private Target target;
-
-  @Override
-  public void execute(Target target) {
-    oldSize = target.getSize();
-    target.setSize(Size.SMALL);
-    this.target = target;
-  }
-
-  @Override
-  public void undo() {
-    if (oldSize != null && target != null) {
-      var temp = target.getSize();
-      target.setSize(oldSize);
-      oldSize = temp;
-    }
-  }
-
-  @Override
-  public void redo() {
-    undo();
-  }
-
-  @Override
-  public String toString() {
-    return "Shrink spell";
+  public RemoteServiceException(String message) {
+    super(message);
   }
 }

circuit-breaker/src/test/java/com/iluwatar/circuitbreaker/AppTest.java

@@ -0,0 +1,135 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.circuitbreaker;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * App Test showing usage of circuit breaker.
+ */
+public class AppTest {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(AppTest.class);
+
+  //Startup delay for delayed service (in seconds)
+  private static final int STARTUP_DELAY = 4;
+
+  //Number of failed requests for circuit breaker to open
+  private static final int FAILURE_THRESHOLD = 1;
+
+  //Time period in seconds for circuit breaker to retry service
+  private static final int RETRY_PERIOD = 2;
+
+  private MonitoringService monitoringService;
+
+  private CircuitBreaker delayedServiceCircuitBreaker;
+
+  private CircuitBreaker quickServiceCircuitBreaker;
+
+  /**
+   * Setup the circuit breakers and services, where {@link DelayedRemoteService} will be start with
+   * a delay of 4 seconds and a {@link QuickRemoteService} responding healthy. Both services are
+   * wrapped in a {@link DefaultCircuitBreaker} implementation with failure threshold of 1 failure
+   * and retry time period of 2 seconds.
+   */
+  @BeforeEach
+  public void setupCircuitBreakers() {
+    var delayedService = new DelayedRemoteService(System.nanoTime(), STARTUP_DELAY);
+    //Set the circuit Breaker parameters
+    delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000,
+        FAILURE_THRESHOLD,
+        RETRY_PERIOD * 1000 * 1000 * 1000);
+
+    var quickService = new QuickRemoteService();
+    //Set the circuit Breaker parameters
+    quickServiceCircuitBreaker = new DefaultCircuitBreaker(quickService, 3000, FAILURE_THRESHOLD,
+        RETRY_PERIOD * 1000 * 1000 * 1000);
+
+    monitoringService = new MonitoringService(delayedServiceCircuitBreaker,
+        quickServiceCircuitBreaker);
+
+  }
+
+  @Test
+  public void testFailure_OpenStateTransition() {
+    //Calling delayed service, which will be unhealthy till 4 seconds
+    assertEquals("Delayed service is down", monitoringService.delayedServiceResponse());
+    //As failure threshold is "1", the circuit breaker is changed to OPEN
+    assertEquals("OPEN", delayedServiceCircuitBreaker.getState());
+    //As circuit state is OPEN, we expect a quick fallback response from circuit breaker.
+    assertEquals("Delayed service is down", monitoringService.delayedServiceResponse());
+
+    //Meanwhile, the quick service is responding and the circuit state is CLOSED
+    assertEquals("Quick Service is working", monitoringService.quickServiceResponse());
+    assertEquals("CLOSED", quickServiceCircuitBreaker.getState());
+
+  }
+
+  @Test
+  public void testFailure_HalfOpenStateTransition() {
+    //Calling delayed service, which will be unhealthy till 4 seconds
+    assertEquals("Delayed service is down", monitoringService.delayedServiceResponse());
+    //As failure threshold is "1", the circuit breaker is changed to OPEN
+    assertEquals("OPEN", delayedServiceCircuitBreaker.getState());
+
+    //Waiting for recovery period of 2 seconds for circuit breaker to retry service.
+    try {
+      LOGGER.info("Waiting 2s for delayed service to become responsive");
+      Thread.sleep(2000);
+    } catch (InterruptedException e) {
+      e.printStackTrace();
+    }
+    //After 2 seconds, the circuit breaker should move to "HALF_OPEN" state and retry fetching response from service again
+    assertEquals("HALF_OPEN", delayedServiceCircuitBreaker.getState());
+
+  }
+
+  @Test
+  public void testRecovery_ClosedStateTransition() {
+    //Calling delayed service, which will be unhealthy till 4 seconds
+    assertEquals("Delayed service is down", monitoringService.delayedServiceResponse());
+    //As failure threshold is "1", the circuit breaker is changed to OPEN
+    assertEquals("OPEN", delayedServiceCircuitBreaker.getState());
+
+    //Waiting for 4 seconds, which is enough for DelayedService to become healthy and respond successfully.
+    try {
+      LOGGER.info("Waiting 4s for delayed service to become responsive");
+      Thread.sleep(4000);
+    } catch (InterruptedException e) {
+      e.printStackTrace();
+    }
+    //As retry period is 2 seconds (<4 seconds of wait), hence the circuit breaker should be back in HALF_OPEN state.
+    assertEquals("HALF_OPEN", delayedServiceCircuitBreaker.getState());
+    //Check the success response from delayed service.
+    assertEquals("Delayed service is working", monitoringService.delayedServiceResponse());
+    //As the response is success, the state should be CLOSED
+    assertEquals("CLOSED", delayedServiceCircuitBreaker.getState());
+  }
+
+}

circuit-breaker/src/test/java/com/iluwatar/circuitbreaker/DefaultCircuitBreakerTest.java

@@ -25,56 +25,60 @@ package com.iluwatar.circuitbreaker;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
+import java.rmi.Remote;
 import org.junit.jupiter.api.Test;
 
 /**
  * Circuit Breaker test
  */
-public class CircuitBreakerTest {
+public class DefaultCircuitBreakerTest {
 
   //long timeout, int failureThreshold, long retryTimePeriod
   @Test
-  public void testSetState() {
-    var circuitBreaker = new CircuitBreaker(1, 1, 100);
+  public void testEvaluateState() {
+    var circuitBreaker = new DefaultCircuitBreaker(null, 1, 1, 100);
     //Right now, failureCount<failureThreshold, so state should be closed
     assertEquals(circuitBreaker.getState(), "CLOSED");
     circuitBreaker.failureCount = 4;
     circuitBreaker.lastFailureTime = System.nanoTime();
-    circuitBreaker.setState();
+    circuitBreaker.evaluateState();
     //Since failureCount>failureThreshold, and lastFailureTime is nearly equal to current time,
     //state should be half-open
     assertEquals(circuitBreaker.getState(), "HALF_OPEN");
     //Since failureCount>failureThreshold, and lastFailureTime is much lesser current time,
     //state should be open
     circuitBreaker.lastFailureTime = System.nanoTime() - 1000 * 1000 * 1000 * 1000;
-    circuitBreaker.setState();
+    circuitBreaker.evaluateState();
     assertEquals(circuitBreaker.getState(), "OPEN");
     //Now set it back again to closed to test idempotency
     circuitBreaker.failureCount = 0;
-    circuitBreaker.setState();
+    circuitBreaker.evaluateState();
     assertEquals(circuitBreaker.getState(), "CLOSED");
   }
 
   @Test
   public void testSetStateForBypass() {
-    var circuitBreaker = new CircuitBreaker(1, 1, 100);
+    var circuitBreaker = new DefaultCircuitBreaker(null, 1, 1, 2000 * 1000 * 1000);
     //Right now, failureCount<failureThreshold, so state should be closed
     //Bypass it and set it to open
-    circuitBreaker.setStateForBypass(State.OPEN);
+    circuitBreaker.setState(State.OPEN);
     assertEquals(circuitBreaker.getState(), "OPEN");
   }
 
   @Test
-  public void testApiResponses() {
-    var circuitBreaker = new CircuitBreaker(1, 1, 100);
-    try {
-      //Call with the paramater start_time set to huge amount of time in past so that service 
-      //replies with "Ok". Also, state is CLOSED in start
-      var serviceStartTime = System.nanoTime() - 60 * 1000 * 1000 * 1000;
-      var response = circuitBreaker.call("delayedService", serviceStartTime);
-      assertEquals(response, "Delayed service is working");
-    } catch (Exception e) {
-      System.out.println(e.getMessage());
-    }
+  public void testApiResponses() throws RemoteServiceException {
+    RemoteService mockService = new RemoteService() {
+      @Override
+      public String call() throws RemoteServiceException {
+        return "Remote Success";
+      }
+    };
+    var circuitBreaker = new DefaultCircuitBreaker(mockService, 1, 1, 100);
+    //Call with the paramater start_time set to huge amount of time in past so that service
+    //replies with "Ok". Also, state is CLOSED in start
+    var serviceStartTime = System.nanoTime() - 60 * 1000 * 1000 * 1000;
+    var response = circuitBreaker.attemptRequest();
+    assertEquals(response, "Remote Success");
+
   }
 }

circuit-breaker/src/test/java/com/iluwatar/circuitbreaker/DelayedRemoteServiceTest.java

@@ -0,0 +1,59 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.circuitbreaker;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Monitoring Service test
+ */
+public class DelayedRemoteServiceTest {
+
+  /**
+   * Testing immediate response of the delayed service.
+   *
+   * @throws RemoteServiceException
+   */
+  @Test
+  public void testDefaultConstructor() throws RemoteServiceException {
+    Assertions.assertThrows(RemoteServiceException.class, () -> {
+      var obj = new DelayedRemoteService();
+      obj.call();
+    });
+  }
+
+  /**
+   * Testing server started in past (2 seconds ago) and with a simulated delay of 1 second.
+   *
+   * @throws RemoteServiceException
+   */
+  @Test
+  public void testParameterizedConstructor() throws RemoteServiceException {
+      var obj = new DelayedRemoteService(System.nanoTime()-2000*1000*1000,1);
+      assertEquals("Delayed service is working",obj.call());
+  }
+}

circuit-breaker/src/test/java/com/iluwatar/circuitbreaker/MonitoringServiceTest.java

@@ -35,28 +35,45 @@ public class MonitoringServiceTest {
   //long timeout, int failureThreshold, long retryTimePeriod
   @Test
   public void testLocalResponse() {
-    var monitoringService = new MonitoringService();
+    var monitoringService = new MonitoringService(null,null);
     var response = monitoringService.localResourceResponse();
     assertEquals(response, "Local Service is working");
   }
 
   @Test
-  public void testRemoteResponse() {
-    var monitoringService = new MonitoringService();
-    var circuitBreaker = new CircuitBreaker(1, 1, 100);
+  public void testDelayedRemoteResponseSuccess() {
+    var delayedService = new DelayedRemoteService(System.nanoTime()-2*1000*1000*1000, 2);
+    var delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000,
+        1,
+        2 * 1000 * 1000 * 1000);
+
+    var monitoringService = new MonitoringService(delayedServiceCircuitBreaker,null);
     //Set time in past to make the server work
-    var serverStartTime = System.nanoTime() / 10;
-    var response = monitoringService.remoteResourceResponse(circuitBreaker, serverStartTime);
+    var response = monitoringService.delayedServiceResponse();
     assertEquals(response, "Delayed service is working");
   }
 
   @Test
-  public void testRemoteResponse2() {
-    var monitoringService = new MonitoringService();
-    var circuitBreaker = new CircuitBreaker(1, 1, 100);
+  public void testDelayedRemoteResponseFailure() {
+    var delayedService = new DelayedRemoteService(System.nanoTime(), 2);
+    var delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000,
+        1,
+        2 * 1000 * 1000 * 1000);
+    var monitoringService = new MonitoringService(delayedServiceCircuitBreaker,null);
     //Set time as current time as initially server fails
-    var serverStartTime = System.nanoTime();
-    var response = monitoringService.remoteResourceResponse(circuitBreaker, serverStartTime);
-    assertEquals(response, "Remote service not responding");
+    var response = monitoringService.delayedServiceResponse();
+    assertEquals(response, "Delayed service is down");
+  }
+
+  @Test
+  public void testQuickRemoteServiceResponse() {
+    var delayedService = new QuickRemoteService();
+    var delayedServiceCircuitBreaker = new DefaultCircuitBreaker(delayedService, 3000,
+        1,
+        2 * 1000 * 1000 * 1000);
+    var monitoringService = new MonitoringService(delayedServiceCircuitBreaker,null);
+    //Set time as current time as initially server fails
+    var response = monitoringService.delayedServiceResponse();
+    assertEquals(response, "Quick Service is working");
   }
 }

command/README.md

@@ -14,14 +14,14 @@ Action, Transaction
 
 ## Intent
 
-Encapsulate a request as an object, thereby letting you parameterize clients with different 
+Encapsulate a request as an object, thereby letting you parameterize clients with different
 requests, queue or log requests, and support undoable operations.
 
 ## Explanation
 Real world example
 
-> There is a wizard casting spells on a goblin. The spells are executed on the goblin one by one. 
-> The first spell shrinks the goblin and the second makes him invisible. Then the wizard reverses 
+> There is a wizard casting spells on a goblin. The spells are executed on the goblin one by one.
+> The first spell shrinks the goblin and the second makes him invisible. Then the wizard reverses
 > the spells one by one. Each spell here is a command object that can be undone.
 
 In plain words
@@ -30,8 +30,8 @@ In plain words
 
 Wikipedia says
 
-> In object-oriented programming, the command pattern is a behavioral design pattern in which an 
-> object is used to encapsulate all information needed to perform an action or trigger an event at 
+> In object-oriented programming, the command pattern is a behavioral design pattern in which an
+> object is used to encapsulate all information needed to perform an action or trigger an event at
 > a later time.
 
 **Programmatic Example**
@@ -41,25 +41,21 @@ Here's the sample code with wizard and goblin. Let's start from the `Wizard` cla
 ```java
 public class Wizard {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
-
   private final Deque<Command> undoStack = new LinkedList<>();
   private final Deque<Command> redoStack = new LinkedList<>();
 
   public Wizard() {}
 
-  public void castSpell(Command command, Target target) {
-    LOGGER.info("{} casts {} at {}", this, command, target);
-    command.execute(target);
-    undoStack.offerLast(command);
+  public void castSpell(Runnable runnable) {
+    runnable.run();
+    undoStack.offerLast(runnable);
   }
 
   public void undoLastSpell() {
     if (!undoStack.isEmpty()) {
       var previousSpell = undoStack.pollLast();
       redoStack.offerLast(previousSpell);
-      LOGGER.info("{} undoes {}", this, previousSpell);
-      previousSpell.undo();
+      previousSpell.run();
     }
   }
 
@@ -67,8 +63,7 @@ public class Wizard {
     if (!redoStack.isEmpty()) {
       var previousSpell = redoStack.pollLast();
       undoStack.offerLast(previousSpell);
-      LOGGER.info("{} redoes {}", this, previousSpell);
-      previousSpell.redo();
+      previousSpell.run();
     }
   }
 
@@ -79,84 +74,7 @@ public class Wizard {
 }
 ```
 
-Next we present the spell hierarchy.
-
-```java
-public interface Command {
-
-  void execute(Target target);
-
-  void undo();
-
-  void redo();
-
-  String toString();
-}
-
-public class InvisibilitySpell implements Command {
-
-  private Target target;
-
-  @Override
-  public void execute(Target target) {
-    target.setVisibility(Visibility.INVISIBLE);
-    this.target = target;
-  }
-
-  @Override
-  public void undo() {
-    if (target != null) {
-      target.setVisibility(Visibility.VISIBLE);
-    }
-  }
-
-  @Override
-  public void redo() {
-    if (target != null) {
-      target.setVisibility(Visibility.INVISIBLE);
-    }
-  }
-
-  @Override
-  public String toString() {
-    return "Invisibility spell";
-  }
-}
-
-public class ShrinkSpell implements Command {
-
-  private Size oldSize;
-  private Target target;
-
-  @Override
-  public void execute(Target target) {
-    oldSize = target.getSize();
-    target.setSize(Size.SMALL);
-    this.target = target;
-  }
-
-  @Override
-  public void undo() {
-    if (oldSize != null && target != null) {
-      var temp = target.getSize();
-      target.setSize(oldSize);
-      oldSize = temp;
-    }
-  }
-
-  @Override
-  public void redo() {
-    undo();
-  }
-
-  @Override
-  public String toString() {
-    return "Shrink spell";
-  }
-}
-```
-
-Finally, we have the goblin who's the target of the spells.
+Next, we have the goblin who's the target of the spells.
 
 ```java
 public abstract class Target {
@@ -203,33 +121,73 @@ public class Goblin extends Target {
     return "Goblin";
   }
 
+  public void changeSize() {
+    var oldSize = getSize() == Size.NORMAL ? Size.SMALL : Size.NORMAL;
+    setSize(oldSize);
+  }
+
+  public void changeVisibility() {
+    var visible = getVisibility() == Visibility.INVISIBLE
+          ? Visibility.VISIBLE : Visibility.INVISIBLE;
+    setVisibility(visible);
+  }
 }
 ```
 
+Finally we have the wizard in main function who casts spell
+
+```java
+public static void main(String[] args) {
+  var wizard = new Wizard();
+  var goblin = new Goblin();
+
+  // casts shrink/unshrink spell
+  wizard.castSpell(goblin::changeSize);
+
+  // casts visible/invisible spell
+  wizard.castSpell(goblin::changeVisibility);
+
+  // undo and redo casts
+   wizard.undoLastSpell();
+   wizard.redoLastSpell();
+```
+
 Here's the whole example in action.
 
 ```java
 var wizard = new Wizard();
 var goblin = new Goblin();
+
 goblin.printStatus();
-wizard.castSpell(new ShrinkSpell(), goblin);
+wizard.castSpell(goblin::changeSize);
 goblin.printStatus();
-wizard.castSpell(new InvisibilitySpell(), goblin);
+
+wizard.castSpell(goblin::changeVisibility);
 goblin.printStatus();
+
 wizard.undoLastSpell();
 goblin.printStatus();
+
+wizard.undoLastSpell();
+goblin.printStatus();
+
+wizard.redoLastSpell();
+goblin.printStatus();
+
+wizard.redoLastSpell();
+goblin.printStatus();
 ```
 
 Here's the program output:
 
 ```java
-// Goblin, [size=normal] [visibility=visible]
-// Wizard casts Shrink spell at Goblin
-// Goblin, [size=small] [visibility=visible]
-// Wizard casts Invisibility spell at Goblin
-// Goblin, [size=small] [visibility=invisible]
-// Wizard undoes Invisibility spell
-// Goblin, [size=small] [visibility=visible]
+Goblin, [size=normal] [visibility=visible]
+Goblin, [size=small] [visibility=visible]
+Goblin, [size=small] [visibility=invisible]
+Goblin, [size=small] [visibility=visible]
+Goblin, [size=normal] [visibility=visible]
+Goblin, [size=small] [visibility=visible]
+Goblin, [size=small] [visibility=invisible]
 ```
 
 ## Class diagram
@@ -240,26 +198,26 @@ Here's the program output:
 
 Use the Command pattern when you want to:
 
-* Parameterize objects by an action to perform. You can express such parameterization in a 
-procedural language with a callback function, that is, a function that's registered somewhere to be 
+* Parameterize objects by an action to perform. You can express such parameterization in a
+procedural language with a callback function, that is, a function that's registered somewhere to be
 called at a later point. Commands are an object-oriented replacement for callbacks.
-* Specify, queue, and execute requests at different times. A Command object can have a lifetime 
-independent of the original request. If the receiver of a request can be represented in an address 
-space-independent way, then you can transfer a command object for the request to a different process 
+* Specify, queue, and execute requests at different times. A Command object can have a lifetime
+independent of the original request. If the receiver of a request can be represented in an address
+space-independent way, then you can transfer a command object for the request to a different process
 and fulfill the request there.
-* Support undo. The Command's execute operation can store state for reversing its effects in the 
-command itself. The Command interface must have an added un-execute operation that reverses the 
-effects of a previous call to execute. The executed commands are stored in a history list. 
-Unlimited-level undo and redo is achieved by traversing this list backwards and forwards calling 
+* Support undo. The Command's execute operation can store state for reversing its effects in the
+command itself. The Command interface must have an added un-execute operation that reverses the
+effects of a previous call to execute. The executed commands are stored in a history list.
+Unlimited-level undo and redo is achieved by traversing this list backwards and forwards calling
 un-execute and execute, respectively.
-* Support logging changes so that they can be reapplied in case of a system crash. By augmenting the 
-Command interface with load and store operations, you can keep a persistent log of changes. 
-Recovering from a crash involves reloading logged commands from disk and re-executing them with 
+* Support logging changes so that they can be reapplied in case of a system crash. By augmenting the
+Command interface with load and store operations, you can keep a persistent log of changes.
+Recovering from a crash involves reloading logged commands from disk and re-executing them with
 the execute operation.
-* Structure a system around high-level operations build on primitive operations. Such a structure is 
-common in information systems that support transactions. A transaction encapsulates a set of changes 
-to data. The Command pattern offers a way to model transactions. Commands have a common interface, 
-letting you invoke all transactions the same way. The pattern also makes it easy to extend the 
+* Structure a system around high-level operations build on primitive operations. Such a structure is
+common in information systems that support transactions. A transaction encapsulates a set of changes
+to data. The Command pattern offers a way to model transactions. Commands have a common interface,
+letting you invoke all transactions the same way. The pattern also makes it easy to extend the
 system with new transactions.
 
 ## Typical Use Case

command/etc/command.ucls

@@ -1,116 +1,89 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true" 
-  realizations="true" associations="true" dependencies="false" nesting-relationships="true">  
-  <class id="1" language="java" name="com.iluwatar.command.ShrinkSpell" project="command" 
-    file="/command/src/main/java/com/iluwatar/command/ShrinkSpell.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="178" width="141" x="-30" y="681"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="2" language="java" name="com.iluwatar.command.Goblin" project="command" 
-    file="/command/src/main/java/com/iluwatar/command/Goblin.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="129" y="1223"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="3" language="java" name="com.iluwatar.command.Wizard" project="command" 
-    file="/command/src/main/java/com/iluwatar/command/Wizard.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="129" y="362"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="4" language="java" name="com.iluwatar.command.Command" project="command" 
-    file="/command/src/main/java/com/iluwatar/command/Command.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="129" y="561"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="5" language="java" name="com.iluwatar.command.InvisibilitySpell" project="command" 
-    file="/command/src/main/java/com/iluwatar/command/InvisibilitySpell.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="160" width="141" x="151" y="681"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="6" language="java" name="com.iluwatar.command.Target" project="command" 
-    file="/command/src/main/java/com/iluwatar/command/Target.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="129" y="1014"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <association id="7">    
-    <end type="SOURCE" refId="3" navigable="false">      
-      <attribute id="8" name="redoStack">        
-        <position height="20" width="67" x="140" y="451"/>      
-      </attribute>      
-      <multiplicity id="9" minimum="0" maximum="2147483647">        
-        <position height="18" width="25" x="221" y="452"/>      
-      </multiplicity>    
-    </end>    
-    <end type="TARGET" refId="4" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <generalization id="10">    
-    <end type="SOURCE" refId="2"/>    
-    <end type="TARGET" refId="6"/>  
-  </generalization>  
-  <association id="11">    
-    <end type="SOURCE" refId="1" navigable="false">      
-      <attribute id="12" name="target"/>      
-      <multiplicity id="13" minimum="0" maximum="1"/>    
-    </end>    
-    <end type="TARGET" refId="6" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <generalization id="14">    
-    <end type="SOURCE" refId="1"/>    
-    <end type="TARGET" refId="4"/>  
-  </generalization>  
-  <association id="15">    
-    <end type="SOURCE" refId="3" navigable="false">      
-      <attribute id="16" name="undoStack">        
-        <position height="20" width="70" x="-17" y="451"/>      
-      </attribute>      
-      <multiplicity id="17" minimum="0" maximum="2147483647">        
-        <position height="18" width="25" x="60" y="452"/>      
-      </multiplicity>    
-    </end>    
-    <end type="TARGET" refId="4" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <generalization id="18">    
-    <end type="SOURCE" refId="5"/>    
-    <end type="TARGET" refId="4"/>  
-  </generalization>  
-  <association id="19">    
-    <end type="SOURCE" refId="5" navigable="false">      
-      <attribute id="20" name="target"/>      
-      <multiplicity id="21" minimum="0" maximum="1"/>    
-    </end>    
-    <end type="TARGET" refId="6" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-    sort-features="false" accessors="true" visibility="true">    
-    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
-    <operations public="true" package="true" protected="true" private="true" static="true"/>  
-  </classifier-display>  
+<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
+  realizations="true" associations="true" dependencies="false" nesting-relationships="true">
+  <class id="2" language="java" name="com.iluwatar.command.Goblin" project="command"
+    file="/command/src/main/java/com/iluwatar/command/Goblin.java" binary="false" corner="BOTTOM_RIGHT">
+    <position height="-1" width="-1" x="129" y="1223"/>
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
+      sort-features="false" accessors="true" visibility="true">
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>
+      <operations public="true" package="true" protected="true" private="true" static="true"/>
+    </display>
+  </class>
+  <class id="3" language="java" name="com.iluwatar.command.Wizard" project="command"
+    file="/command/src/main/java/com/iluwatar/command/Wizard.java" binary="false" corner="BOTTOM_RIGHT">
+    <position height="-1" width="-1" x="129" y="362"/>
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
+      sort-features="false" accessors="true" visibility="true">
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>
+      <operations public="true" package="true" protected="true" private="true" static="true"/>
+    </display>
+  </class>
+  <class id="6" language="java" name="com.iluwatar.command.Target" project="command"
+    file="/command/src/main/java/com/iluwatar/command/Target.java" binary="false" corner="BOTTOM_RIGHT">
+    <position height="-1" width="-1" x="129" y="1014"/>
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
+      sort-features="false" accessors="true" visibility="true">
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>
+      <operations public="true" package="true" protected="true" private="true" static="true"/>
+    </display>
+  </class>
+  <association id="7">
+    <end type="SOURCE" refId="3" navigable="false">
+      <attribute id="8" name="redoStack">
+        <position height="20" width="67" x="140" y="451"/>
+      </attribute>
+      <multiplicity id="9" minimum="0" maximum="2147483647">
+        <position height="18" width="25" x="221" y="452"/>
+      </multiplicity>
+    </end>
+    <end type="TARGET" refId="4" navigable="true"/>
+    <display labels="true" multiplicity="true"/>
+  </association>
+  <generalization id="10">
+    <end type="SOURCE" refId="2"/>
+    <end type="TARGET" refId="6"/>
+  </generalization>
+  <association id="11">
+    <end type="SOURCE" refId="1" navigable="false">
+      <attribute id="12" name="target"/>
+      <multiplicity id="13" minimum="0" maximum="1"/>
+    </end>
+    <end type="TARGET" refId="6" navigable="true"/>
+    <display labels="true" multiplicity="true"/>
+  </association>
+  <generalization id="14">
+    <end type="SOURCE" refId="1"/>
+    <end type="TARGET" refId="4"/>
+  </generalization>
+  <association id="15">
+    <end type="SOURCE" refId="3" navigable="false">
+      <attribute id="16" name="undoStack">
+        <position height="20" width="70" x="-17" y="451"/>
+      </attribute>
+      <multiplicity id="17" minimum="0" maximum="2147483647">
+        <position height="18" width="25" x="60" y="452"/>
+      </multiplicity>
+    </end>
+    <end type="TARGET" refId="4" navigable="true"/>
+    <display labels="true" multiplicity="true"/>
+  </association>
+  <generalization id="18">
+    <end type="SOURCE" refId="5"/>
+    <end type="TARGET" refId="4"/>
+  </generalization>
+  <association id="19">
+    <end type="SOURCE" refId="5" navigable="false">
+      <attribute id="20" name="target"/>
+      <multiplicity id="21" minimum="0" maximum="1"/>
+    </end>
+    <end type="TARGET" refId="6" navigable="true"/>
+    <display labels="true" multiplicity="true"/>
+  </association>
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
+    sort-features="false" accessors="true" visibility="true">
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>
+    <operations public="true" package="true" protected="true" private="true" static="true"/>
+  </classifier-display>
   <association-display labels="true" multiplicity="true"/>
-</class-diagram>
+</class-diagram>

command/etc/command.urm.puml

@@ -4,33 +4,11 @@ package com.iluwatar.command {
     + App()
     + main(args : String[]) {static}
   }
-  interface Command {
-    + Command()
-    + execute(Target) {abstract}
-    + redo() {abstract}
-    + toString() : String {abstract}
-    + undo() {abstract}
-  }
   class Goblin {
     + Goblin()
     + toString() : String
-  }
-  class InvisibilitySpell {
-    - target : Target
-    + InvisibilitySpell()
-    + execute(target : Target)
-    + redo()
-    + toString() : String
-    + undo()
-  }
-  class ShrinkSpell {
-    - oldSize : Size
-    - target : Target
-    + ShrinkSpell()
-    + execute(target : Target)
-    + redo()
-    + toString() : String
-    + undo()
+    + changeSize()
+    + changeVisibility()
   }
   enum Size {
     + NORMAL {static}
@@ -62,22 +40,19 @@ package com.iluwatar.command {
   }
   class Wizard {
     - LOGGER : Logger {static}
-    - redoStack : Deque<Command>
-    - undoStack : Deque<Command>
+    - redoStack : Deque<Runnable>
+    - undoStack : Deque<Runnable>
     + Wizard()
-    + castSpell(command : Command, target : Target)
+    + castSpell(Runnable : runnable)
     + redoLastSpell()
     + toString() : String
     + undoLastSpell()
   }
 }
 Target -->  "-size" Size
-Wizard -->  "-undoStack" Command
-ShrinkSpell -->  "-oldSize" Size
-InvisibilitySpell -->  "-target" Target
-ShrinkSpell -->  "-target" Target
+Wizard -->  "-changeSize" Goblin
+Wizard -->  "-changeVisibility" Goblin
 Target -->  "-visibility" Visibility
 Goblin --|> Target
-InvisibilitySpell ..|> Command
-ShrinkSpell ..|> Command
+App --> "castSpell" Wizard
 @enduml

command/src/main/java/com/iluwatar/command/App.java

@@ -30,12 +30,10 @@ package com.iluwatar.command;
  *
  * <p>Four terms always associated with the command pattern are command, receiver, invoker and
  * client. A command object (spell) knows about the receiver (target) and invokes a method of the
- * receiver. Values for parameters of the receiver method are stored in the command. The receiver
- * then does the work. An invoker object (wizard) knows how to execute a command, and optionally
- * does bookkeeping about the command execution. The invoker does not know anything about a concrete
- * command, it knows only about command interface. Both an invoker object and several command
- * objects are held by a client object (app). The client decides which commands to execute at which
- * points. To execute a command, it passes the command object to the invoker object.
+ * receiver. An invoker object (wizard) receives a reference to the command to be executed and
+ * optionally does bookkeeping about the command execution. The invoker does not know anything
+ * about how the command is executed. The client decides which commands to execute at which
+ * points. To execute a command, it passes a reference of the function to the invoker object.
  *
  * <p>In other words, in this example the wizard casts spells on the goblin. The wizard keeps track
  * of the previous spells cast, so it is easy to undo them. In addition, the wizard keeps track of
@@ -54,10 +52,10 @@ public class App {
 
     goblin.printStatus();
 
-    wizard.castSpell(new ShrinkSpell(), goblin);
+    wizard.castSpell(goblin::changeSize);
     goblin.printStatus();
 
-    wizard.castSpell(new InvisibilitySpell(), goblin);
+    wizard.castSpell(goblin::changeVisibility);
     goblin.printStatus();
 
     wizard.undoLastSpell();

command/src/main/java/com/iluwatar/command/Goblin.java

@@ -37,5 +37,4 @@ public class Goblin extends Target {
   public String toString() {
     return "Goblin";
   }
-
 }

command/src/main/java/com/iluwatar/command/Target.java

@@ -62,4 +62,21 @@ public abstract class Target {
   public void printStatus() {
     LOGGER.info("{}, [size={}] [visibility={}]", this, getSize(), getVisibility());
   }
+
+  /**
+   * Changes the size of the target.
+   */
+  public void changeSize() {
+    var oldSize = getSize() == Size.NORMAL ? Size.SMALL : Size.NORMAL;
+    setSize(oldSize);
+  }
+
+  /**
+   * Changes the visibility of the target.
+   */
+  public void changeVisibility() {
+    var visible = getVisibility() == Visibility.INVISIBLE
+            ? Visibility.VISIBLE : Visibility.INVISIBLE;
+    setVisibility(visible);
+  }
 }

command/src/main/java/com/iluwatar/command/Wizard.java

@@ -25,30 +25,24 @@ package com.iluwatar.command;
 
 import java.util.Deque;
 import java.util.LinkedList;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 
 /**
  * Wizard is the invoker of the commands.
  */
 public class Wizard {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
-
-  private final Deque<Command> undoStack = new LinkedList<>();
-  private final Deque<Command> redoStack = new LinkedList<>();
+  private final Deque<Runnable> undoStack = new LinkedList<>();
+  private final Deque<Runnable> redoStack = new LinkedList<>();
 
   public Wizard() {
-    // comment to ignore sonar issue: LEVEL critical
   }
 
   /**
    * Cast spell.
    */
-  public void castSpell(Command command, Target target) {
-    LOGGER.info("{} casts {} at {}", this, command, target);
-    command.execute(target);
-    undoStack.offerLast(command);
+  public void castSpell(Runnable runnable) {
+    runnable.run();
+    undoStack.offerLast(runnable);
   }
 
   /**
@@ -58,8 +52,7 @@ public class Wizard {
     if (!undoStack.isEmpty()) {
       var previousSpell = undoStack.pollLast();
       redoStack.offerLast(previousSpell);
-      LOGGER.info("{} undoes {}", this, previousSpell);
-      previousSpell.undo();
+      previousSpell.run();
     }
   }
 
@@ -70,8 +63,7 @@ public class Wizard {
     if (!redoStack.isEmpty()) {
       var previousSpell = redoStack.pollLast();
       undoStack.offerLast(previousSpell);
-      LOGGER.info("{} redoes {}", this, previousSpell);
-      previousSpell.redo();
+      previousSpell.run();
     }
   }
 

command/src/test/java/com/iluwatar/command/CommandTest.java

@@ -56,10 +56,10 @@ public class CommandTest {
     var wizard = new Wizard();
     var goblin = new Goblin();
 
-    wizard.castSpell(new ShrinkSpell(), goblin);
+    wizard.castSpell(goblin::changeSize);
     verifyGoblin(goblin, GOBLIN, Size.SMALL, Visibility.VISIBLE);
 
-    wizard.castSpell(new InvisibilitySpell(), goblin);
+    wizard.castSpell(goblin::changeVisibility);
     verifyGoblin(goblin, GOBLIN, Size.SMALL, Visibility.INVISIBLE);
 
     wizard.undoLastSpell();

data-transfer-object-enum-impl/README.md

@@ -0,0 +1,54 @@
+---
+layout: pattern
+title: Data Transfer Object
+folder: data-transfer-object
+permalink: /patterns/data-transfer-object/
+categories: Architectural
+tags:
+ - Performance
+---
+
+## Intent
+
+Pass data with multiple attributes in one shot from client to server, to avoid multiple calls to 
+remote server. 
+
+## Explanation
+
+Real world example
+
+> We need to fetch information about customers from remote database. Instead of querying the 
+> attributes one at a time, we use DTOs to transfer all the relevant attributes in a single shot.     
+
+In plain words
+
+> Using DTO relevant information can be fetched with a single backend query. 
+
+Wikipedia says
+
+> In the field of programming a data transfer object (DTO) is an object that carries data between 
+> processes. The motivation for its use is that communication between processes is usually done 
+> resorting to remote interfaces (e.g. web services), where each call is an expensive operation. 
+> Because the majority of the cost of each call is related to the round-trip time between the client 
+> and the server, one way of reducing the number of calls is to use an object (the DTO) that 
+> aggregates the data that would have been transferred by the several calls, but that is served by 
+> one call only.
+
+## Class diagram
+
+![alt text](./etc/dto-enum-uml.png "data-transfer-object")
+
+## Applicability
+
+Use the Data Transfer Object pattern when:
+
+* The client is asking for multiple information. And the information is related.
+* When you want to boost the performance to get resources.
+* You want reduced number of remote calls.
+
+## Credits
+
+* [Design Pattern - Transfer Object Pattern](https://www.tutorialspoint.com/design_pattern/transfer_object_pattern.htm)
+* [Data Transfer Object](https://msdn.microsoft.com/en-us/library/ff649585.aspx)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=f27d2644fbe5026ea448791a8ad09c94)
+* [Patterns of Enterprise Application Architecture](https://www.amazon.com/gp/product/0321127420/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321127420&linkCode=as2&tag=javadesignpat-20&linkId=014237a67c9d46f384b35e10151956bd)

data-transfer-object-enum-impl/etc/data-transfer-object-enum-impl.urm.puml

@@ -0,0 +1,129 @@
+@startuml
+package com.iluwatar.datatransferenum {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+  class Product {
+    - cost : Double
+    - id : Long
+    - name : String
+    - price : Double
+    - supplier : String
+    + Product()
+    + getCost() : Double
+    + getId() : Long
+    + getName() : String
+    + getPrice() : Double
+    + getSupplier() : String
+    + setCost(cost : Double) : Product
+    + setId(id : Long) : Product
+    + setName(name : String) : Product
+    + setPrice(price : Double) : Product
+    + setSupplier(supplier : String) : Product
+    + toString() : String
+  }
+  enum ProductDTO {
+    + valueOf(name : String) : ProductDTO {static}
+    + values() : ProductDTO[] {static}
+  }
+  -interface Cost {
+    + getCost() : Double {abstract}
+  }
+  -interface Id {
+    + getId() : Long {abstract}
+  }
+  -interface Name {
+    + getName() : String {abstract}
+  }
+  -interface Price {
+    + getPrice() : Double {abstract}
+  }
+  enum Request {
+    + valueOf(name : String) : Request {static}
+    + values() : Request[] {static}
+  }
+  class Create {
+    - cost : Double
+    - name : String
+    - price : Double
+    - supplier : String
+    + Create()
+    + getCost() : Double
+    + getName() : String
+    + getPrice() : Double
+    + getSupplier() : String
+    + setCost(cost : Double) : Create
+    + setName(name : String) : Create
+    + setPrice(price : Double) : Create
+    + setSupplier(supplier : String) : Create
+  }
+  enum Response {
+    + valueOf(name : String) : Response {static}
+    + values() : Response[] {static}
+  }
+  class Private {
+    - cost : Double
+    - id : Long
+    - name : String
+    - price : Double
+    + Private()
+    + getCost() : Double
+    + getId() : Long
+    + getName() : String
+    + getPrice() : Double
+    + setCost(cost : Double) : Private
+    + setId(id : Long) : Private
+    + setName(name : String) : Private
+    + setPrice(price : Double) : Private
+    + toString() : String
+  }
+  class Public {
+    - id : Long
+    - name : String
+    - price : Double
+    + Public()
+    + getId() : Long
+    + getName() : String
+    + getPrice() : Double
+    + setId(id : Long) : Public
+    + setName(name : String) : Public
+    + setPrice(price : Double) : Public
+    + toString() : String
+  }
+  -interface Supplier {
+    + getSupplier() : String {abstract}
+  }
+  class ProductResource {
+    - products : List<Product>
+    + ProductResource(products : List<Product>)
+    + getAllProductsForAdmin() : List<Private>
+    + getAllProductsForCustomer() : List<Public>
+    + getProducts() : List<Product>
+    + save(createProductDTO : Create)
+  }
+}
+Create ..+ Request
+Request ..+ ProductDTO
+Private ..+ Response
+Supplier ..+ ProductDTO
+Name ..+ ProductDTO
+ProductResource -->  "-products" Product
+Public ..+ Response
+Id ..+ ProductDTO
+Price ..+ ProductDTO
+Response ..+ ProductDTO
+Cost ..+ ProductDTO
+Create ..|> Name 
+Create ..|> Price 
+Create ..|> Cost 
+Create ..|> Supplier 
+Private ..|> Id 
+Private ..|> Name 
+Private ..|> Price 
+Private ..|> Cost 
+Public ..|> Id 
+Public ..|> Name 
+Public ..|> Price 
+@enduml

data-transfer-object-enum-impl/pom.xml

@@ -0,0 +1,61 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2019 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.24.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>data-transfer-object-enum-impl</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <configuration>
+                            <archive>
+                                <manifest>
+                                    <mainClass>com.iluwatar.datatransferenum.App</mainClass>
+                                </manifest>
+                            </archive>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

data-transfer-object-enum-impl/src/main/java/com/iluwatar/datatransferenum/App.java

@@ -0,0 +1,59 @@
+package com.iluwatar.datatransferenum;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The Data Transfer Object pattern is a design pattern in which an data transfer object is used to
+ * serve related information together to avoid multiple call for each piece of information.
+ *
+ * <p>In this example, ({@link App}) as as product details consumer i.e. client to
+ * request for product details to server.
+ *
+ * <p>productResource ({@link ProductResource}) act as server to serve product information. And
+ * The productDto ({@link ProductDto} is data transfer object to share product information.
+ */
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Method as act client and request to server for details.
+   *
+   * @param args program argument.
+   */
+  public static void main(String[] args) {
+    Product tv =
+        new Product().setId(1L).setName("TV").setSupplier("Sony").setPrice(1000D).setCost(1090D);
+    Product microwave =
+        new Product().setId(2L).setName("microwave").setSupplier("Delonghi").setPrice(1000D)
+            .setCost(1090D);
+    Product refrigerator =
+        new Product().setId(3L).setName("refrigerator").setSupplier("Botsch").setPrice(1000D)
+            .setCost(1090D);
+    Product airConditioner =
+        new Product().setId(4L).setName("airConditioner").setSupplier("LG").setPrice(1000D)
+            .setCost(1090D);
+    List<Product> products =
+        new ArrayList<>(Arrays.asList(tv, microwave, refrigerator, airConditioner));
+    ProductResource productResource = new ProductResource(products);
+
+    LOGGER.info("####### List of products including sensitive data just for admins: \n {}",
+        Arrays.toString(productResource.getAllProductsForAdmin().toArray()));
+    LOGGER.info("####### List of products for customers: \n {}",
+        Arrays.toString(productResource.getAllProductsForCustomer().toArray()));
+
+    LOGGER.info("####### Going to save Sony PS5 ...");
+    ProductDto.Request.Create createProductRequestDto = new ProductDto.Request.Create()
+        .setName("PS5")
+        .setCost(1000D)
+        .setPrice(1220D)
+        .setSupplier("Sony");
+    productResource.save(createProductRequestDto);
+    LOGGER.info("####### List of products after adding PS5: {}",
+        Arrays.toString(productResource.getProducts().toArray()));
+  }
+}

data-transfer-object-enum-impl/src/main/java/com/iluwatar/datatransferenum/Product.java

@@ -0,0 +1,91 @@
+package com.iluwatar.datatransferenum;
+
+/**
+ * {@link Product} is a entity class for product entity. This class act as entity in the demo.
+ */
+public final class Product {
+  private Long id;
+  private String name;
+  private Double price;
+  private Double cost;
+  private String supplier;
+
+  /**
+   * Constructor.
+   *
+   * @param id       product id
+   * @param name     product name
+   * @param price    product price
+   * @param cost     product cost
+   * @param supplier product supplier
+   */
+  public Product(Long id, String name, Double price, Double cost, String supplier) {
+    this.id = id;
+    this.name = name;
+    this.price = price;
+    this.cost = cost;
+    this.supplier = supplier;
+  }
+
+  /**
+   * Constructor.
+   */
+  public Product() {
+  }
+
+  public Long getId() {
+    return id;
+  }
+
+  public Product setId(Long id) {
+    this.id = id;
+    return this;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public Product setName(String name) {
+    this.name = name;
+    return this;
+  }
+
+  public Double getPrice() {
+    return price;
+  }
+
+  public Product setPrice(Double price) {
+    this.price = price;
+    return this;
+  }
+
+  public Double getCost() {
+    return cost;
+  }
+
+  public Product setCost(Double cost) {
+    this.cost = cost;
+    return this;
+  }
+
+  public String getSupplier() {
+    return supplier;
+  }
+
+  public Product setSupplier(String supplier) {
+    this.supplier = supplier;
+    return this;
+  }
+
+  @Override
+  public String toString() {
+    return "Product{"
+        + "id=" + id
+        + ", name='" + name + '\''
+        + ", price=" + price
+        + ", cost=" + cost
+        + ", supplier='" + supplier + '\''
+        + '}';
+  }
+}

data-transfer-object-enum-impl/src/main/java/com/iluwatar/datatransferenum/ProductDto.java

@@ -0,0 +1,264 @@
+package com.iluwatar.datatransferenum;
+
+/**
+ * {@link ProductDto} is a data transfer object POJO.
+ * Instead of sending individual information to
+ * client We can send related information together in POJO.
+ *
+ * <p>Dto will not have any business logic in it.
+ */
+public enum ProductDto {
+  ;
+
+  /**
+   * This is Request class which consist of Create or any other request DTO's
+   * you might want to use in your API.
+   */
+  public enum Request {
+    ;
+
+    /**
+     * This is Create dto class for requesting create new product.
+     */
+    public static final class Create implements Name, Price, Cost, Supplier {
+      private String name;
+      private Double price;
+      private Double cost;
+      private String supplier;
+
+      @Override
+      public String getName() {
+        return name;
+      }
+
+      public Create setName(String name) {
+        this.name = name;
+        return this;
+      }
+
+      @Override
+      public Double getPrice() {
+        return price;
+      }
+
+      public Create setPrice(Double price) {
+        this.price = price;
+        return this;
+      }
+
+      @Override
+      public Double getCost() {
+        return cost;
+      }
+
+      public Create setCost(Double cost) {
+        this.cost = cost;
+        return this;
+      }
+
+      @Override
+      public String getSupplier() {
+        return supplier;
+      }
+
+      public Create setSupplier(String supplier) {
+        this.supplier = supplier;
+        return this;
+      }
+    }
+  }
+
+  /**
+   * This is Response class which consist of any response DTO's
+   * you might want to provide to your clients.
+   */
+  public enum Response {
+    ;
+
+    /**
+     * This is Public dto class for API response with the lowest data security.
+     */
+    public static final class Public implements Id, Name, Price {
+      private Long id;
+      private String name;
+      private Double price;
+
+      @Override
+      public Long getId() {
+        return id;
+      }
+
+      public Public setId(Long id) {
+        this.id = id;
+        return this;
+      }
+
+      @Override
+      public String getName() {
+        return name;
+      }
+
+      public Public setName(String name) {
+        this.name = name;
+        return this;
+      }
+
+      @Override
+      public Double getPrice() {
+        return price;
+      }
+
+      public Public setPrice(Double price) {
+        this.price = price;
+        return this;
+      }
+
+      @Override
+      public String toString() {
+        return "Public{"
+            + "id="
+            + id
+            + ", name='"
+            + name
+            + '\''
+            + ", price="
+            + price
+            + '}';
+      }
+    }
+
+    /**
+     * This is Private dto class for API response with the highest data security.
+     */
+    public static final class Private implements Id, Name, Price, Cost {
+      private Long id;
+      private String name;
+      private Double price;
+      private Double cost;
+
+      @Override
+      public Long getId() {
+        return id;
+      }
+
+      public Private setId(Long id) {
+        this.id = id;
+        return this;
+      }
+
+      @Override
+      public String getName() {
+        return name;
+      }
+
+      public Private setName(String name) {
+        this.name = name;
+        return this;
+      }
+
+      @Override
+      public Double getPrice() {
+        return price;
+      }
+
+      public Private setPrice(Double price) {
+        this.price = price;
+        return this;
+      }
+
+      @Override
+      public Double getCost() {
+        return cost;
+      }
+
+      public Private setCost(Double cost) {
+        this.cost = cost;
+        return this;
+      }
+
+      @Override
+      public String toString() {
+        return "Private{"
+            +
+            "id="
+            + id
+            +
+            ", name='"
+            + name
+            + '\''
+            +
+            ", price="
+            + price
+            +
+            ", cost="
+            + cost
+            +
+            '}';
+      }
+    }
+  }
+
+  /**
+   * Use this interface whenever you want to provide the product Id in your DTO.
+   */
+  private interface Id {
+    /**
+     * Unique identifier of the product.
+     *
+     * @return : id of the product.
+     */
+    Long getId();
+  }
+
+  /**
+   * Use this interface whenever you want to provide the product Name in your DTO.
+   */
+  private interface Name {
+    /**
+     * The name of the product.
+     *
+     * @return : name of the product.
+     */
+    String getName();
+  }
+
+  /**
+   * Use this interface whenever you want to provide the product Price in your DTO.
+   */
+  private interface Price {
+    /**
+     * The amount we sell a product for.
+     * <b>This data is not confidential</b>
+     *
+     * @return : price of the product.
+     */
+    Double getPrice();
+  }
+
+  /**
+   * Use this interface whenever you want to provide the product Cost in your DTO.
+   */
+  private interface Cost {
+    /**
+     * The amount that it costs us to purchase this product
+     * For the amount we sell a product for, see the {@link Price Price} parameter.
+     * <b>This data is confidential</b>
+     *
+     * @return : cost of the product.
+     */
+    Double getCost();
+  }
+
+  /**
+   * Use this interface whenever you want to provide the product Supplier in your DTO.
+   */
+  private interface Supplier {
+    /**
+     * The name of supplier of the product or its manufacturer.
+     * <b>This data is highly confidential</b>
+     *
+     * @return : supplier of the product.
+     */
+    String getSupplier();
+  }
+}

data-transfer-object-enum-impl/src/main/java/com/iluwatar/datatransferenum/ProductResource.java

@@ -0,0 +1,71 @@
+package com.iluwatar.datatransferenum;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * The resource class which serves product information. This class act as server in the demo. Which
+ * has all product details.
+ */
+public class ProductResource {
+  private final List<Product> products;
+
+  /**
+   * Initialise resource with existing products.
+   *
+   * @param products initialize resource with existing products. Act as database.
+   */
+  public ProductResource(final List<Product> products) {
+    this.products = products;
+  }
+
+  /**
+   * Get all products.
+   *
+   * @return : all products in list but in the scheme of private dto.
+   */
+  public List<ProductDto.Response.Private> getAllProductsForAdmin() {
+    return products
+        .stream()
+        .map(p -> new ProductDto.Response.Private().setId(p.getId()).setName(p.getName())
+            .setCost(p.getCost())
+            .setPrice(p.getPrice()))
+        .collect(Collectors.toList());
+  }
+
+  /**
+   * Get all products.
+   *
+   * @return : all products in list but in the scheme of public dto.
+   */
+  public List<ProductDto.Response.Public> getAllProductsForCustomer() {
+    return products
+        .stream()
+        .map(p -> new ProductDto.Response.Public().setId(p.getId()).setName(p.getName())
+            .setPrice(p.getPrice()))
+        .collect(Collectors.toList());
+  }
+
+  /**
+   * Save new product.
+   *
+   * @param createProductDto save new product to list.
+   */
+  public void save(ProductDto.Request.Create createProductDto) {
+    products.add(new Product()
+        .setId((long) (products.size() + 1))
+        .setName(createProductDto.getName())
+        .setSupplier(createProductDto.getSupplier())
+        .setPrice(createProductDto.getPrice())
+        .setCost(createProductDto.getCost()));
+  }
+
+  /**
+   * List of all products in an entity representation.
+   *
+   * @return : all the products entity that stored in the products list
+   */
+  public List<Product> getProducts() {
+    return products;
+  }
+}

data-transfer-object-enum-impl/src/test/java/com/iluwatar/datatransferenum/AppTest.java

@@ -0,0 +1,44 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.datatransferenum;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+
+import org.junit.jupiter.api.Test;
+
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   * <p>
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
+  @Test
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[] {}));
+  }
+}

factory/README.md

@@ -81,7 +81,7 @@ public enum CarType {
 }
 ```
 Then we have the static method `getCar` to create car objects encapsulated in the factory class 
-`CarSimpleFactory`.
+`CarsFactory`.
 
 ```java
 public class CarsFactory {
@@ -98,7 +98,7 @@ Now on the client code we can create different types of cars using the factory c
 var car1 = CarsFactory.getCar(CarType.FORD);
 var car2 = CarsFactory.getCar(CarType.FERRARI);
 LOGGER.info(car1.getDescription());
-LOGGER.info(car2.getDescription());;
+LOGGER.info(car2.getDescription());
 ```
 
 Program output:
@@ -119,13 +119,23 @@ and manage it.
 
 Pros
 
-* Allows keeping all objects creation in one place and avoid of spreading 'new' key value across codebase.
-* Allows to writs loosely coupled code. Some of its main advantages include better testability, easy-to-understand code, swappable components, scalability and isolated features.
+* Allows keeping all objects creation in one place and avoid of spreading 'new' keyword across codebase.
+* Allows to write loosely coupled code. Some of its main advantages include better testability, easy-to-understand code, swappable components, scalability and isolated features.
 
 Cons
 
 * The code becomes more complicated than it should be. 
 
+## Real world examples
+
+* [java.util.Calendar#getInstance()](https://docs.oracle.com/javase/8/docs/api/java/util/Calendar.html#getInstance--)
+* [java.util.ResourceBundle#getBundle()](https://docs.oracle.com/javase/8/docs/api/java/util/ResourceBundle.html#getBundle-java.lang.String-)
+* [java.text.NumberFormat#getInstance()](https://docs.oracle.com/javase/8/docs/api/java/text/NumberFormat.html#getInstance--)
+* [java.nio.charset.Charset#forName()](https://docs.oracle.com/javase/8/docs/api/java/nio/charset/Charset.html#forName-java.lang.String-)
+* [java.net.URLStreamHandlerFactory#createURLStreamHandler(String)](https://docs.oracle.com/javase/8/docs/api/java/net/URLStreamHandlerFactory.html) (Returns different singleton objects, depending on a protocol)
+* [java.util.EnumSet#of()](https://docs.oracle.com/javase/8/docs/api/java/util/EnumSet.html#of(E))
+* [javax.xml.bind.JAXBContext#createMarshaller()](https://docs.oracle.com/javase/8/docs/api/javax/xml/bind/JAXBContext.html#createMarshaller--) and other similar methods.
+
 ## Related patterns
 
 * [Factory Method](https://java-design-patterns.com/patterns/factory-method/)

fr/README.md

@@ -0,0 +1,60 @@
+<!-- Cette ligne doit restée vide pour des raisons de formatage
+    afin qu'on puisse avoir un affichage agréable comme sur un 
+    site web par exemple -->
+
+# Les patrons de conception implémentés en Java
+
+![Java CI with Maven](https://github.com/iluwatar/java-design-patterns/workflows/Java%20CI%20with%20Maven/badge.svg)
+[![License MIT](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/iluwatar/java-design-patterns/master/LICENSE.md)
+[![Lines of Code](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=ncloc)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns) 
+[![Coverage](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=coverage)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns)
+[![Join the chat at https://gitter.im/iluwatar/java-design-patterns](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/iluwatar/java-design-patterns?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+<!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
+[![All Contributors](https://img.shields.io/badge/all_contributors-148-orange.svg?style=flat-square)](#contributors-)
+<!-- ALL-CONTRIBUTORS-BADGE:END -->
+
+# Introduction
+
+Le patron de conception est la meilleure formatalisation qu'un programmeur
+peut utiliser pour résoudre un problème lors d'une conception d'une application/system.
+
+Le patron de conception (design pattern) est là pour accélérer le processus de 
+développement en fournissant des paradigmes éprouvés.
+
+La réutilisation de patron de conception aide à prévenir des problèmes subtiles mais
+qui sont à l'origine de problèmes majeures, comme cette pratique augmente la lisibilitée
+du code par les développeurs/architectes familiers avec l'utilisation de ces concepts.
+
+# Commencer
+
+Ce site présente des modèles de conception Java. Les solutions ont été développées par des
+développeurs et architectes expérimentés de la communauté open source. Les modèles peuvent être parcourus par leurs descriptions de haut niveau ou en regardant leur code source. Les exemples de code source sont bien commentés et peuvent être considérés comme tutoriels de programmation sur la façon d'implémenter un modèle spécifique. Nous utilisons le plus technologies Java open source éprouvées au combat.
+
+Avant de plonger dans le matériau, vous devez vous familiariser avec divers
+[Principes de conception de logiciels](https://java-design-patterns.com/principles/).
+
+Toutes les conceptions doivent être aussi simples que possible. Vous devriez commencer par KISS, YAGNI,
+et faire la chose la plus simple qui pourrait éventuellement fonctionner principes. Complexité et
+les modèles ne devraient être introduits que lorsqu'ils sont nécessaires pour
+extensibilité.
+
+Une fois que vous êtes familiarisé avec ces concepts, vous pouvez commencer à explorer
+[modèles de conception disponibles](https://java-design-patterns.com/patterns/) par tout
+des approches suivantes
+
+ - Recherchez un modèle spécifique par son nom. Vous n'en trouvez pas? Veuillez signaler un nouveau modèle [ici](https://github.com/iluwatar/java-design-patterns/issues).
+ - Utilisation de balises telles que `Performance`, `Gang of Four` ou `Data access`.
+ - Utilisation des catégories de modèles, `Créatif`, `Comportemental` et autres.
+
+J'espère que les solutions orientées objet présentées sur ce site vous seront utiles
+dans vos architectures et ayez autant de plaisir à les apprendre que nous en avons eu à les développer.
+
+# Comment contribuer
+
+Si vous souhaitez contribuer au projet, vous trouverez les informations pertinentes dans
+notre [wiki développeur](https://github.com/iluwatar/java-design-patterns/wiki). Nous aiderons
+vous et répondez à vos questions dans le [Gitter chatroom](https://gitter.im/iluwatar/java-design-patterns).
+
+# Licence
+
+Ce projet est concédé sous les termes de la licence MIT.

game-loop/src/test/java/com/iluwatar/gameloop/AppTest.java

@@ -23,7 +23,7 @@
 
 package com.iluwatar.gameloop;
 
-import org.junit.Test;
+import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 

game-loop/src/test/java/com/iluwatar/gameloop/FixedStepGameLoopTest.java

@@ -23,10 +23,11 @@
 
 package com.iluwatar.gameloop;
 
-import org.junit.After;
-import org.junit.Assert;
-import org.junit.Before;
-import org.junit.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 
 /**
  * FixedStepGameLoop unit test class.
@@ -35,12 +36,12 @@ public class FixedStepGameLoopTest {
 
   private FixedStepGameLoop gameLoop;
 
-  @Before
+  @BeforeEach
   public void setup() {
     gameLoop = new FixedStepGameLoop();
   }
 
-  @After
+  @AfterEach
   public void tearDown() {
     gameLoop = null;
   }
@@ -48,7 +49,7 @@ public class FixedStepGameLoopTest {
   @Test
   public void testUpdate() {
     gameLoop.update();
-    Assert.assertEquals(0.01f, gameLoop.controller.getBulletPosition(), 0);
+    assertEquals(0.01f, gameLoop.controller.getBulletPosition(), 0);
   }
 
 }

game-loop/src/test/java/com/iluwatar/gameloop/FrameBasedGameLoopTest.java

@@ -23,10 +23,10 @@
 
 package com.iluwatar.gameloop;
 
-import org.junit.After;
-import org.junit.Assert;
-import org.junit.Before;
-import org.junit.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 
 /**
  * FrameBasedGameLoop unit test class.
@@ -35,19 +35,19 @@ public class FrameBasedGameLoopTest {
 
   private FrameBasedGameLoop gameLoop;
 
-  @Before
+  @BeforeEach
   public void setup() {
     gameLoop = new FrameBasedGameLoop();
   }
 
-  @After
+  @AfterEach
   public void tearDown() {
     gameLoop = null;
   }
 
-  @Test
+  @org.junit.jupiter.api.Test
   public void testUpdate() {
     gameLoop.update();
-    Assert.assertEquals(0.5f, gameLoop.controller.getBulletPosition(), 0);
+    assertEquals(0.5f, gameLoop.controller.getBulletPosition(), 0);
   }
 }

game-loop/src/test/java/com/iluwatar/gameloop/GameControllerTest.java

@@ -23,34 +23,33 @@
 
 package com.iluwatar.gameloop;
 
-import org.junit.After;
-import org.junit.Assert;
-import org.junit.Before;
-import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 
 public class GameControllerTest {
 
   private GameController controller;
 
-  @Before
+  @BeforeEach
   public void setup() {
     controller = new GameController();
   }
 
-  @After
+  @AfterEach
   public void tearDown() {
     controller = null;
   }
 
-  @Test
+  @org.junit.jupiter.api.Test
   public void testMoveBullet() {
     controller.moveBullet(1.5f);
-    Assert.assertEquals(1.5f, controller.bullet.getPosition(), 0);
+    Assertions.assertEquals(1.5f, controller.bullet.getPosition(), 0);
   }
 
-  @Test
+  @org.junit.jupiter.api.Test
   public void testGetBulletPosition() {
-    Assert.assertEquals(controller.bullet.getPosition(), controller.getBulletPosition(), 0);
+    Assertions.assertEquals(controller.bullet.getPosition(), controller.getBulletPosition(), 0);
   }
 
 }

game-loop/src/test/java/com/iluwatar/gameloop/GameLoopTest.java

@@ -23,10 +23,9 @@
 
 package com.iluwatar.gameloop;
 
-import org.junit.After;
-import org.junit.Assert;
-import org.junit.Before;
-import org.junit.Test;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
 
 /**
  * GameLoop unit test class.
@@ -38,7 +37,7 @@ public class GameLoopTest {
   /**
    * Create mock implementation of GameLoop.
    */
-  @Before
+  @BeforeEach
   public void setup() {
     gameLoop = new GameLoop() {
       @Override
@@ -46,26 +45,26 @@ public class GameLoopTest {
     };
   }
 
-  @After
+  @AfterEach
   public void tearDown() {
     gameLoop = null;
   }
 
-  @Test
+  @org.junit.jupiter.api.Test
   public void testRun() {
     gameLoop.run();
-    Assert.assertEquals(GameStatus.RUNNING, gameLoop.status);
+    Assertions.assertEquals(GameStatus.RUNNING, gameLoop.status);
   }
 
-  @Test
+  @org.junit.jupiter.api.Test
   public void testStop() {
     gameLoop.stop();
-    Assert.assertEquals(GameStatus.STOPPED, gameLoop.status);
+    Assertions.assertEquals(GameStatus.STOPPED, gameLoop.status);
   }
 
-  @Test
+  @org.junit.jupiter.api.Test
   public void testIsGameRunning() {
-    Assert.assertFalse(gameLoop.isGameRunning());
+    Assertions.assertFalse(gameLoop.isGameRunning());
   }
 
 }

game-loop/src/test/java/com/iluwatar/gameloop/VariableStepGameLoopTest.java

@@ -23,11 +23,9 @@
 
 package com.iluwatar.gameloop;
 
-import java.lang.reflect.InvocationTargetException;
-import org.junit.After;
-import org.junit.Assert;
-import org.junit.Before;
-import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 
 /**
  * VariableStepGameLoop unit test class.
@@ -36,19 +34,19 @@ public class VariableStepGameLoopTest {
 
   private VariableStepGameLoop gameLoop;
 
-  @Before
+  @BeforeEach
   public void setup() {
     gameLoop = new VariableStepGameLoop();
   }
 
-  @After
+  @AfterEach
   public void tearDown() {
     gameLoop = null;
   }
 
-  @Test
+  @org.junit.jupiter.api.Test
   public void testUpdate() {
     gameLoop.update(20L);
-    Assert.assertEquals(0.01f, gameLoop.controller.getBulletPosition(), 0);
+    Assertions.assertEquals(0.01f, gameLoop.controller.getBulletPosition(), 0);
   }
 }

iterator/README.md

@@ -140,6 +140,10 @@ Use the Iterator pattern
 * To support multiple traversals of aggregate objects.
 * To provide a uniform interface for traversing different aggregate structures.
 
+## Tutorials
+
+* [How to Use Iterator?](http://www.tutorialspoint.com/java/java_using_iterator.htm)
+
 ## Real world examples
 
 * [java.util.Iterator](http://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html)

ko/README.md

@@ -0,0 +1,263 @@
+<!-- the line below needs to be an empty line C: (its because kramdown isnt
+     that smart and dearly wants an empty line before a heading to be able to
+     display it as such, e.g. website) -->
+
+# 자바로 구현된 디자인 패턴
+
+![Java CI with Maven](https://github.com/iluwatar/java-design-patterns/workflows/Java%20CI%20with%20Maven/badge.svg)[](https://raw.githubusercontent.com/iluwatar/java-design-patterns/master/LICENSE.md)![License MIT](https://img.shields.io/badge/license-MIT-blue.svg)[](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns)![Join the chat at https://gitter.im/iluwatar/java-design-patterns](https://badges.gitter.im/Join%20Chat.svg)
+
+<!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
+
+[](#contributors-)![All Contributors](https://img.shields.io/badge/all_contributors-133-orange.svg?style=flat-square)
+
+<!-- ALL-CONTRIBUTORS-BADGE:END -->
+
+# 소개
+
+디자인 패턴은 프로그래머가 응용 프로그램이나 시스템을 디자인 할 때 일반적인 문제를 해결하는 데 사용할 수있는 가장 공식화 된 방법입니다.
+
+디자인 패턴은 테스트되고 입증 된 개발 패러다임을 제공하여 개발 프로세스 속도를 높일 수 있습니다.
+
+디자인 패턴을 재사용하면 주요 문제를 유발하는 미묘한 이슈들을 방지하는데 도움이 되며 또한 패턴에 익숙한 코더와 아키텍트의 코드 가독성도 향상됩니다.
+
+# 시작하기
+
+이 사이트는 Java 디자인 패턴을 보여줍니다. 이 솔루션은 오픈 소스 커뮤니티의 경험이 많은 프로그래머와 설계자가 개발했습니다. 패턴은 높은 수준의 설명이나 소스 코드를 통해 찾아 볼 수 있습니다. 소스 코드 예제는 잘 설명되어 있으며 특정 패턴을 구현하는 방법을 알려주는 프로그래밍 튜토리얼로 생각할 수 있습니다. 우리는 가장 널리 알려지고 실무에서 입증된 오픈 소스 Java 기술을 사용합니다.
+
+자료를 살펴보기 전에 다양한 [소프트웨어 설계 원칙](https://java-design-patterns.com/principles/)을 숙지해야합니다.
+
+모든 디자인은 가능한 한 단순해야합니다. 당신은 KISS, YAGNI로 시작해야하며, 원칙을 작동 할 수 있는 가장 단순한 일을 해야합니다. 복잡성과 패턴은 실용적인 확장성을 위해 필요할 때만 도입되어야합니다.
+
+이러한 개념에 익숙해지면 다음 접근 방식 중 하나를 이용하여 [사용 가능한 디자인 패턴](https://java-design-patterns.com/patterns/)으로 드릴다운 할 수 있습니다.
+
+- 이름으로 특정 패턴을 검색합니다. 찾을 수 없습니까? [여기](https://github.com/iluwatar/java-design-patterns/issues)에서 새 패턴을 보고하십시오.
+- `Performance`, `Gang of Four` 또는 `Data access` 태그 사용.
+- 패턴 카테고리, `Creational`, `Behavioral` 및 기타 사용
+
+이 사이트에 제시된 객체 지향 솔루션이 여러분의 아키텍처에서 유용하고 우리가 개발 한 것만큼 재미있게 배우기를 바랍니다.
+
+# 기여하는 방법
+
+프로젝트에 기여할 의향이 있다면 [developer wiki](https://github.com/iluwatar/java-design-patterns/wiki)에서 관련 정보를 찾을 수 있습니다. [Gitter chatroom](https://gitter.im/iluwatar/java-design-patterns)에서 귀하를 돕고 질문에 답변 해 드리겠습니다.
+
+# 특허
+
+이 프로젝트는 MIT 라이센스 조건에 따라 라이센스가 부여됩니다.
+
+# 기여자
+
+<!-- ALL-CONTRIBUTORS-LIST:START - Do not remove or modify this section -->
+
+<!-- prettier-ignore-start -->
+
+<!-- markdownlint-disable -->
+
+<table>
+  <tr>
+    <td align="center"><a href="https://github.com/iluwatar"><img src="https://avatars1.githubusercontent.com/u/582346?v=4" width="100px;" alt=""><br><sub><b>Ilkka Seppälä</b></sub></a></td>
+    <td align="center"><a href="https://github.com/amit1307"><img src="https://avatars0.githubusercontent.com/u/23420222?v=4" width="100px;" alt=""><br><sub><b>amit1307</b></sub></a></td>
+    <td align="center"><a href="https://github.com/npathai"><img src="https://avatars2.githubusercontent.com/u/1792515?v=4" width="100px;" alt=""><br><sub><b>Narendra Pathai</b></sub></a></td>
+    <td align="center"><a href="https://github.com/fluxw42"><img src="https://avatars1.githubusercontent.com/u/1545460?v=4" width="100px;" alt=""><br><sub><b>Jeroen Meulemeester</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://www.joemccarthy.co.uk"><img src="https://avatars0.githubusercontent.com/u/4526195?v=4" width="100px;" alt=""><br><sub><b>Joseph McCarthy</b></sub></a></td>
+    <td align="center"><a href="https://github.com/thomasoss"><img src="https://avatars1.githubusercontent.com/u/22516154?v=4" width="100px;" alt=""><br><sub><b>Thomas</b></sub></a></td>
+    <td align="center"><a href="https://github.com/anuragagarwal561994"><img src="https://avatars1.githubusercontent.com/u/6075379?v=4" width="100px;" alt=""><br><sub><b>Anurag Agarwal</b></sub></a></td>
+    <td align="center"><a href="https://markusmo3.github.io"><img src="https://avatars1.githubusercontent.com/u/3317416?v=4" width="100px;" alt=""><br><sub><b>Markus Moser</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://twitter.com/i_sabiq"><img src="https://avatars1.githubusercontent.com/u/19510920?v=4" width="100px;" alt=""><br><sub><b>Sabiq Ihab</b></sub></a></td>
+    <td align="center"><a href="http://inbravo.github.io"><img src="https://avatars3.githubusercontent.com/u/5253764?v=4" width="100px;" alt=""><br><sub><b>Amit Dixit</b></sub></a></td>
+    <td align="center"><a href="https://github.com/piyushchaudhari04"><img src="https://avatars3.githubusercontent.com/u/10268029?v=4" width="100px;" alt=""><br><sub><b>Piyush Kailash Chaudhari</b></sub></a></td>
+    <td align="center"><a href="https://github.com/joshzambales"><img src="https://avatars1.githubusercontent.com/u/8704552?v=4" width="100px;" alt=""><br><sub><b>joshzambales</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/Crossy147"><img src="https://avatars2.githubusercontent.com/u/7272996?v=4" width="100px;" alt=""><br><sub><b>Kamil Pietruszka</b></sub></a></td>
+    <td align="center"><a href="http://cs.joensuu.fi/~zkhayda"><img src="https://avatars2.githubusercontent.com/u/660742?v=4" width="100px;" alt=""><br><sub><b>Zafar Khaydarov</b></sub></a></td>
+    <td align="center"><a href="https://kemitix.github.io/"><img src="https://avatars1.githubusercontent.com/u/1147749?v=4" width="100px;" alt=""><br><sub><b>Paul Campbell</b></sub></a></td>
+    <td align="center"><a href="https://github.com/Argyro-Sioziou"><img src="https://avatars0.githubusercontent.com/u/22822639?v=4" width="100px;" alt=""><br><sub><b>Argyro Sioziou</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/TylerMcConville"><img src="https://avatars0.githubusercontent.com/u/4946449?v=4" width="100px;" alt=""><br><sub><b>TylerMcConville</b></sub></a></td>
+    <td align="center"><a href="https://github.com/saksham93"><img src="https://avatars1.githubusercontent.com/u/37399540?v=4" width="100px;" alt=""><br><sub><b>saksham93</b></sub></a></td>
+    <td align="center"><a href="https://github.com/nikhilbarar"><img src="https://avatars2.githubusercontent.com/u/37332144?v=4" width="100px;" alt=""><br><sub><b>nikhilbarar</b></sub></a></td>
+    <td align="center"><a href="http://colinbut.com"><img src="https://avatars2.githubusercontent.com/u/10725674?v=4" width="100px;" alt=""><br><sub><b>Colin But</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/ruslanpa"><img src="https://avatars2.githubusercontent.com/u/1503411?v=4" width="100px;" alt=""><br><sub><b>Ruslan</b></sub></a></td>
+    <td align="center"><a href="https://github.com/JuhoKang"><img src="https://avatars1.githubusercontent.com/u/4745294?v=4" width="100px;" alt=""><br><sub><b>Juho Kang</b></sub></a></td>
+    <td align="center"><a href="https://github.com/dheeraj-mummareddy"><img src="https://avatars2.githubusercontent.com/u/7002230?v=4" width="100px;" alt=""><br><sub><b>Dheeraj Mummareddy</b></sub></a></td>
+    <td align="center"><a href="https://www.bernardosulzbach.com"><img src="https://avatars0.githubusercontent.com/u/8271090?v=4" width="100px;" alt=""><br><sub><b>Bernardo Sulzbach</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/4lexis"><img src="https://avatars0.githubusercontent.com/u/19871727?v=4" width="100px;" alt=""><br><sub><b>Aleksandar Dudukovic</b></sub></a></td>
+    <td align="center"><a href="https://www.yusufaytas.com"><img src="https://avatars2.githubusercontent.com/u/1049483?v=4" width="100px;" alt=""><br><sub><b>Yusuf Aytaş</b></sub></a></td>
+    <td align="center"><a href="http://futurehomes.hu"><img src="https://avatars2.githubusercontent.com/u/1001491?v=4" width="100px;" alt=""><br><sub><b>Mihály Kuprivecz</b></sub></a></td>
+    <td align="center"><a href="https://github.com/kapinuss"><img src="https://avatars0.githubusercontent.com/u/17639945?v=4" width="100px;" alt=""><br><sub><b>Stanislav Kapinus</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/gvsharma"><img src="https://avatars1.githubusercontent.com/u/6648152?v=4" width="100px;" alt=""><br><sub><b>GVSharma</b></sub></a></td>
+    <td align="center"><a href="https://github.com/SrdjanPaunovic"><img src="https://avatars1.githubusercontent.com/u/22815104?v=4" width="100px;" alt=""><br><sub><b>Srđan Paunović</b></sub></a></td>
+    <td align="center"><a href="https://sideris.xyz/"><img src="https://avatars3.githubusercontent.com/u/5484694?v=4" width="100px;" alt=""><br><sub><b>Petros G. Sideris</b></sub></a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/pramodgupta3/"><img src="https://avatars1.githubusercontent.com/u/2184241?v=4" width="100px;" alt=""><br><sub><b>Pramod Gupta</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://amarnath510.github.io/portfolio"><img src="https://avatars0.githubusercontent.com/u/4599623?v=4" width="100px;" alt=""><br><sub><b>Amarnath Chandana</b></sub></a></td>
+    <td align="center"><a href="https://github.com/Anurag870"><img src="https://avatars1.githubusercontent.com/u/6295975?v=4" width="100px;" alt=""><br><sub><b>Anurag870</b></sub></a></td>
+    <td align="center"><a href="http://theerroris.me"><img src="https://avatars0.githubusercontent.com/u/1685953?v=4" width="100px;" alt=""><br><sub><b>Wes Gilleland</b></sub></a></td>
+    <td align="center"><a href="https://github.com/Harshrajsinh"><img src="https://avatars2.githubusercontent.com/u/22811531?v=4" width="100px;" alt=""><br><sub><b>Harshraj Thakor</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/MaVdbussche"><img src="https://avatars1.githubusercontent.com/u/26136934?v=4" width="100px;" alt=""><br><sub><b>Martin Vandenbussche</b></sub></a></td>
+    <td align="center"><a href="https://alexsomai.com"><img src="https://avatars1.githubusercontent.com/u/5720977?v=4" width="100px;" alt=""><br><sub><b>Alexandru Somai</b></sub></a></td>
+    <td align="center"><a href="https://github.com/amogozov"><img src="https://avatars3.githubusercontent.com/u/7372215?v=4" width="100px;" alt=""><br><sub><b>Artur Mogozov</b></sub></a></td>
+    <td align="center"><a href="https://github.com/anthonycampbell"><img src="https://avatars3.githubusercontent.com/u/10249255?v=4" width="100px;" alt=""><br><sub><b>anthony</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://ccygnus.com/"><img src="https://avatars1.githubusercontent.com/u/9342724?v=4" width="100px;" alt=""><br><sub><b>Christian Cygnus</b></sub></a></td>
+    <td align="center"><a href="https://about.me/dzmitryh"><img src="https://avatars2.githubusercontent.com/u/5390492?v=4" width="100px;" alt=""><br><sub><b>Dima Gubin</b></sub></a></td>
+    <td align="center"><a href="https://github.com/jjjimenez100"><img src="https://avatars3.githubusercontent.com/u/22243493?v=4" width="100px;" alt=""><br><sub><b>Joshua Jimenez</b></sub></a></td>
+    <td align="center"><a href="http://about.me/kaiwinter"><img src="https://avatars0.githubusercontent.com/u/110982?v=4" width="100px;" alt=""><br><sub><b>Kai Winter</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/lbroman"><img src="https://avatars1.githubusercontent.com/u/86007?v=4" width="100px;" alt=""><br><sub><b>lbroman</b></sub></a></td>
+    <td align="center"><a href="https://przemeknowak.com"><img src="https://avatars1.githubusercontent.com/u/3254609?v=4" width="100px;" alt=""><br><sub><b>Przemek</b></sub></a></td>
+    <td align="center"><a href="https://github.com/prafful1"><img src="https://avatars0.githubusercontent.com/u/14350274?v=4" width="100px;" alt=""><br><sub><b>Prafful Agarwal</b></sub></a></td>
+    <td align="center"><a href="https://github.com/sankypanhale"><img src="https://avatars1.githubusercontent.com/u/6478783?v=4" width="100px;" alt=""><br><sub><b>Sanket Panhale</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/staillebois"><img src="https://avatars0.githubusercontent.com/u/23701200?v=4" width="100px;" alt=""><br><sub><b>staillebois</b></sub></a></td>
+    <td align="center"><a href="https://github.com/valdar-hu"><img src="https://avatars3.githubusercontent.com/u/17962817?v=4" width="100px;" alt=""><br><sub><b>Krisztián Nagy</b></sub></a></td>
+    <td align="center"><a href="https://www.vanogrid.com"><img src="https://avatars0.githubusercontent.com/u/4307918?v=4" width="100px;" alt=""><br><sub><b>Alexander Ivanov</b></sub></a></td>
+    <td align="center"><a href="https://github.com/yosfik"><img src="https://avatars3.githubusercontent.com/u/4850270?v=4" width="100px;" alt=""><br><sub><b>Yosfik Alqadri</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/7agustibm"><img src="https://avatars0.githubusercontent.com/u/8149332?v=4" width="100px;" alt=""><br><sub><b>Agustí Becerra Milà</b></sub></a></td>
+    <td align="center"><a href="https://github.com/Juaanma"><img src="https://avatars3.githubusercontent.com/u/7390500?v=4" width="100px;" alt=""><br><sub><b>Juan Manuel Suárez</b></sub></a></td>
+    <td align="center"><a href="http://www.devsedge.net/"><img src="https://avatars0.githubusercontent.com/u/9956006?v=4" width="100px;" alt=""><br><sub><b>Luigi Cortese</b></sub></a></td>
+    <td align="center"><a href="https://github.com/Rzeposlaw"><img src="https://avatars2.githubusercontent.com/u/18425745?v=4" width="100px;" alt=""><br><sub><b>Katarzyna Rzepecka</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://adamski.pro"><img src="https://avatars1.githubusercontent.com/u/6537430?v=4" width="100px;" alt=""><br><sub><b>adamski.pro</b></sub></a></td>
+    <td align="center"><a href="https://github.com/baislsl"><img src="https://avatars0.githubusercontent.com/u/17060584?v=4" width="100px;" alt=""><br><sub><b>Shengli Bai</b></sub></a></td>
+    <td align="center"><a href="https://github.com/besok"><img src="https://avatars2.githubusercontent.com/u/29834592?v=4" width="100px;" alt=""><br><sub><b>Boris</b></sub></a></td>
+    <td align="center"><a href="https://github.com/dmitraver"><img src="https://avatars3.githubusercontent.com/u/1798156?v=4" width="100px;" alt=""><br><sub><b>Dmitry Avershin</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/fanofxiaofeng"><img src="https://avatars0.githubusercontent.com/u/3983683?v=4" width="100px;" alt=""><br><sub><b>靳阳</b></sub></a></td>
+    <td align="center"><a href="https://github.com/hoangnam2261"><img src="https://avatars2.githubusercontent.com/u/31692990?v=4" width="100px;" alt=""><br><sub><b>hoangnam2261</b></sub></a></td>
+    <td align="center"><a href="https://github.com/jarpit96"><img src="https://avatars2.githubusercontent.com/u/10098713?v=4" width="100px;" alt=""><br><sub><b>Arpit Jain</b></sub></a></td>
+    <td align="center"><a href="http://joningi.net"><img src="https://avatars2.githubusercontent.com/u/6115148?v=4" width="100px;" alt=""><br><sub><b>Jón Ingi Sveinbjörnsson</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/kirill-vlasov"><img src="https://avatars3.githubusercontent.com/u/16112495?v=4" width="100px;" alt=""><br><sub><b>Kirill Vlasov</b></sub></a></td>
+    <td align="center"><a href="http://mitchell-irvin.com"><img src="https://avatars0.githubusercontent.com/u/16233245?v=4" width="100px;" alt=""><br><sub><b>Mitchell Irvin</b></sub></a></td>
+    <td align="center"><a href="https://ranjeet-floyd.github.io"><img src="https://avatars0.githubusercontent.com/u/1992972?v=4" width="100px;" alt=""><br><sub><b>Ranjeet</b></sub></a></td>
+    <td align="center"><a href="https://alwayswithme.github.io"><img src="https://avatars3.githubusercontent.com/u/3234786?v=4" width="100px;" alt=""><br><sub><b>PhoenixYip</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/MSaifAsif"><img src="https://avatars1.githubusercontent.com/u/6280554?v=4" width="100px;" alt=""><br><sub><b>M Saif Asif</b></sub></a></td>
+    <td align="center"><a href="https://github.com/kanwarpreet25"><img src="https://avatars0.githubusercontent.com/u/39183641?v=4" width="100px;" alt=""><br><sub><b>kanwarpreet25</b></sub></a></td>
+    <td align="center"><a href="http://leonmak.me"><img src="https://avatars3.githubusercontent.com/u/13071508?v=4" width="100px;" alt=""><br><sub><b>Leon Mak</b></sub></a></td>
+    <td align="center"><a href="http://www.wramdemark.se"><img src="https://avatars2.githubusercontent.com/u/7052193?v=4" width="100px;" alt=""><br><sub><b>Per Wramdemark</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/waisuan"><img src="https://avatars2.githubusercontent.com/u/10975700?v=4" width="100px;" alt=""><br><sub><b>Evan Sia Wai Suan</b></sub></a></td>
+    <td align="center"><a href="https://github.com/AnaghaSasikumar"><img src="https://avatars2.githubusercontent.com/u/42939261?v=4" width="100px;" alt=""><br><sub><b>AnaghaSasikumar</b></sub></a></td>
+    <td align="center"><a href="https://christofferh.com"><img src="https://avatars1.githubusercontent.com/u/767643?v=4" width="100px;" alt=""><br><sub><b>Christoffer Hamberg</b></sub></a></td>
+    <td align="center"><a href="https://github.com/dgruntz"><img src="https://avatars0.githubusercontent.com/u/1516800?v=4" width="100px;" alt=""><br><sub><b>Dominik Gruntz</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://bitbucket.org/hannespernpeintner/"><img src="https://avatars3.githubusercontent.com/u/1679437?v=4" width="100px;" alt=""><br><sub><b>Hannes</b></sub></a></td>
+    <td align="center"><a href="https://github.com/leogtzr"><img src="https://avatars0.githubusercontent.com/u/1211969?v=4" width="100px;" alt=""><br><sub><b>Leo Gutiérrez Ramírez</b></sub></a></td>
+    <td align="center"><a href="https://github.com/npczwh"><img src="https://avatars0.githubusercontent.com/u/14066422?v=4" width="100px;" alt=""><br><sub><b>Zhang WH</b></sub></a></td>
+    <td align="center"><a href="https://github.com/oconnelc"><img src="https://avatars0.githubusercontent.com/u/1112973?v=4" width="100px;" alt=""><br><sub><b>Christopher O'Connell</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/giorgosmav21"><img src="https://avatars2.githubusercontent.com/u/22855493?v=4" width="100px;" alt=""><br><sub><b>George Mavroeidis</b></sub></a></td>
+    <td align="center"><a href="https://github.com/hbothra15"><img src="https://avatars1.githubusercontent.com/u/7418012?v=4" width="100px;" alt=""><br><sub><b>Hemant Bothra</b></sub></a></td>
+    <td align="center"><a href="https://www.kevinpeters.net/about/"><img src="https://avatars1.githubusercontent.com/u/12736734?v=4" width="100px;" alt=""><br><sub><b>Kevin Peters</b></sub></a></td>
+    <td align="center"><a href="https://llorllale.github.io/"><img src="https://avatars1.githubusercontent.com/u/2019896?v=4" width="100px;" alt=""><br><sub><b>George Aristy</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/mookkiah"><img src="https://avatars1.githubusercontent.com/u/8975264?v=4" width="100px;" alt=""><br><sub><b>Mahendran Mookkiah</b></sub></a></td>
+    <td align="center"><a href="https://github.com/Azureyjt"><img src="https://avatars2.githubusercontent.com/u/18476317?v=4" width="100px;" alt=""><br><sub><b>Azureyjt</b></sub></a></td>
+    <td align="center"><a href="https://github.com/vehpsr"><img src="https://avatars2.githubusercontent.com/u/3133265?v=4" width="100px;" alt=""><br><sub><b>gans</b></sub></a></td>
+    <td align="center"><a href="https://github.com/ThatGuyWithTheHat"><img src="https://avatars0.githubusercontent.com/u/24470582?v=4" width="100px;" alt=""><br><sub><b>Matt</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://www.linkedin.com/in/gopinathlangote/"><img src="https://avatars2.githubusercontent.com/u/10210778?v=4" width="100px;" alt=""><br><sub><b>Gopinath Langote</b></sub></a></td>
+    <td align="center"><a href="https://github.com/hoswey"><img src="https://avatars3.githubusercontent.com/u/3689445?v=4" width="100px;" alt=""><br><sub><b>Hoswey</b></sub></a></td>
+    <td align="center"><a href="https://github.com/amit2103"><img src="https://avatars3.githubusercontent.com/u/7566692?v=4" width="100px;" alt=""><br><sub><b>Amit Pandey</b></sub></a></td>
+    <td align="center"><a href="https://github.com/gwildor28"><img src="https://avatars0.githubusercontent.com/u/16000365?v=4" width="100px;" alt=""><br><sub><b>gwildor28</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://t.me/paul_docker"><img src="https://avatars1.githubusercontent.com/u/2404785?v=4" width="100px;" alt=""><br><sub><b>田浩</b></sub></a></td>
+    <td align="center"><a href="https://twitter.com/StPitsios"><img src="https://avatars1.githubusercontent.com/u/6773603?v=4" width="100px;" alt=""><br><sub><b>Stamatis Pitsios</b></sub></a></td>
+    <td align="center"><a href="https://github.com/qza"><img src="https://avatars3.githubusercontent.com/u/233149?v=4" width="100px;" alt=""><br><sub><b>qza</b></sub></a></td>
+    <td align="center"><a href="http://tschis.github.io"><img src="https://avatars1.githubusercontent.com/u/20662669?v=4" width="100px;" alt=""><br><sub><b>Rodolfo Forte</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/ankurkaushal"><img src="https://avatars2.githubusercontent.com/u/2236616?v=4" width="100px;" alt=""><br><sub><b>Ankur Kaushal</b></sub></a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/ovidijus-okinskas/"><img src="https://avatars0.githubusercontent.com/u/20372387?v=4" width="100px;" alt=""><br><sub><b>Ovidijus Okinskas</b></sub></a></td>
+    <td align="center"><a href="https://github.com/robertt240"><img src="https://avatars1.githubusercontent.com/u/9137432?v=4" width="100px;" alt=""><br><sub><b>Robert Kasperczyk</b></sub></a></td>
+    <td align="center"><a href="https://github.com/trautonen"><img src="https://avatars3.githubusercontent.com/u/1641063?v=4" width="100px;" alt=""><br><sub><b>Tapio Rautonen</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://vk.com/yuri.orlov"><img src="https://avatars0.githubusercontent.com/u/1595733?v=4" width="100px;" alt=""><br><sub><b>Yuri Orlov</b></sub></a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/varunu28/"><img src="https://avatars0.githubusercontent.com/u/7676016?v=4" width="100px;" alt=""><br><sub><b>Varun Upadhyay</b></sub></a></td>
+    <td align="center"><a href="https://github.com/PalAditya"><img src="https://avatars2.githubusercontent.com/u/25523604?v=4" width="100px;" alt=""><br><sub><b>Aditya Pal</b></sub></a></td>
+    <td align="center"><a href="https://github.com/grzesiekkedzior"><img src="https://avatars3.githubusercontent.com/u/23739158?v=4" width="100px;" alt=""><br><sub><b>grzesiekkedzior</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/sivasubramanim"><img src="https://avatars2.githubusercontent.com/u/51107434?v=4" width="100px;" alt=""><br><sub><b>Sivasubramani M</b></sub></a></td>
+    <td align="center"><a href="https://github.com/d4gg4d"><img src="https://avatars2.githubusercontent.com/u/99457?v=4" width="100px;" alt=""><br><sub><b>Sami Airaksinen</b></sub></a></td>
+    <td align="center"><a href="https://github.com/vertti"><img src="https://avatars0.githubusercontent.com/u/557751?v=4" width="100px;" alt=""><br><sub><b>Janne Sinivirta</b></sub></a></td>
+    <td align="center"><a href="https://github.com/Bobo1239"><img src="https://avatars1.githubusercontent.com/u/2302947?v=4" width="100px;" alt=""><br><sub><b>Boris-Chengbiao Zhou</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://jahhein.github.io"><img src="https://avatars2.githubusercontent.com/u/10779515?v=4" width="100px;" alt=""><br><sub><b>Jacob Hein</b></sub></a></td>
+    <td align="center"><a href="https://github.com/iamrichardjones"><img src="https://avatars3.githubusercontent.com/u/14842151?v=4" width="100px;" alt=""><br><sub><b>Richard Jones</b></sub></a></td>
+    <td align="center"><a href="https://rachelcarmena.github.io"><img src="https://avatars0.githubusercontent.com/u/22792183?v=4" width="100px;" alt=""><br><sub><b>Rachel M. Carmena</b></sub></a></td>
+    <td align="center"><a href="https://zd-zero.github.io"><img src="https://avatars0.githubusercontent.com/u/21978370?v=4" width="100px;" alt=""><br><sub><b>Zaerald Denze Lungos</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://webpro.nl"><img src="https://avatars1.githubusercontent.com/u/456426?v=4" width="100px;" alt=""><br><sub><b>Lars Kappert</b></sub></a></td>
+    <td align="center"><a href="https://xiaod.info"><img src="https://avatars2.githubusercontent.com/u/21277644?v=4" width="100px;" alt=""><br><sub><b>Mike Liu</b></sub></a></td>
+    <td align="center"><a href="https://github.com/charlesfinley"><img src="https://avatars1.githubusercontent.com/u/6307904?v=4" width="100px;" alt=""><br><sub><b>Matt Dolan</b></sub></a></td>
+    <td align="center"><a href="https://github.com/MananS77"><img src="https://avatars3.githubusercontent.com/u/21033516?v=4" width="100px;" alt=""><br><sub><b>Manan</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/nishant"><img src="https://avatars2.githubusercontent.com/u/15331971?v=4" width="100px;" alt=""><br><sub><b>Nishant Arora</b></sub></a></td>
+    <td align="center"><a href="https://github.com/raja-peeyush-kumar-singh"><img src="https://avatars0.githubusercontent.com/u/5496024?v=4" width="100px;" alt=""><br><sub><b>Peeyush</b></sub></a></td>
+    <td align="center"><a href="https://github.com/ravening"><img src="https://avatars1.githubusercontent.com/u/10645273?v=4" width="100px;" alt=""><br><sub><b>Rakesh</b></sub></a></td>
+    <td align="center"><a href="https://github.com/vINCENT8888801"><img src="https://avatars0.githubusercontent.com/u/8037883?v=4" width="100px;" alt=""><br><sub><b>Wei Seng</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://www.linkedin.com/in/ashish-trivedi-218379135/"><img src="https://avatars3.githubusercontent.com/u/23194128?v=4" width="100px;" alt=""><br><sub><b>Ashish Trivedi</b></sub></a></td>
+    <td align="center"><a href="https://rayyounghong.com"><img src="https://avatars1.githubusercontent.com/u/41055099?v=4" width="100px;" alt=""><br><sub><b>洪月阳</b></sub></a></td>
+    <td align="center"><a href="https://xdvrx1.github.io/"><img src="https://avatars0.githubusercontent.com/u/47092464?v=4" width="100px;" alt=""><br><sub><b>xdvrx1</b></sub></a></td>
+    <td align="center"><a href="http://subho.xyz"><img src="https://avatars0.githubusercontent.com/u/13291222?v=4" width="100px;" alt=""><br><sub><b>Subhrodip Mohanta</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/nahteb"><img src="https://avatars3.githubusercontent.com/u/13121570?v=4" width="100px;" alt=""><br><sub><b>Bethan Palmer</b></sub></a></td>
+    <td align="center"><a href="https://github.com/ToxicDreamz"><img src="https://avatars0.githubusercontent.com/u/45225562?v=4" width="100px;" alt=""><br><sub><b>Toxic Dreamz</b></sub></a></td>
+    <td align="center"><a href="http://www.edycutjong.com"><img src="https://avatars1.githubusercontent.com/u/1098102?v=4" width="100px;" alt=""><br><sub><b>Edy Cu Tjong</b></sub></a></td>
+    <td align="center"><a href="https://github.com/mkrzywanski"><img src="https://avatars0.githubusercontent.com/u/15279585?v=4" width="100px;" alt=""><br><sub><b>Michał Krzywański</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://www.stefan-birkner.de"><img src="https://avatars1.githubusercontent.com/u/711349?v=4" width="100px;" alt=""><br><sub><b>Stefan Birkner</b></sub></a></td>
+    <td align="center"><a href="https://github.com/fedorskvorcov"><img src="https://avatars3.githubusercontent.com/u/43882212?v=4" width="100px;" alt=""><br><sub><b>Fedor Skvorcov</b></sub></a></td>
+    <td align="center"><a href="https://github.com/samilAyoub"><img src="https://avatars0.githubusercontent.com/u/61546990?v=4" width="100px;" alt=""><br><sub><b>samilAyoub</b></sub></a></td>
+    <td align="center"><a href="https://github.com/vdlald"><img src="https://avatars0.githubusercontent.com/u/29997701?v=4" width="100px;" alt=""><br><sub><b>Vladislav Golubinov</b></sub></a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/swarajsaaj"><img src="https://avatars2.githubusercontent.com/u/6285049?v=4" width="100px;" alt=""><br><sub><b>Swaraj</b></sub></a></td>
+  </tr>
+</table>
+
+<!-- markdownlint-enable -->
+
+<!-- prettier-ignore-end -->
+
+<!-- ALL-CONTRIBUTORS-LIST:END -->

ko/adapter/README.md

@@ -0,0 +1,134 @@
+---
+layout: pattern
+title: Adapter
+folder: adapter
+permalink: "/patterns/adapter/"
+categories: Structural
+tags:
+- Gang of Four
+---
+
+## 또한 ~으로 알려진
+
+Wrapper
+
+## 의도
+
+클래스의 인터페이스를 클라이언트가 기대하는 다른 인터페이스로 변환합니다. adapter를 사용하면 호환되지 않는 인터페이스로 인해 같이 쓸 수 없는 클래스를 함께 작동 할 수 있습니다.
+
+## 설명
+
+예시
+
+> 메모리 카드에 몇 장의 사진이 있고 컴퓨터로 전송해야한다고 생각하십시오. 이들을 전송하려면 컴퓨터에 메모리 카드를 연결할 수 있도록 컴퓨터 포트와 호환되는 어댑터가 필요합니다. 이 경우 카드 리더는 어댑터입니다. 또 다른 예는 유명한 전원 어댑터입니다. 세 갈래 플러그는 두 갈래 콘센트에 연결할 수 없습니다. 두 갈래 콘센트와 호환되는 전원 어댑터를 사용해야합니다. 또 다른 예는 한 사람이 말한 단어를 다른 사람에게 번역하는 번역가입니다.
+
+평범하게 말하자면
+
+> adapter 패턴을 사용하면 호환되지 않는 개체를 adapter에 연결하여 다른 클래스와 호환되도록 할 수 있습니다.
+
+Wikipedia 말에 의하면
+
+> 소프트웨어 엔지니어링에서 adapter 패턴은 기존 클래스의 인터페이스를 다른 인터페이스로 사용할 수 있도록 하는 소프트웨어 디자인 패턴입니다. 소스 코드를 수정하지 않고 기존 클래스가 다른 클래스와 함께 작동하도록 만드는 데 자주 사용됩니다.
+
+**프로그램 코드 예제**
+
+조정 보트만 사용할 수 있고 전혀 항해할 수 없는 선장을 생각해보십시오.
+
+먼저 `RowingBoat` 및 `FishingBoat` 인터페이스가 있습니다.
+
+```java
+public interface RowingBoat {
+  void row();
+}
+
+public class FishingBoat {
+  private static final Logger LOGGER = LoggerFactory.getLogger(FishingBoat.class);
+  public void sail() {
+    LOGGER.info("The fishing boat is sailing");
+  }
+}
+```
+
+그리고 선장은 `RowingBoat` 인터페이스를 이동할 수 있게 구현했습니다.
+
+```java
+public class Captain {
+
+  private final RowingBoat rowingBoat;
+  // default constructor and setter for rowingBoat
+  public Captain(RowingBoat rowingBoat) {
+    this.rowingBoat = rowingBoat;
+  }
+
+  public void row() {
+    rowingBoat.row();
+  }
+}
+```
+
+이제 해적이오고 있고 우리 선장이 탈출해야하는데 어선만 있습니다. 선장이 조정 보트 기술로 어선을 조작 할 수있는 adapter를 만들어야합니다.
+
+```java
+public class FishingBoatAdapter implements RowingBoat {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(FishingBoatAdapter.class);
+
+  private final FishingBoat boat;
+
+  public FishingBoatAdapter() {
+    boat = new FishingBoat();
+  }
+
+  @Override
+  public void row() {
+    boat.sail();
+  }
+}
+```
+
+이제 `Captain` 은 `FishingBoat` 를 사용하여 해적을 탈출 할 수 있습니다.
+
+```java
+var captain = new Captain(new FishingBoatAdapter());
+captain.row();
+```
+
+## 클레스 다이어그램
+
+![alt text](https://github.com/warp125/java-design-patterns/blob/master/adapter/etc/adapter.urm.png?raw=true)
+
+## 적용 가능성
+
+다음과 같은 경우 adapter 패턴을 사용합니다.
+
+- 기존 클래스를 사용 하려는데 해당 인터페이스가 필요한 클래스와 일치하지 않습니다.
+- 관련이 없거나 예상치 못한 클래스, 즉 호환되는 인터페이스가 반드시 필요하지 않은 클래스와 협력하는 재사용 가능한 클래스를 만들고 싶습니다.
+- 기존의 여러 하위 클래스를 사용해야하지만 모든 하위 클래스를 하위 클래스로 지정하여 인터페이스를 조정하는 것은 비현실적입니다. 개체 adapter는 부모 클래스의 인터페이스를 조정할 수 있습니다.
+- 타사 라이브러리를 사용하는 대부분의 응용 프로그램은 adapter를 응용 프로그램과 타사 라이브러리 사이의 중간 계층으로 사용하여 라이브러리에서 응용 프로그램을 분리합니다. 다른 라이브러리를 사용해야하는 경우 애플리케이션 코드를 변경할 필요없이 새 라이브러리 용 adapter만 필요합니다.
+
+## 결과 :
+
+클래스 및 개체 adapter에는 서로 다른 장단점이 있습니다. <br>클래스 adapter
+
+- 구체적인 Adaptee 클래스를 커밋하여 Adaptee를 Target에 적용합니다. 결과적으로 클래스와 모든 하위 클래스를 조정하려는 경우 클래스 adapter가 작동하지 않습니다.
+- adapter는 Adaptee의 하위 클래스이기 때문에 Adaptee의 일부 동작을 오버라이드합니다.
+- 하나의 객체만 생성하고 adaptee를 얻기위해 위해 추가 포인터 간접 지정이 필요하지 않습니다.
+
+개체 adapter
+
+- 하나의 adapter가 많은 Adaptees, 즉 Adaptee 자체와 모든 하위 클래스 (있는 경우)와 함께 작동하도록합시다. adapter는 한 번에 모든 어댑터에 기능을 추가 할 수도 있습니다.
+- Adaptee 동작을 오버라이드하기가 더 어렵습니다. Adaptee를 서브 클래싱하고 어댑터가 Adaptee 자체가 아닌 서브 클래스를 참조하도록 해야합니다.
+
+## 실제 사례
+
+- [java.util.Arrays#asList()](http://docs.oracle.com/javase/8/docs/api/java/util/Arrays.html#asList%28T...%29)
+- [java.util.Collections#list()](https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#list-java.util.Enumeration-)
+- [java.util.Collections#enumeration()](https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#enumeration-java.util.Collection-)
+- [javax.xml.bind.annotation.adapters.XMLAdapter](http://docs.oracle.com/javase/8/docs/api/javax/xml/bind/annotation/adapters/XmlAdapter.html#marshal-BoundType-)
+
+## 크레딧
+
+- [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+- [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)
+- [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+- [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)

ko/factory/README.md

@@ -0,0 +1,128 @@
+---
+layout: pattern
+title: Factory
+folder: factory
+permalink: "/patterns/factory/"
+categories: Creational
+tags:
+- Gang of Four
+---
+
+## 또한 ~으로 알려진
+
+- Simple Factory
+- Static Factory Method
+
+## 의도
+
+구현 논리를 숨기고 클라이언트 코드가 새 객체를 초기화하는 대신 사용에 집중하도록하기 위해 factory라는 클래스에 캡슐화 된 정적 메서드를 제공합니다.
+
+## 설명
+
+예시
+
+> SQLServer에 연결된 웹 응용 프로그램이 있지만 이제 Oracle로 전환하려고 합니다. 기존 소스 코드를 수정하지 않고 이를 수행하려면 주어진 데이터베이스에 대한 연결을 생성하기 위해 정적 메서드를 호출 할 수 있는 Simple Factory 패턴을 구현해야합니다.
+
+Wikipedia 말에 의하면
+
+> factory는 다른 객체를 생성하기위한 객체입니다. 공식적으로 factory는 다양한 프로토 타입 또는 클래스의 객체를 반환하는 함수 또는 메서드입니다.
+
+**프로그램 코드 예제**
+
+우리는 인터페이스 `Car` 와 두 가지 구현 `Ford` 와 `Ferrari`을 가지고 있습니다.
+
+```java
+public interface Car {
+  String getDescription();
+}
+
+public class Ford implements Car {
+
+  static final String DESCRIPTION = "This is Ford.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+}
+
+public class Ferrari implements Car {
+   
+  static final String DESCRIPTION = "This is Ferrari.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+}
+```
+
+열거형은 우리가 지원하는 자동차 유형을 나타냅니다 ( `Ford` 및 `Ferrari` ).
+
+```java
+public enum CarType {
+  
+  FORD(Ford::new),
+  FERRARI(Ferrari::new);
+  
+  private final Supplier<Car> constructor;
+  
+  CarType(Supplier<Car> constructor) {
+    this.constructor = constructor;
+  }
+  
+  public Supplier<Car> getConstructor() {
+    return this.constructor;
+  }
+}
+```
+
+그런 다음 factory 클래스 `CarsFactory` 캡슐화 된 자동차 객체를 만드는 정적 메서드 `getCar` 가 있습니다.
+
+```java
+public class CarsFactory {
+  
+  public static Car getCar(CarType type) {
+    return type.getConstructor().get();
+  }
+}
+```
+
+이제 클라이언트 코드에서 factory 클래스를 사용하여 다양한 유형의 자동차를 만들 수 있습니다.
+
+```java
+var car1 = CarsFactory.getCar(CarType.FORD);
+var car2 = CarsFactory.getCar(CarType.FERRARI);
+LOGGER.info(car1.getDescription());
+LOGGER.info(car2.getDescription());;
+```
+
+프로그램 출력 :
+
+```java
+This is Ford.
+This Ferrari.
+```
+
+## 클래스 다이어그램
+
+![alt text](https://github.com/iluwatar/java-design-patterns/blob/master/factory/etc/factory.urm.png)
+
+## 적용 가능성
+
+객체 생성 및 관리 방법이 아닌 객체 생성에만 관심이있을 때 Simple Factory 패턴을 사용합니다.
+
+장점
+
+- 모든 객체 생성을 한곳에 유지하고 코드베이스에 '새'키 값이 확산되는 것을 방지합니다.
+- 느슨하게 결합 된 코드를 작성할 수 있습니다. 주요 장점 중 일부는 더 나은 테스트 가능성, 이해하기 쉬운 코드, 교체 가능한 구성 요소, 확장성 및 격리된 기능을 포함합니다.
+
+단점
+
+- 코드는 생각보다 복잡해집니다.
+
+## 관련 패턴
+
+- [Factory Method](https://java-design-patterns.com/patterns/factory-method/)
+- [Factory Kit](https://java-design-patterns.com/patterns/factory-kit/)
+- [Abstract Factory](https://java-design-patterns.com/patterns/abstract-factory/)

ko/observer/README.md

@@ -0,0 +1,165 @@
+---
+layout: pattern
+title: Observer
+folder: observer
+permalink: "/patterns/observer/"
+categories: Behavioral
+tags:
+- Gang Of Four
+- Reactive
+---
+
+## 또한 ~으로 알려진
+
+Dependents, Publish-Subscribe
+
+## 의도
+
+하나의 개체가 상태를 변경하면 모든 종속 항목에 알림이 전송되고 자동으로 업데이트 되도록 개체간의 일대 다 종속성을 정의합니다.
+
+## 설명
+
+예시
+
+> 멀리 떨어진 땅에는 호빗과 오크 종족이 살고 있습니다. 둘 다 대부분 야외에 있으므로 날씨 변화를 면밀히 따릅니다. 끊임없이 날씨를 관찰하고 있다고 말할 수 있습니다.
+
+평범하게 말하자면
+
+> observer로 등록하여 개체의 상태 변경을 수신합니다.
+
+Wikipedia 말에 의하면
+
+> observer 패턴은 주제라고하는 객체가 observer라고 하는 종속 항목 목록을 유지하고 일반적으로 메서드 중 하나를 호출하여 상태 변경을 자동으로 알리는 소프트웨어 디자인 패턴입니다.
+
+**프로그램 코드 예제**
+
+먼저 `WeatherObserver` 인터페이스와 우리의 종족 `Orcs` 와 `Hobbits` 소개하겠습니다.
+
+```java
+public interface WeatherObserver {
+
+  void update(WeatherType currentWeather);
+}
+
+public class Orcs implements WeatherObserver {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Orcs.class);
+
+  @Override
+  public void update(WeatherType currentWeather) {
+    LOGGER.info("The orcs are facing " + currentWeather.getDescription() + " weather now");
+  }
+}
+
+public class Hobbits implements WeatherObserver {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Hobbits.class);
+
+  @Override
+  public void update(WeatherType currentWeather) {
+    switch (currentWeather) {
+      LOGGER.info("The hobbits are facing " + currentWeather.getDescription() + " weather now");
+    }
+  }
+}
+```
+
+그리고 끊임없이 변화하는 `Weather` 가 있습니다.
+
+```java
+public class Weather {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Weather.class);
+
+  private WeatherType currentWeather;
+  private final List<WeatherObserver> observers;
+
+  public Weather() {
+    observers = new ArrayList<>();
+    currentWeather = WeatherType.SUNNY;
+  }
+
+  public void addObserver(WeatherObserver obs) {
+    observers.add(obs);
+  }
+
+  public void removeObserver(WeatherObserver obs) {
+    observers.remove(obs);
+  }
+
+  /**
+   * Makes time pass for weather.
+   */
+  public void timePasses() {
+    var enumValues = WeatherType.values();
+    currentWeather = enumValues[(currentWeather.ordinal() + 1) % enumValues.length];
+    LOGGER.info("The weather changed to {}.", currentWeather);
+    notifyObservers();
+  }
+
+  private void notifyObservers() {
+    for (var obs : observers) {
+      obs.update(currentWeather);
+    }
+  }
+}
+```
+
+여기에 전체 예제가 있습니다.
+
+```java
+    var weather = new Weather();
+    weather.addObserver(new Orcs());
+    weather.addObserver(new Hobbits());
+    weather.timePasses();
+    weather.timePasses();
+    weather.timePasses();
+    weather.timePasses();
+```
+
+프로그램 출력 :
+
+```
+The weather changed to rainy.
+The orcs are facing rainy weather now
+The hobbits are facing rainy weather now
+The weather changed to windy.
+The orcs are facing windy weather now
+The hobbits are facing windy weather now
+The weather changed to cold.
+The orcs are facing cold weather now
+The hobbits are facing cold weather now
+The weather changed to sunny.
+The orcs are facing sunny weather now
+The hobbits are facing sunny weather now
+```
+
+## 클래스 다이어그램
+
+![alt text](https://github.com/iluwatar/java-design-patterns/blob/master/observer/etc/observer.png)
+
+## 적용 가능성
+
+다음 상황에서 관찰자 패턴을 사용하십시오.
+
+- 추상화에 두 가지 측면이 있을 때 하나는 다른 하나에 종속됩니다. 이러한 측면을 별도의 개체에 캡슐화하면 독립적으로 변경하고 재사용 할 수 있습니다.
+- 한 개체를 변경하려면 얼마나 다른 개체를 변경해야 하는지 알 수 없는 경우입니다.
+- 개체가 다른 개체가 누구인지 가정하지 않고 알릴 수 있어야하는 경우. 즉, 개체가 단단히 결합되는 것을 원하지 않습니다.
+
+## 일반적인 사용 사례
+
+- 한 개체를 변경하면 다른 개체도 변경됩니다.
+
+## 실제 사례
+
+- [java.util.Observer](http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html)
+- [java.util.EventListener](http://docs.oracle.com/javase/8/docs/api/java/util/EventListener.html)
+- [javax.servlet.http.HttpSessionBindingListener](http://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionBindingListener.html)
+- [RxJava](https://github.com/ReactiveX/RxJava)
+
+## 크레딧
+
+- [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+- [Java Generics and Collections](https://www.amazon.com/gp/product/0596527756/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596527756&linkCode=as2&tag=javadesignpat-20&linkId=246e5e2c26fe1c3ada6a70b15afcb195)
+- [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+- [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)

ko/prototype/README.md

@@ -0,0 +1,87 @@
+---
+layout: pattern
+title: Prototype
+folder: prototype
+permalink: "/patterns/prototype/"
+categories: Creational
+tags:
+- Gang Of Four
+- Instantiation
+---
+
+## 의도
+
+prototype 인스턴스를 사용하여 만들 개체의 종류를 지정하고  prototype을 복사하여 새 개체를 만듭니다.
+
+## 설명
+
+먼저 성능 이점을 얻기 위해 prototype 패턴이 사용되지 않는다는 점에 유의해야합니다. 오직 prototype 인스턴스에서 새 개체를 만드는 데만 사용됩니다.
+
+예시
+
+> 복제된 양인 돌리에 대해 기억나십니까? 자세한 내용은 다루지 않겠습니다만 여기서 핵심은 복제에 관한 것입니다.
+
+평범한 말하자면
+
+> 복제를 통해 기존 개체를 기반으로 개체를 만듭니다.
+
+Wikipedia 말에 의하면
+
+> prototype 패턴은 소프트웨어 개발에서 생성 디자인 패턴입니다. 생성 할 개체의 유형이 새 개체를 생성하기 위해 복제되는 prototype 인스턴스에 의해 결정될 때 사용됩니다.
+
+간단히 말해, 객체를 처음부터 만들고 설정하는 문제를 겪는 대신 기존 객체의 복사본을 만들어 필요에 맞게 수정할 수 있습니다.
+
+**프로그램 코드 예제**
+
+Java에서는 `Cloneable` 을 구현하고 `Object` 에서 `clone` 을 오버라이딩하여 쉽게 수행 할 수 있습니다.
+
+```java
+class Sheep implements Cloneable {
+  private String name;
+  public Sheep(String name) { this.name = name; }
+  public void setName(String name) { this.name = name; }
+  public String getName() { return name; }
+  @Override
+  public Sheep clone() {
+    try {
+      return (Sheep)super.clone();
+    } catch(CloneNotSuportedException) {
+      throw new InternalError();
+    }
+  }
+}
+```
+
+그런 다음 아래와 같이 복제할 수 있습니다.
+
+```java
+var original = new Sheep("Jolly");
+System.out.println(original.getName()); // Jolly
+
+// Clone and modify what is required
+var cloned = original.clone();
+cloned.setName("Dolly");
+System.out.println(cloned.getName()); // Dolly
+```
+
+## 클래스 다이어그램
+
+![alt text](https://github.com/iluwatar/java-design-patterns/blob/master/prototype/etc/prototype.urm.png)
+
+## 적용 가능성
+
+시스템이 제품의 생성, 구성, 표현 및 표현 방식에 독립적이어야 할 때 prototype 패턴을 사용
+
+- 인스턴스화할 클래스가 런타임에 지정되는 경우 (예 : 동적 로딩)
+- 제품의 클래스 계층 구조와 유사한 팩토리의 클래스 계층 구조 구축을 방지해야할 경우
+- 클래스의 인스턴스가 몇 가지 다른 상태 조합 중 하나만 가질 수 있는 경우. 적절한 상태로 매번 클래스를 수동으로 인스턴스화하는 것보다 해당하는 수의 prototype을 설치하고 복제하는 것이 더 편리 할 수 있습니다.
+- 복제에 비해 개체 생성 비용이 많이 드는 경우.
+
+## 실제 사례
+
+- [java.lang.Object#clone()](https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#clone%28%29)
+
+## 크레딧
+
+- [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+- [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)

ko/singleton/README.md

@@ -0,0 +1,84 @@
+---
+layout: pattern
+title: Singleton
+folder: singleton
+permalink: "/patterns/singleton/"
+categories: Creational
+tags:
+- Gang of Four
+---
+
+## 의도
+
+클래스에 인스턴스가 하나만 있는지 확인하고 이에 대한 전역 access point을 제공합니다.
+
+## 설명
+
+예시
+
+> 마법사들이 마법을 연구하는 상아탑은 단 하나뿐입니다. 마법사는 항상 동일한 마법의 상아탑을 사용합니다. 여기서 상아탑은 singleton입니다.
+
+평범하게 말하자면
+
+> 특정 클래스의 개체가 하나만 생성되도록합니다.
+
+Wikipedia 말에 의하면
+
+> 소프트웨어 엔지니어링에서 singleton 패턴은 클래스의 인스턴스화를 하나의 객체로 제한하는 소프트웨어 디자인 패턴입니다. 이는 시스템 전체에서 작업을 조정하는 데 정확히 하나의 개체가 필요할 때 유용합니다.
+
+**프로그램 코드 예제**
+
+Joshua Bloch, Effective Java 2nd Edition p.18
+
+> 단일 요소 열거형은 singleton을 구현하는 가장 좋은 방법입니다.
+
+```java
+public enum EnumIvoryTower {
+  INSTANCE
+}
+```
+
+그런 다음 사용하려면 :
+
+```java
+var enumIvoryTower1 = EnumIvoryTower.INSTANCE;
+var enumIvoryTower2 = EnumIvoryTower.INSTANCE;
+assertEquals(enumIvoryTower1, enumIvoryTower2); // true
+```
+
+## 클래스 다이어그램
+
+![alt text](https://github.com/iluwatar/java-design-patterns/blob/master/singleton/etc/singleton.urm.png)
+
+## 적용 가능성
+
+다음과 같은 경우 Singleton 패턴을 사용합니다.
+
+- 정확히 하나의 클래스 인스턴스가 있어야하며 잘 알려진 access point에서 클라이언트에 접근할 수 있어야합니다.
+- 단일 인스턴스가 서브 클래싱으로 확장 가능해야하고 클라이언트가 코드를 수정하지 않고 확장 인스턴스를 사용할 수 있어야 하는 경우
+
+## 일반적인 사용 사례
+
+- 로깅 클래스
+- 데이터베이스에 대한 연결 관리
+- 파일 관리자
+
+## 실제 사례
+
+- [java.lang.Runtime#getRuntime()](http://docs.oracle.com/javase/8/docs/api/java/lang/Runtime.html#getRuntime%28%29)
+- [java.awt.Desktop#getDesktop()](http://docs.oracle.com/javase/8/docs/api/java/awt/Desktop.html#getDesktop--)
+- [java.lang.System#getSecurityManager()](http://docs.oracle.com/javase/8/docs/api/java/lang/System.html#getSecurityManager--)
+
+## 결과
+
+- 자체 생성 및 수명주기를 제어하여 SRP (Single Responsibility Principle)를 위반합니다.
+- 이 개체가 사용하는 개체와 리소스가 할당 해제되는 것을 방지하는 전역 공유 인스턴스를 사용하도록 권장합니다.
+- 밀접하게 연결된 코드를 만듭니다. Singleton의 클라이언트는 테스트하기가 어려워집니다.
+- Singleton의 하위 클래스를 만드는 것이 거의 불가능합니다.
+
+## 크레딧
+
+- [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+- [Effective Java](https://www.amazon.com/gp/product/0134685997/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0134685997&linkCode=as2&tag=javadesignpat-20&linkId=4e349f4b3ff8c50123f8147c828e53eb)
+- [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+- [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)

pom.xml

@@ -89,6 +89,7 @@
         <module>state</module>
         <module>strategy</module>
         <module>template-method</module>
+        <module>version-number</module>
         <module>visitor</module>
         <module>double-checked-locking</module>
         <module>servant</module>
@@ -194,9 +195,12 @@
         <module>strangler</module>
         <module>arrange-act-assert</module>
         <module>transaction-script</module>
+        <module>registry</module>
         <module>filterer</module>
         <module>factory</module>
         <module>separated-interface</module>
+        <module>data-transfer-object-enum-impl</module>
+        <module>special-case</module>
     </modules>
 
     <repositories>

registry/README.md

@@ -0,0 +1,86 @@
+---
+layout: pattern
+title: Registry
+folder: registry
+permalink: /patterns/registry/
+categories: Creational
+tags:
+ - Instantiation
+---
+
+## Intent
+Stores the objects of a single class and provide a global point of access to them. 
+Similar to Multiton pattern, only difference is that in a registry there is no restriction on the number of objects.
+
+## Explanation
+
+In Plain Words
+
+> Registry is a well-known object that other objects can use to find common objects and services.
+
+**Programmatic Example**
+Below is a `Customer` Class
+
+```java
+public class Customer {
+
+  private final String id;
+  private final String name;
+
+  public Customer(String id, String name) {
+    this.id = id;
+    this.name = name;
+  }
+
+  public String getId() {
+    return id;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+}
+```
+
+This registry of the `Customer` objects is `CustomerRegistry`
+```java
+public final class CustomerRegistry {
+
+  private static final CustomerRegistry instance = new CustomerRegistry();
+
+  public static CustomerRegistry getInstance() {
+    return instance;
+  }
+
+  private final Map<String, Customer> customerMap;
+
+  private CustomerRegistry() {
+    customerMap = new ConcurrentHashMap<>();
+  }
+
+  public Customer addCustomer(Customer customer) {
+    return customerMap.put(customer.getId(), customer);
+  }
+
+  public Customer getCustomer(String id) {
+    return customerMap.get(id);
+  }
+
+}
+```
+
+## Class diagram
+![Registry](./etc/registry.png)
+
+## Applicability
+Use Registry pattern when 
+
+* client wants reference of some object, so client can lookup for that object in the object's registry.
+
+## Consequences
+Large number of bulky objects added to registry would result in a lot of memory consumption as objects in the registry are not garbage collected.
+
+## Credits
+* https://www.martinfowler.com/eaaCatalog/registry.html
+* https://wiki.c2.com/?RegistryPattern

registry/etc/registry.urm.puml

@@ -0,0 +1,21 @@
+@startuml
+package com.iluwatar.registry {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+  class Customer {
+    - id : String
+    - name : String
+    + getId() : String
+    + getName() : String
+    + toString() : String
+  }
+  class CustomerRegistry {
+    + addCustomer(customer : Customer)
+    + getCustomer(id : String)
+  }
+}
+Customer --> "-addCustomer" CustomerRegistry
+@enduml

registry/pom.xml

@@ -0,0 +1,46 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.24.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>registry</artifactId>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-assembly-plugin</artifactId>
+        <executions>
+          <execution>
+            <configuration>
+              <archive>
+                <manifest>
+                  <mainClass>com.iluwatar.registry.App</mainClass>
+                </manifest>
+              </archive>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+
+</project>

registry/src/main/java/com/iluwatar/registry/App.java

@@ -0,0 +1,27 @@
+package com.iluwatar.registry;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args
+   */
+  public static void main(String[] args) {
+    CustomerRegistry customerRegistry = CustomerRegistry.getInstance();
+    var john = new Customer("1", "John");
+    customerRegistry.addCustomer(john);
+
+    var julia = new Customer("2", "Julia");
+    customerRegistry.addCustomer(julia);
+
+    LOGGER.info("John {}", customerRegistry.getCustomer("1"));
+    LOGGER.info("Julia {}", customerRegistry.getCustomer("2"));
+  }
+
+}

registry/src/main/java/com/iluwatar/registry/Customer.java

@@ -0,0 +1,28 @@
+package com.iluwatar.registry;
+
+public class Customer {
+
+  private final String id;
+  private final String name;
+
+  public Customer(String id, String name) {
+    this.id = id;
+    this.name = name;
+  }
+
+  public String getId() {
+    return id;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  @Override
+  public String toString() {
+    return "Customer{"
+        + "id='" + id + '\''
+        + ", name='" + name + '\''
+        + '}';
+  }
+}

registry/src/main/java/com/iluwatar/registry/CustomerRegistry.java

@@ -0,0 +1,28 @@
+package com.iluwatar.registry;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public final class CustomerRegistry {
+
+  private static final CustomerRegistry instance = new CustomerRegistry();
+
+  public static CustomerRegistry getInstance() {
+    return instance;
+  }
+
+  private final Map<String, Customer> customerMap;
+
+  private CustomerRegistry() {
+    customerMap = new ConcurrentHashMap<>();
+  }
+
+  public Customer addCustomer(Customer customer) {
+    return customerMap.put(customer.getId(), customer);
+  }
+
+  public Customer getCustomer(String id) {
+    return customerMap.get(id);
+  }
+
+}

registry/src/test/java/com/iluwatar/registry/CustomerRegistryTest.java

@@ -0,0 +1,44 @@
+package com.iluwatar.registry;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+public class CustomerRegistryTest {
+
+  private static CustomerRegistry customerRegistry;
+
+  @BeforeAll
+  public static void setUp() {
+    customerRegistry = CustomerRegistry.getInstance();
+  }
+
+  @Test
+  public void shouldBeAbleToAddAndQueryCustomerObjectFromRegistry() {
+    Customer john = new Customer("1", "john");
+    Customer julia = new Customer("2", "julia");
+
+    customerRegistry.addCustomer(john);
+    customerRegistry.addCustomer(julia);
+
+    Customer customerWithId1 = customerRegistry.getCustomer("1");
+    assertNotNull(customerWithId1);
+    assertEquals("1", customerWithId1.getId());
+    assertEquals("john", customerWithId1.getName());
+
+    Customer customerWithId2 = customerRegistry.getCustomer("2");
+    assertNotNull(customerWithId2);
+    assertEquals("2", customerWithId2.getId());
+    assertEquals("julia", customerWithId2.getName());
+  }
+
+  @Test
+  public void shouldReturnNullWhenQueriedCustomerIsNotInRegistry() {
+    Customer customerWithId5 = customerRegistry.getCustomer("5");
+    assertNull(customerWithId5);
+  }
+
+}

special-case/README.md

@@ -0,0 +1,368 @@
+---
+layout: pattern
+title: Special Case
+folder: special-case
+permalink: /patterns/special-case/
+categories: Behavioral
+tags:
+ - Extensibility
+---
+
+## Intent
+
+Define some special cases, and encapsulates them into subclasses that provide different special behaviors.
+
+## Explanation
+
+Real world example
+
+> In an e-commerce system, presentation layer expects application layer to produce certain view model.
+> We have a successful scenario, in which receipt view model contains actual data from the purchase,
+> and a couple of failure scenarios.
+
+In plain words
+
+> Special Case pattern allows returning non-null real objects that perform special behaviors.
+
+In [Patterns of Enterprise Application Architecture](https://martinfowler.com/books/eaa.html) says 
+the difference from Null Object Pattern
+
+> If you’ll pardon the unresistable pun, I see Null Object as special case of Special Case.
+
+**Programmatic Example**
+
+To focus on the pattern itself, we implement DB and maintenance lock of the e-commerce system by the singleton instance.
+
+```java
+public class Db {
+  private static Db instance;
+  private Map<String, User> userName2User;
+  private Map<User, Account> user2Account;
+  private Map<String, Product> itemName2Product;
+
+  public static Db getInstance() {
+    if (instance == null) {
+      synchronized (Db.class) {
+        if (instance == null) {
+          instance = new Db();
+          instance.userName2User = new HashMap<>();
+          instance.user2Account = new HashMap<>();
+          instance.itemName2Product = new HashMap<>();
+        }
+      }
+    }
+    return instance;
+  }
+
+  public void seedUser(String userName, Double amount) {
+    User user = new User(userName);
+    instance.userName2User.put(userName, user);
+    Account account = new Account(amount);
+    instance.user2Account.put(user, account);
+  }
+
+  public void seedItem(String itemName, Double price) {
+    Product item = new Product(price);
+    itemName2Product.put(itemName, item);
+  }
+
+  public User findUserByUserName(String userName) {
+    if (!userName2User.containsKey(userName)) {
+      return null;
+    }
+    return userName2User.get(userName);
+  }
+
+  public Account findAccountByUser(User user) {
+    if (!user2Account.containsKey(user)) {
+      return null;
+    }
+    return user2Account.get(user);
+  }
+
+  public Product findProductByItemName(String itemName) {
+    if (!itemName2Product.containsKey(itemName)) {
+      return null;
+    }
+    return itemName2Product.get(itemName);
+  }
+
+  public class User {
+    private String userName;
+
+    public User(String userName) {
+      this.userName = userName;
+    }
+
+    public String getUserName() {
+      return userName;
+    }
+
+    public ReceiptDto purchase(Product item) {
+      return new ReceiptDto(item.getPrice());
+    }
+  }
+
+  public class Account {
+    private Double amount;
+
+    public Account(Double amount) {
+      this.amount = amount;
+    }
+
+    public MoneyTransaction withdraw(Double price) {
+      if (price > amount) {
+        return null;
+      }
+      return new MoneyTransaction(amount, price);
+    }
+
+    public Double getAmount() {
+      return amount;
+    }
+  }
+
+  public class Product {
+    private Double price;
+
+    public Product(Double price) {
+      this.price = price;
+    }
+
+    public Double getPrice() {
+      return price;
+    }
+  }
+}
+
+public class MaintenanceLock {
+  private static final Logger LOGGER = LoggerFactory.getLogger(MaintenanceLock.class);
+
+  private static MaintenanceLock instance;
+  private boolean lock = true;
+
+  public static MaintenanceLock getInstance() {
+    if (instance == null) {
+      synchronized (MaintenanceLock.class) {
+        if (instance == null) {
+          instance = new MaintenanceLock();
+        }
+      }
+    }
+    return instance;
+  }
+
+  public boolean isLock() {
+    return lock;
+  }
+
+  public void setLock(boolean lock) {
+    this.lock = lock;
+    LOGGER.info("Maintenance lock is set to: " + lock);
+  }
+}
+```
+
+Let's first introduce presentation layer, the receipt view model interface and its implementation of successful scenario.
+
+```java
+public interface ReceiptViewModel {
+  void show();
+}
+
+public class ReceiptDto implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(ReceiptDto.class);
+
+  private Double price;
+
+  public ReceiptDto(Double price) {
+    this.price = price;
+  }
+
+  public Double getPrice() {
+    return price;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Receipt: " + price + " paid");
+  }
+}
+```
+
+And here are the implementations of failure scenarios, which are the special cases.
+
+```java
+public class DownForMaintenance implements ReceiptViewModel {
+  private static final Logger LOGGER = LoggerFactory.getLogger(DownForMaintenance.class);
+
+  @Override
+  public void show() {
+    LOGGER.info("Down for maintenance");
+  }
+}
+
+public class InvalidUser implements ReceiptViewModel {
+  private static final Logger LOGGER = LoggerFactory.getLogger(InvalidUser.class);
+
+  private final String userName;
+
+  public InvalidUser(String userName) {
+    this.userName = userName;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Invalid user: " + userName);
+  }
+}
+
+public class OutOfStock implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(OutOfStock.class);
+
+  private String userName;
+  private String itemName;
+
+  public OutOfStock(String userName, String itemName) {
+    this.userName = userName;
+    this.itemName = itemName;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Out of stock: " + itemName + " for user = " + userName + " to buy");
+  }
+}
+
+public class InsufficientFunds implements ReceiptViewModel {
+  private static final Logger LOGGER = LoggerFactory.getLogger(InsufficientFunds.class);
+
+  private String userName;
+  private Double amount;
+  private String itemName;
+
+  public InsufficientFunds(String userName, Double amount, String itemName) {
+    this.userName = userName;
+    this.amount = amount;
+    this.itemName = itemName;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Insufficient funds: " + amount + " of user: " + userName
+        + " for buying item: " + itemName);
+  }
+}
+```
+
+Second, here's the application layer, the application services implementation and the domain services implementation.
+
+```java
+public class ApplicationServicesImpl implements ApplicationServices {
+  private DomainServicesImpl domain = new DomainServicesImpl();
+
+  @Override
+  public ReceiptViewModel loggedInUserPurchase(String userName, String itemName) {
+    if (isDownForMaintenance()) {
+      return new DownForMaintenance();
+    }
+    return this.domain.purchase(userName, itemName);
+  }
+
+  private boolean isDownForMaintenance() {
+    return MaintenanceLock.getInstance().isLock();
+  }
+}
+
+public class DomainServicesImpl implements DomainServices {
+  public ReceiptViewModel purchase(String userName, String itemName) {
+    Db.User user = Db.getInstance().findUserByUserName(userName);
+    if (user == null) {
+      return new InvalidUser(userName);
+    }
+
+    Db.Account account = Db.getInstance().findAccountByUser(user);
+    return purchase(user, account, itemName);
+  }
+
+  private ReceiptViewModel purchase(Db.User user, Db.Account account, String itemName) {
+    Db.Product item = Db.getInstance().findProductByItemName(itemName);
+    if (item == null) {
+      return new OutOfStock(user.getUserName(), itemName);
+    }
+
+    ReceiptDto receipt = user.purchase(item);
+    MoneyTransaction transaction = account.withdraw(receipt.getPrice());
+    if (transaction == null) {
+      return new InsufficientFunds(user.getUserName(), account.getAmount(), itemName);
+    }
+
+    return receipt;
+  }
+}
+```
+
+Finally, the client send requests the application services to get the presentation view.
+
+```java
+    // DB seeding
+    LOGGER.info("Db seeding: " + "1 user: {\"ignite1771\", amount = 1000.0}, "
+        + "2 products: {\"computer\": price = 800.0, \"car\": price = 20000.0}");
+    Db.getInstance().seedUser("ignite1771", 1000.0);
+    Db.getInstance().seedItem("computer", 800.0);
+    Db.getInstance().seedItem("car", 20000.0);
+
+    var applicationServices = new ApplicationServicesImpl();
+    ReceiptViewModel receipt;
+
+    LOGGER.info("[REQUEST] User: " + "abc123" + " buy product: " + "tv");
+    receipt = applicationServices.loggedInUserPurchase("abc123", "tv");
+    receipt.show();
+    MaintenanceLock.getInstance().setLock(false);
+    LOGGER.info("[REQUEST] User: " + "abc123" + " buy product: " + "tv");
+    receipt = applicationServices.loggedInUserPurchase("abc123", "tv");
+    receipt.show();
+    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "tv");
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "tv");
+    receipt.show();
+    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "car");
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "car");
+    receipt.show();
+    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "computer");
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "computer");
+    receipt.show();
+```
+
+Program output of every request:
+
+```
+    Down for maintenance
+    Invalid user: abc123
+    Out of stock: tv for user = ignite1771 to buy
+    Insufficient funds: 1000.0 of user: ignite1771 for buying item: car
+    Receipt: 800.0 paid    
+```
+
+## Class diagram
+
+![alt text](./etc/special_case_urm.png "Special Case")
+
+## Applicability
+
+Use the Special Case pattern when
+
+* You have multiple places in the system that have the same behavior after a conditional check 
+for a particular class instance, or the same behavior after a null check.
+* Return a real object that performs the real behavior, instead of a null object that performs nothing.
+
+## Tutorial 
+
+* [Special Case Tutorial](https://www.codinghelmet.com/articles/reduce-cyclomatic-complexity-special-case)
+
+## Credits
+
+* [How to Reduce Cyclomatic Complexity Part 2: Special Case Pattern](https://www.codinghelmet.com/articles/reduce-cyclomatic-complexity-special-case)
+* [Patterns of Enterprise Application Architecture](https://martinfowler.com/books/eaa.html)
+* [Special Case](https://www.martinfowler.com/eaaCatalog/specialCase.html)

special-case/etc/special-case.urm.puml

@@ -0,0 +1,119 @@
+@startuml
+left to right direction
+package com.iluwatar.specialcase {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+  interface ApplicationServices {
+    + loggedInUserPurchase(String, String) : ReceiptViewModel {abstract}
+  }
+  class ApplicationServicesImpl {
+    - domain : DomainServicesImpl
+    + ApplicationServicesImpl()
+    - isDownForMaintenance() : boolean
+    + loggedInUserPurchase(userName : String, itemName : String) : ReceiptViewModel
+  }
+  class Db {
+    - instance : Db {static}
+    - itemName2Product : Map<String, Product>
+    - user2Account : Map<User, Account>
+    - userName2User : Map<String, User>
+    + Db()
+    + findAccountByUser(user : User) : Account
+    + findProductByItemName(itemName : String) : Product
+    + findUserByUserName(userName : String) : User
+    + getInstance() : Db {static}
+    + seedItem(itemName : String, price : Double)
+    + seedUser(userName : String, amount : Double)
+  }
+  class Account {
+    - amount : Double
+    + Account(this$0 : Double)
+    + getAmount() : Double
+    + withdraw(price : Double) : MoneyTransaction
+  }
+  class Product {
+    - price : Double
+    + Product(this$0 : Double)
+    + getPrice() : Double
+  }
+  class User {
+    - userName : String
+    + User(this$0 : String)
+    + getUserName() : String
+    + purchase(item : Product) : ReceiptDto
+  }
+  interface DomainServices {
+  }
+  class DomainServicesImpl {
+    + DomainServicesImpl()
+    - purchase(user : User, account : Account, itemName : String) : ReceiptViewModel
+    + purchase(userName : String, itemName : String) : ReceiptViewModel
+  }
+  class DownForMaintenance {
+    - LOGGER : Logger {static}
+    + DownForMaintenance()
+    + show()
+  }
+  class InsufficientFunds {
+    - LOGGER : Logger {static}
+    - amount : Double
+    - itemName : String
+    - userName : String
+    + InsufficientFunds(userName : String, amount : Double, itemName : String)
+    + show()
+  }
+  class InvalidUser {
+    - LOGGER : Logger {static}
+    - userName : String
+    + InvalidUser(userName : String)
+    + show()
+  }
+  class MaintenanceLock {
+    - LOGGER : Logger {static}
+    - instance : MaintenanceLock {static}
+    - lock : boolean
+    + MaintenanceLock()
+    + getInstance() : MaintenanceLock {static}
+    + isLock() : boolean
+    + setLock(lock : boolean)
+  }
+  class MoneyTransaction {
+    - amount : Double
+    - price : Double
+    + MoneyTransaction(amount : Double, price : Double)
+  }
+  class OutOfStock {
+    - LOGGER : Logger {static}
+    - itemName : String
+    - userName : String
+    + OutOfStock(userName : String, itemName : String)
+    + show()
+  }
+  class ReceiptDto {
+    - LOGGER : Logger {static}
+    - price : Double
+    + ReceiptDto(price : Double)
+    + getPrice() : Double
+    + show()
+  }
+  interface ReceiptViewModel {
+    + show() {abstract}
+  }
+}
+User --+ Db
+Product --+ Db
+MaintenanceLock -->  "-instance" MaintenanceLock
+Db -->  "-instance" Db
+ApplicationServicesImpl -->  "-domain" DomainServicesImpl
+Account --+ Db
+ApplicationServicesImpl ..|> ApplicationServices 
+DomainServicesImpl ..|> DomainServices 
+DownForMaintenance ..|> ReceiptViewModel 
+InsufficientFunds ..|> ReceiptViewModel 
+InvalidUser ..|> ReceiptViewModel 
+OutOfStock ..|> ReceiptViewModel 
+ReceiptDto ..|> ReceiptViewModel 
+@enduml

special-case/pom.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>java-design-patterns</artifactId>
+        <groupId>com.iluwatar</groupId>
+        <version>1.24.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>special-case</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+
+</project>

special-case/src/main/java/com/iluwatar/specialcase/App.java

@@ -0,0 +1,47 @@
+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * <p>The Special Case Pattern is a software design pattern that encapsulates particular cases
+ * into subclasses that provide special behaviors.</p>
+ *
+ * <p>In this example ({@link ReceiptViewModel}) encapsulates all particular cases.</p>
+ */
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Program entry point.
+   */
+  public static void main(String[] args) {
+    // DB seeding
+    LOGGER.info("Db seeding: " + "1 user: {\"ignite1771\", amount = 1000.0}, "
+        + "2 products: {\"computer\": price = 800.0, \"car\": price = 20000.0}");
+    Db.getInstance().seedUser("ignite1771", 1000.0);
+    Db.getInstance().seedItem("computer", 800.0);
+    Db.getInstance().seedItem("car", 20000.0);
+
+    final var applicationServices = new ApplicationServicesImpl();
+    ReceiptViewModel receipt;
+
+    LOGGER.info("[REQUEST] User: " + "abc123" + " buy product: " + "tv");
+    receipt = applicationServices.loggedInUserPurchase("abc123", "tv");
+    receipt.show();
+    MaintenanceLock.getInstance().setLock(false);
+    LOGGER.info("[REQUEST] User: " + "abc123" + " buy product: " + "tv");
+    receipt = applicationServices.loggedInUserPurchase("abc123", "tv");
+    receipt.show();
+    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "tv");
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "tv");
+    receipt.show();
+    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "car");
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "car");
+    receipt.show();
+    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "computer");
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "computer");
+    receipt.show();
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/ApplicationServices.java

@@ -0,0 +1,6 @@
+package com.iluwatar.specialcase;
+
+public interface ApplicationServices {
+
+  ReceiptViewModel loggedInUserPurchase(String userName, String itemName);
+}

special-case/src/main/java/com/iluwatar/specialcase/ApplicationServicesImpl.java

@@ -0,0 +1,18 @@
+package com.iluwatar.specialcase;
+
+public class ApplicationServicesImpl implements ApplicationServices {
+
+  private DomainServicesImpl domain = new DomainServicesImpl();
+
+  @Override
+  public ReceiptViewModel loggedInUserPurchase(String userName, String itemName) {
+    if (isDownForMaintenance()) {
+      return new DownForMaintenance();
+    }
+    return this.domain.purchase(userName, itemName);
+  }
+
+  private boolean isDownForMaintenance() {
+    return MaintenanceLock.getInstance().isLock();
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/Db.java

@@ -0,0 +1,150 @@
+package com.iluwatar.specialcase;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class Db {
+
+  private static Db instance;
+  private Map<String, User> userName2User;
+  private Map<User, Account> user2Account;
+  private Map<String, Product> itemName2Product;
+
+  /**
+   * Get the instance of Db.
+   *
+   * @return singleton instance of Db class
+   */
+  public static Db getInstance() {
+    if (instance == null) {
+      synchronized (Db.class) {
+        if (instance == null) {
+          instance = new Db();
+          instance.userName2User = new HashMap<>();
+          instance.user2Account = new HashMap<>();
+          instance.itemName2Product = new HashMap<>();
+        }
+      }
+    }
+    return instance;
+  }
+
+  /**
+   * Seed a user into Db.
+   *
+   * @param userName of the user
+   * @param amount of the user's account
+   */
+  public void seedUser(String userName, Double amount) {
+    User user = new User(userName);
+    instance.userName2User.put(userName, user);
+    Account account = new Account(amount);
+    instance.user2Account.put(user, account);
+  }
+
+  /**
+   * Seed an item into Db.
+   *
+   * @param itemName of the item
+   * @param price of the item
+   */
+  public void seedItem(String itemName, Double price) {
+    Product item = new Product(price);
+    itemName2Product.put(itemName, item);
+  }
+
+  /**
+   * Find a user with the userName.
+   *
+   * @param userName of the user
+   * @return instance of User
+   */
+  public User findUserByUserName(String userName) {
+    if (!userName2User.containsKey(userName)) {
+      return null;
+    }
+    return userName2User.get(userName);
+  }
+
+  /**
+   * Find an account of the user.
+   *
+   * @param user in Db
+   * @return instance of Account of the user
+   */
+  public Account findAccountByUser(User user) {
+    if (!user2Account.containsKey(user)) {
+      return null;
+    }
+    return user2Account.get(user);
+  }
+
+  /**
+   * Find a product with the itemName.
+   *
+   * @param itemName of the item
+   * @return instance of Product
+   */
+  public Product findProductByItemName(String itemName) {
+    if (!itemName2Product.containsKey(itemName)) {
+      return null;
+    }
+    return itemName2Product.get(itemName);
+  }
+
+  public class User {
+
+    private String userName;
+
+    public User(String userName) {
+      this.userName = userName;
+    }
+
+    public String getUserName() {
+      return userName;
+    }
+
+    public ReceiptDto purchase(Product item) {
+      return new ReceiptDto(item.getPrice());
+    }
+  }
+
+  public class Account {
+
+    private Double amount;
+
+    public Account(Double amount) {
+      this.amount = amount;
+    }
+
+    /**
+     * Withdraw the price of the item from the account.
+     *
+     * @param price of the item
+     * @return instance of MoneyTransaction
+     */
+    public MoneyTransaction withdraw(Double price) {
+      if (price > amount) {
+        return null;
+      }
+      return new MoneyTransaction(amount, price);
+    }
+
+    public Double getAmount() {
+      return amount;
+    }
+  }
+
+  public class Product {
+
+    private Double price;
+
+    public Product(Double price) {
+      this.price = price;
+    }
+
+    public Double getPrice() {
+      return price;
+    }
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/DomainServices.java

@@ -0,0 +1,4 @@
+package com.iluwatar.specialcase;
+
+public interface DomainServices {
+}

special-case/src/main/java/com/iluwatar/specialcase/DomainServicesImpl.java

@@ -0,0 +1,46 @@
+package com.iluwatar.specialcase;
+
+public class DomainServicesImpl implements DomainServices {
+
+  /**
+   * Domain purchase with userName and itemName, with validation for userName.
+   *
+   * @param userName of the user
+   * @param itemName of the item
+   * @return instance of ReceiptViewModel
+   */
+  public ReceiptViewModel purchase(String userName, String itemName) {
+    Db.User user = Db.getInstance().findUserByUserName(userName);
+    if (user == null) {
+      return new InvalidUser(userName);
+    }
+
+    Db.Account account = Db.getInstance().findAccountByUser(user);
+    return purchase(user, account, itemName);
+  }
+
+  /**
+   * Domain purchase with user, account and itemName,
+   * with validation for whether product is out of stock
+   * and whether user has insufficient funds in the account.
+   *
+   * @param user in Db
+   * @param account in Db
+   * @param itemName of the item
+   * @return instance of ReceiptViewModel
+   */
+  private ReceiptViewModel purchase(Db.User user, Db.Account account, String itemName) {
+    Db.Product item = Db.getInstance().findProductByItemName(itemName);
+    if (item == null) {
+      return new OutOfStock(user.getUserName(), itemName);
+    }
+
+    ReceiptDto receipt = user.purchase(item);
+    MoneyTransaction transaction = account.withdraw(receipt.getPrice());
+    if (transaction == null) {
+      return new InsufficientFunds(user.getUserName(), account.getAmount(), itemName);
+    }
+
+    return receipt;
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/DownForMaintenance.java

@@ -0,0 +1,14 @@
+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class DownForMaintenance implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(DownForMaintenance.class);
+
+  @Override
+  public void show() {
+    LOGGER.info("Down for maintenance");
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/InsufficientFunds.java

@@ -0,0 +1,32 @@
+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class InsufficientFunds implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(InsufficientFunds.class);
+
+  private String userName;
+  private Double amount;
+  private String itemName;
+
+  /**
+   * Constructor of InsufficientFunds.
+   *
+   * @param userName of the user
+   * @param amount of the user's account
+   * @param itemName of the item
+   */
+  public InsufficientFunds(String userName, Double amount, String itemName) {
+    this.userName = userName;
+    this.amount = amount;
+    this.itemName = itemName;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Insufficient funds: " + amount + " of user: " + userName
+        + " for buying item: " + itemName);
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/InvalidUser.java

@@ -0,0 +1,20 @@
+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class InvalidUser implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(InvalidUser.class);
+
+  private final String userName;
+
+  public InvalidUser(String userName) {
+    this.userName = userName;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Invalid user: " + userName);
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/MaintenanceLock.java

@@ -0,0 +1,37 @@
+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class MaintenanceLock {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(MaintenanceLock.class);
+
+  private static MaintenanceLock instance;
+  private boolean lock = true;
+
+  /**
+   * Get the instance of MaintenanceLock.
+   *
+   * @return singleton instance of MaintenanceLock
+   */
+  public static MaintenanceLock getInstance() {
+    if (instance == null) {
+      synchronized (MaintenanceLock.class) {
+        if (instance == null) {
+          instance = new MaintenanceLock();
+        }
+      }
+    }
+    return instance;
+  }
+
+  public boolean isLock() {
+    return lock;
+  }
+
+  public void setLock(boolean lock) {
+    this.lock = lock;
+    LOGGER.info("Maintenance lock is set to: " + lock);
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/MoneyTransaction.java

@@ -0,0 +1,12 @@
+package com.iluwatar.specialcase;
+
+public class MoneyTransaction {
+
+  private Double amount;
+  private Double price;
+
+  public MoneyTransaction(Double amount, Double price) {
+    this.amount = amount;
+    this.price = price;
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/OutOfStock.java

@@ -0,0 +1,22 @@
+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class OutOfStock implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(OutOfStock.class);
+
+  private String userName;
+  private String itemName;
+
+  public OutOfStock(String userName, String itemName) {
+    this.userName = userName;
+    this.itemName = itemName;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Out of stock: " + itemName + " for user = " + userName + " to buy");
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/ReceiptDto.java

@@ -0,0 +1,24 @@
+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class ReceiptDto implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(ReceiptDto.class);
+
+  private Double price;
+
+  public ReceiptDto(Double price) {
+    this.price = price;
+  }
+
+  public Double getPrice() {
+    return price;
+  }
+
+  @Override
+  public void show() {
+    LOGGER.info("Receipt: " + price + " paid");
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/ReceiptViewModel.java

@@ -0,0 +1,6 @@
+package com.iluwatar.specialcase;
+
+public interface ReceiptViewModel {
+
+  void show();
+}

special-case/src/test/java/com/iluwatar/specialcase/AppTest.java

@@ -0,0 +1,16 @@
+package com.iluwatar.specialcase;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Application test.
+ */
+public class AppTest {
+
+  @Test
+  void shouldExecuteWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+  }
+}

special-case/src/test/java/com/iluwatar/specialcase/SpecialCasesTest.java

@@ -0,0 +1,120 @@
+package com.iluwatar.specialcase;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.slf4j.LoggerFactory;
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.BeforeEach;
+
+/**
+ * Special cases unit tests. (including the successful scenario {@link ReceiptDto})
+ */
+public class SpecialCasesTest {
+  private static ApplicationServices applicationServices;
+  private static ReceiptViewModel receipt;
+
+  @BeforeAll
+  static void beforeAll() {
+    Db.getInstance().seedUser("ignite1771", 1000.0);
+    Db.getInstance().seedItem("computer", 800.0);
+    Db.getInstance().seedItem("car", 20000.0);
+
+    applicationServices = new ApplicationServicesImpl();
+  }
+
+  @BeforeEach
+  public void beforeEach() {
+    MaintenanceLock.getInstance().setLock(false);
+  }
+
+  @Test
+  public void testDownForMaintenance() {
+    final Logger LOGGER = (Logger) LoggerFactory.getLogger(DownForMaintenance.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+    LOGGER.addAppender(listAppender);
+
+    MaintenanceLock.getInstance().setLock(true);
+    receipt = applicationServices.loggedInUserPurchase(null, null);
+    receipt.show();
+
+    List<ILoggingEvent> loggingEventList = listAppender.list;
+    assertEquals("Down for maintenance", loggingEventList.get(0).getMessage());
+    assertEquals(Level.INFO, loggingEventList.get(0).getLevel());
+  }
+
+  @Test
+  public void testInvalidUser() {
+    final Logger LOGGER = (Logger) LoggerFactory.getLogger(InvalidUser.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+    LOGGER.addAppender(listAppender);
+
+    receipt = applicationServices.loggedInUserPurchase("a", null);
+    receipt.show();
+
+    List<ILoggingEvent> loggingEventList = listAppender.list;
+    assertEquals("Invalid user: a", loggingEventList.get(0).getMessage());
+    assertEquals(Level.INFO, loggingEventList.get(0).getLevel());
+  }
+
+  @Test
+  public void testOutOfStock() {
+    final Logger LOGGER = (Logger) LoggerFactory.getLogger(OutOfStock.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+    LOGGER.addAppender(listAppender);
+
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "tv");
+    receipt.show();
+
+    List<ILoggingEvent> loggingEventList = listAppender.list;
+    assertEquals("Out of stock: tv for user = ignite1771 to buy"
+        , loggingEventList.get(0).getMessage());
+    assertEquals(Level.INFO, loggingEventList.get(0).getLevel());
+  }
+
+  @Test
+  public void testInsufficientFunds() {
+    final Logger LOGGER = (Logger) LoggerFactory.getLogger(InsufficientFunds.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+    LOGGER.addAppender(listAppender);
+
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "car");
+    receipt.show();
+
+    List<ILoggingEvent> loggingEventList = listAppender.list;
+    assertEquals("Insufficient funds: 1000.0 of user: ignite1771 for buying item: car"
+        , loggingEventList.get(0).getMessage());
+    assertEquals(Level.INFO, loggingEventList.get(0).getLevel());
+  }
+
+  @Test
+  public void testReceiptDto() {
+    final Logger LOGGER = (Logger) LoggerFactory.getLogger(ReceiptDto.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+    LOGGER.addAppender(listAppender);
+
+    receipt = applicationServices.loggedInUserPurchase("ignite1771", "computer");
+    receipt.show();
+
+    List<ILoggingEvent> loggingEventList = listAppender.list;
+    assertEquals("Receipt: 800.0 paid"
+        , loggingEventList.get(0).getMessage());
+    assertEquals(Level.INFO, loggingEventList.get(0).getLevel());
+  }
+}

version-number/README.md

@@ -0,0 +1,169 @@
+---
+layout: pattern
+title: Version Number
+folder: versionnumber
+permalink: /patterns/versionnumber/
+description: Entity versioning with version number
+
+categories:
+ - Concurrency
+
+tags:
+ - Data access
+ - Microservices
+---
+
+## Name / classification
+
+Version Number.
+
+## Also known as
+
+Entity Versioning, Optimistic Locking.
+
+## Intent
+
+Resolve concurrency conflicts when multiple clients are trying to update same entity simultaneously.
+
+## Explanation
+
+Real world example
+
+> Alice and Bob are working on the book, which stored in the database. Our heroes are making
+> changes simultaneously, and we need some mechanism to prevent them from overwriting each other.
+
+In plain words
+
+> Version Number pattern grants protection against concurrent updates to same entity.
+
+Wikipedia says
+
+> Optimistic concurrency control assumes that multiple transactions can frequently complete
+> without interfering with each other. While running, transactions use data resources without
+> acquiring locks on those resources. Before committing, each transaction verifies that no other
+> transaction has modified the data it has read. If the check reveals conflicting modifications,
+> the committing transaction rolls back and can be restarted.
+
+**Programmatic Example**
+
+We have a `Book` entity, which is versioned, and has a copy-constructor:
+
+```java
+public class Book {
+  private long id;
+  private String title = "";
+  private String author = "";
+
+  private long version = 0; // version number
+
+  public Book(Book book) {
+    this.id = book.id;
+    this.title = book.title;
+    this.author = book.author;
+    this.version = book.version;
+  }
+
+  // getters and setters are omitted here
+}
+```
+
+We also have `BookRepository`, which implements concurrency control:
+
+```java
+public class BookRepository {
+  private final Map<Long, Book> collection = new HashMap<>();
+
+  public void update(Book book) throws BookNotFoundException, VersionMismatchException {
+    if (!collection.containsKey(book.getId())) {
+      throw new BookNotFoundException("Not found book with id: " + book.getId());
+    }
+
+    var latestBook = collection.get(book.getId());
+    if (book.getVersion() != latestBook.getVersion()) {
+      throw new VersionMismatchException(
+        "Tried to update stale version " + book.getVersion()
+          + " while actual version is " + latestBook.getVersion()
+      );
+    }
+
+    // update version, including client representation - modify by reference here
+    book.setVersion(book.getVersion() + 1);
+
+    // save book copy to repository
+    collection.put(book.getId(), new Book(book));
+  }
+
+  public Book get(long bookId) throws BookNotFoundException {
+    if (!collection.containsKey(bookId)) {
+      throw new BookNotFoundException("Not found book with id: " + bookId);
+    }
+
+    // return copy of the book
+    return new Book(collection.get(bookId));
+  }
+}
+```
+
+Here's the concurrency control in action:
+
+```java
+var bookId = 1;
+// Alice and Bob took the book concurrently
+final var aliceBook = bookRepository.get(bookId);
+final var bobBook = bookRepository.get(bookId);
+
+aliceBook.setTitle("Kama Sutra"); // Alice has updated book title
+bookRepository.update(aliceBook); // and successfully saved book in database
+LOGGER.info("Alice updates the book with new version {}", aliceBook.getVersion());
+
+// now Bob has the stale version of the book with empty title and version = 0
+// while actual book in database has filled title and version = 1
+bobBook.setAuthor("Vatsyayana Mallanaga"); // Bob updates the author
+try {
+  LOGGER.info("Bob tries to update the book with his version {}", bobBook.getVersion());
+  bookRepository.update(bobBook); // Bob tries to save his book to database
+} catch (VersionMismatchException e) {
+  // Bob update fails, and book in repository remained untouchable
+  LOGGER.info("Exception: {}", e.getMessage());
+  // Now Bob should reread actual book from repository, do his changes again and save again
+}
+```
+
+Program output:
+
+```java
+Alice updates the book with new version 1
+Bob tries to update the book with his version 0
+Exception: Tried to update stale version 0 while actual version is 1
+```
+
+## Class diagram
+
+![alt text](./etc/version-number.urm.png "Version Number pattern class diagram")
+
+## Applicability
+
+Use Version Number for:
+
+* resolving concurrent write-access to the data
+* strong data consistency
+
+## Tutorials
+* [Version Number Pattern Tutorial](http://www.java2s.com/Tutorial/Java/0355__JPA/VersioningEntity.htm)
+
+## Known uses
+ * [Hibernate](https://vladmihalcea.com/jpa-entity-version-property-hibernate/)
+ * [Elasticsearch](https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-index_.html#index-versioning)
+ * [Apache Solr](https://lucene.apache.org/solr/guide/6_6/updating-parts-of-documents.html)
+
+## Consequences
+Version Number pattern allows to implement a concurrency control, which is usually done
+via Optimistic Offline Lock pattern.
+
+## Related patterns
+* [Optimistic Offline Lock](https://martinfowler.com/eaaCatalog/optimisticOfflineLock.html)
+
+## Credits
+* [Optimistic Locking in JPA](https://www.baeldung.com/jpa-optimistic-locking)
+* [JPA entity versioning](https://www.byteslounge.com/tutorials/jpa-entity-versioning-version-and-optimistic-locking)
+* [J2EE Design Patterns](http://ommolketab.ir/aaf-lib/axkwht7wxrhvgs2aqkxse8hihyu9zv.pdf)

version-number/etc/version-number.urm.puml

@@ -0,0 +1,32 @@
+@startuml
+package com.iluwatar.versionnumber {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+  class Book {
+    - author : String
+    - id : long
+    - title : String
+    - version : long
+    + Book()
+    + Book(book : Book)
+    + getAuthor() : String
+    + getId() : long
+    + getTitle() : String
+    + getVersion() : long
+    + setAuthor(author : String)
+    + setId(id : long)
+    + setTitle(title : String)
+    + setVersion(version : long)
+  }
+  class BookRepository {
+    - collection : Map<Long, Book>
+    + BookRepository()
+    + add(book : Book)
+    + get(bookId : long) : Book
+    + update(book : Book)
+  }
+}
+@enduml

version-number/pom.xml

@@ -0,0 +1,66 @@
+<?xml version="1.0"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2019 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.24.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>version-number</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-assembly-plugin</artifactId>
+        <executions>
+          <execution>
+            <configuration>
+              <archive>
+                <manifest>
+                  <mainClass>com.iluwatar.versionnumber.App</mainClass>
+                </manifest>
+              </archive>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>

version-number/src/main/java/com/iluwatar/versionnumber/App.java

@@ -0,0 +1,84 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.versionnumber;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The Version Number pattern helps to resolve concurrency conflicts in applications.
+ * Usually these conflicts arise in database operations, when multiple clients are trying
+ * to update the same record simultaneously.
+ * Resolving such conflicts requires determining whether an object has changed.
+ * For this reason we need a version number that is incremented with each change
+ * to the underlying data, e.g. database. The version number can be used by repositories
+ * to check for external changes and to report concurrency issues to the users.
+ *
+ * <p>In this example we show how Alice and Bob will try to update the {@link Book}
+ * and save it simultaneously to {@link BookRepository}, which represents a typical database.
+ *
+ * <p>As in real databases, each client operates with copy of the data instead of original data
+ * passed by reference, that's why we are using {@link Book} copy-constructor here.
+ */
+public class App {
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args
+   */
+  public static void main(String[] args) throws
+      BookDuplicateException,
+      BookNotFoundException,
+      VersionMismatchException {
+    var bookId = 1;
+
+    var bookRepository = new BookRepository();
+    var book = new Book();
+    book.setId(bookId);
+    bookRepository.add(book); // adding a book with empty title and author
+    LOGGER.info("An empty book with version {} was added to repository", book.getVersion());
+
+    // Alice and Bob took the book concurrently
+    final var aliceBook = bookRepository.get(bookId);
+    final var bobBook = bookRepository.get(bookId);
+
+    aliceBook.setTitle("Kama Sutra"); // Alice has updated book title
+    bookRepository.update(aliceBook); // and successfully saved book in database
+    LOGGER.info("Alice updates the book with new version {}", aliceBook.getVersion());
+
+    // now Bob has the stale version of the book with empty title and version = 0
+    // while actual book in database has filled title and version = 1
+    bobBook.setAuthor("Vatsyayana Mallanaga"); // Bob updates the author
+    try {
+      LOGGER.info("Bob tries to update the book with his version {}", bobBook.getVersion());
+      bookRepository.update(bobBook); // Bob tries to save his book to database
+    } catch (VersionMismatchException e) {
+      // Bob update fails, and book in repository remained untouchable
+      LOGGER.info("Exception: {}", e.getMessage());
+      // Now Bob should reread actual book from repository, do his changes again and save again
+    }
+  }
+}

version-number/src/main/java/com/iluwatar/versionnumber/Book.java

@@ -0,0 +1,78 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.versionnumber;
+
+public class Book {
+  private long id;
+  private String title = "";
+  private String author = "";
+
+  private long version = 0; // version number
+
+  public Book() {
+
+  }
+
+  /**
+   * We need this copy constructor to copy book representation in {@link BookRepository}.
+   */
+  public Book(Book book) {
+    this.id = book.id;
+    this.title = book.title;
+    this.author = book.author;
+    this.version = book.version;
+  }
+
+  public long getId() {
+    return id;
+  }
+
+  public void setId(long id) {
+    this.id = id;
+  }
+
+  public String getTitle() {
+    return title;
+  }
+
+  public void setTitle(String title) {
+    this.title = title;
+  }
+
+  public String getAuthor() {
+    return author;
+  }
+
+  public void setAuthor(String author) {
+    this.author = author;
+  }
+
+  public long getVersion() {
+    return version;
+  }
+
+  public void setVersion(long version) {
+    this.version = version;
+  }
+}

version-number/src/main/java/com/iluwatar/versionnumber/BookDuplicateException.java

@@ -0,0 +1,33 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.versionnumber;
+
+/**
+ * When someone has tried to add a book which repository already have.
+ */
+public class BookDuplicateException extends Exception {
+  public BookDuplicateException(String message) {
+    super(message);
+  }
+}

version-number/src/main/java/com/iluwatar/versionnumber/BookNotFoundException.java

@@ -0,0 +1,33 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.versionnumber;
+
+/**
+ * Client has tried to make an operation with book which repository does not have.
+ */
+public class BookNotFoundException extends Exception {
+  public BookNotFoundException(String message) {
+    super(message);
+  }
+}

version-number/src/main/java/com/iluwatar/versionnumber/BookRepository.java

@@ -0,0 +1,86 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.versionnumber;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * This repository represents simplified database.
+ * As a typical database do, repository operates with copies of object.
+ * So client and repo has different copies of book, which can lead to concurrency conflicts
+ * as much as in real databases.
+ */
+public class BookRepository {
+  private final Map<Long, Book> collection = new HashMap<>();
+
+  /**
+   * Adds book to collection.
+   * Actually we are putting copy of book (saving a book by value, not by reference);
+   */
+  public void add(Book book) throws BookDuplicateException {
+    if (collection.containsKey(book.getId())) {
+      throw new BookDuplicateException("Duplicated book with id: " + book.getId());
+    }
+
+    // add copy of the book
+    collection.put(book.getId(), new Book(book));
+  }
+
+  /**
+   * Updates book in collection only if client has modified the latest version of the book.
+   */
+  public void update(Book book) throws BookNotFoundException, VersionMismatchException {
+    if (!collection.containsKey(book.getId())) {
+      throw new BookNotFoundException("Not found book with id: " + book.getId());
+    }
+
+    var latestBook = collection.get(book.getId());
+    if (book.getVersion() != latestBook.getVersion()) {
+      throw new VersionMismatchException(
+        "Tried to update stale version " + book.getVersion()
+          + " while actual version is " + latestBook.getVersion()
+      );
+    }
+
+    // update version, including client representation - modify by reference here
+    book.setVersion(book.getVersion() + 1);
+
+    // save book copy to repository
+    collection.put(book.getId(), new Book(book));
+  }
+
+  /**
+   * Returns book representation to the client.
+   * Representation means we are returning copy of the book.
+   */
+  public Book get(long bookId) throws BookNotFoundException {
+    if (!collection.containsKey(bookId)) {
+      throw new BookNotFoundException("Not found book with id: " + bookId);
+    }
+
+    // return copy of the book
+    return new Book(collection.get(bookId));
+  }
+}

version-number/src/main/java/com/iluwatar/versionnumber/VersionMismatchException.java

@@ -1,37 +1,33 @@
-/*
- * The MIT License
- * Copyright © 2014-2019 Ilkka Seppälä
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
-
-package com.iluwatar.command;
-
-/**
- * Interface for Commands.
- */
-public interface Command {
-  void execute(Target target);
-
-  void undo();
-
-  void redo();
-
-  String toString();
-}
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.versionnumber;
+
+/**
+ * Client has tried to update a stale version of the book.
+ */
+public class VersionMismatchException extends Exception {
+  public VersionMismatchException(String message) {
+    super(message);
+  }
+}

version-number/src/test/java/com/iluwatar/versionnumber/AppTest.java

@@ -21,37 +21,26 @@
  * THE SOFTWARE.
  */
 
-package com.iluwatar.command;
+package com.iluwatar.versionnumber;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 
 /**
- * InvisibilitySpell is a concrete command.
+ * Application test
  */
-public class InvisibilitySpell implements Command {
+class AppTest {
 
-  private Target target;
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
-  @Override
-  public void execute(Target target) {
-    target.setVisibility(Visibility.INVISIBLE);
-    this.target = target;
-  }
-
-  @Override
-  public void undo() {
-    if (target != null) {
-      target.setVisibility(Visibility.VISIBLE);
-    }
-  }
-
-  @Override
-  public void redo() {
-    if (target != null) {
-      target.setVisibility(Visibility.INVISIBLE);
-    }
-  }
-
-  @Override
-  public String toString() {
-    return "Invisibility spell";
+  @Test
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

version-number/src/test/java/com/iluwatar/versionnumber/BookRepositoryTest.java

@@ -0,0 +1,87 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.versionnumber;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Tests for {@link BookRepository}
+ */
+class BookRepositoryTest {
+  private final long bookId = 1;
+  private final BookRepository bookRepository = new BookRepository();
+
+  @BeforeEach
+  public void setUp() throws BookDuplicateException {
+    var book = new Book();
+    book.setId(bookId);
+    bookRepository.add(book);
+  }
+
+  @Test
+  void testDefaultVersionRemainsZeroAfterAdd() throws BookNotFoundException {
+    var book = bookRepository.get(bookId);
+    assertEquals(0, book.getVersion());
+  }
+
+  @Test
+  void testAliceAndBobHaveDifferentVersionsAfterAliceUpdate() throws BookNotFoundException, VersionMismatchException {
+    final var aliceBook = bookRepository.get(bookId);
+    final var bobBook = bookRepository.get(bookId);
+
+    aliceBook.setTitle("Kama Sutra");
+    bookRepository.update(aliceBook);
+
+    assertEquals(1, aliceBook.getVersion());
+    assertEquals(0, bobBook.getVersion());
+    var actualBook = bookRepository.get(bookId);
+    assertEquals(aliceBook.getVersion(), actualBook.getVersion());
+    assertEquals(aliceBook.getTitle(), actualBook.getTitle());
+    assertNotEquals(aliceBook.getTitle(), bobBook.getTitle());
+  }
+
+  @Test
+  void testShouldThrowVersionMismatchExceptionOnStaleUpdate() throws BookNotFoundException, VersionMismatchException {
+    final var aliceBook = bookRepository.get(bookId);
+    final var bobBook = bookRepository.get(bookId);
+
+    aliceBook.setTitle("Kama Sutra");
+    bookRepository.update(aliceBook);
+
+    bobBook.setAuthor("Vatsyayana Mallanaga");
+    try {
+      bookRepository.update(bobBook);
+    } catch (VersionMismatchException e) {
+      assertEquals(0, bobBook.getVersion());
+      var actualBook = bookRepository.get(bookId);
+      assertEquals(1, actualBook.getVersion());
+      assertEquals(aliceBook.getVersion(), actualBook.getVersion());
+      assertEquals("", bobBook.getTitle());
+      assertNotEquals(aliceBook.getAuthor(), bobBook.getAuthor());
+    }
+  }
+}

zh/bridge/README.md

@@ -0,0 +1,205 @@
+---
+layout: pattern
+title: Bridge
+folder: bridge
+permalink: /patterns/bridge/
+categories: Structural
+tags:
+ - Gang of Four
+---
+
+## 又被称为
+
+手柄/身体模式
+
+## 目的
+
+将抽象与其实现分离，以便二者可以独立变化。
+
+## 解释
+
+真实世界例子
+
+> 考虑一下你拥有一种具有不同附魔的武器，并且应该允许将具有不同附魔的不同武器混合使用。 你会怎么做？ 为每个附魔创建每种武器的多个副本，还是只是创建单独的附魔并根据需要为武器设置它？ 桥接模式使您可以进行第二次操作。
+
+通俗的说
+
+> 桥接模式是一个更推荐组合而不是继承的模式。将实现细节从一个层次结构推送到具有单独层次结构的另一个对象。
+
+维基百科说
+
+> 桥接模式是软件工程中使用的一种设计模式，旨在“将抽象与其实现分离，从而使两者可以独立变化”
+
+**程序示例**
+
+翻译一下上面的武器示例。下面我们有武器的类层级：
+
+```java
+public interface Weapon {
+  void wield();
+  void swing();
+  void unwield();
+  Enchantment getEnchantment();
+}
+
+public class Sword implements Weapon {
+
+  private final Enchantment enchantment;
+
+  public Sword(Enchantment enchantment) {
+    this.enchantment = enchantment;
+  }
+
+  @Override
+  public void wield() {
+    LOGGER.info("The sword is wielded.");
+    enchantment.onActivate();
+  }
+
+  @Override
+  public void swing() {
+    LOGGER.info("The sword is swinged.");
+    enchantment.apply();
+  }
+
+  @Override
+  public void unwield() {
+    LOGGER.info("The sword is unwielded.");
+    enchantment.onDeactivate();
+  }
+
+  @Override
+  public Enchantment getEnchantment() {
+    return enchantment;
+  }
+}
+
+public class Hammer implements Weapon {
+
+  private final Enchantment enchantment;
+
+  public Hammer(Enchantment enchantment) {
+    this.enchantment = enchantment;
+  }
+
+  @Override
+  public void wield() {
+    LOGGER.info("The hammer is wielded.");
+    enchantment.onActivate();
+  }
+
+  @Override
+  public void swing() {
+    LOGGER.info("The hammer is swinged.");
+    enchantment.apply();
+  }
+
+  @Override
+  public void unwield() {
+    LOGGER.info("The hammer is unwielded.");
+    enchantment.onDeactivate();
+  }
+
+  @Override
+  public Enchantment getEnchantment() {
+    return enchantment;
+  }
+}
+```
+
+这里是单独的附魔类结构：
+
+```java
+public interface Enchantment {
+  void onActivate();
+  void apply();
+  void onDeactivate();
+}
+
+public class FlyingEnchantment implements Enchantment {
+
+  @Override
+  public void onActivate() {
+    LOGGER.info("The item begins to glow faintly.");
+  }
+
+  @Override
+  public void apply() {
+    LOGGER.info("The item flies and strikes the enemies finally returning to owner's hand.");
+  }
+
+  @Override
+  public void onDeactivate() {
+    LOGGER.info("The item's glow fades.");
+  }
+}
+
+public class SoulEatingEnchantment implements Enchantment {
+
+  @Override
+  public void onActivate() {
+    LOGGER.info("The item spreads bloodlust.");
+  }
+
+  @Override
+  public void apply() {
+    LOGGER.info("The item eats the soul of enemies.");
+  }
+
+  @Override
+  public void onDeactivate() {
+    LOGGER.info("Bloodlust slowly disappears.");
+  }
+}
+```
+
+这里是两种层次结构的实践：
+
+```java
+var enchantedSword = new Sword(new SoulEatingEnchantment());
+enchantedSword.wield();
+enchantedSword.swing();
+enchantedSword.unwield();
+// The sword is wielded.
+// The item spreads bloodlust.
+// The sword is swinged.
+// The item eats the soul of enemies.
+// The sword is unwielded.
+// Bloodlust slowly disappears.
+
+var hammer = new Hammer(new FlyingEnchantment());
+hammer.wield();
+hammer.swing();
+hammer.unwield();
+// The hammer is wielded.
+// The item begins to glow faintly.
+// The hammer is swinged.
+// The item flies and strikes the enemies finally returning to owner's hand.
+// The hammer is unwielded.
+// The item's glow fades.
+```
+
+
+
+## 类图
+
+![alt text](../../bridge/etc/bridge.urm.png "Bridge class diagram")
+
+## 适用性
+
+使用桥接模式当
+
+* 你想永久性的避免抽象和他的实现之间的绑定。有可能是这种情况，当实现需要被选择或者在运行时切换。
+* 抽象和他们的实现应该能通过写子类来扩展。这种情况下，桥接模式让你可以组合不同的抽象和实现并独立的扩展他们。
+* 对抽象的实现的改动应当不会对客户产生影响；也就是说，他们的代码不必重新编译。
+* 你有种类繁多的类。这样的类层次结构表明需要将一个对象分为两部分。Rumbaugh 使用术语“嵌套归纳”来指代这种类层次结构。
+* 你想在多个对象间分享一种实现（可能使用引用计数），这个事实应该对客户隐藏。一个简单的示例是Coplien的String类，其中多个对象可以共享同一字符串表示形式
+
+## 教程
+
+* [Bridge Pattern Tutorial](https://www.journaldev.com/1491/bridge-design-pattern-java)
+
+## 鸣谢
+
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)

zh/callback/README.md

@@ -0,0 +1,79 @@
+---
+layout: pattern
+title: Callback
+folder: callback
+permalink: /patterns/callback/
+categories: Idiom
+tags:
+ - Reactive
+---
+
+## 目的
+回调是一部分被当为参数来传递给其他代码的可执行代码，接收方的代码可以在一些方便的时候来调用它。
+
+## 解释
+
+真实世界例子
+
+> 我们需要被通知当执行的任务结束时。我们为调用者传递一个回调方法然后等它调用通知我们。
+
+通俗的讲
+
+
+> 回调是一个用来传递给调用者的方法，它将在定义的时刻被调用。 
+
+维基百科说
+
+> 在计算机编程中，回调又被称为“稍后调用”函数，可以是任何可执行的代码用来作为参数传递给其他代码；其它代码被期望在给定时间内调用回调方法。
+
+**编程示例**
+
+回调是一个只有一个方法的简单接口。
+
+```java
+public interface Callback {
+
+  void call();
+}
+```
+
+下面我们定义一个任务它将在任务执行完成后执行回调。
+
+```java
+public abstract class Task {
+
+  final void executeWith(Callback callback) {
+    execute();
+    Optional.ofNullable(callback).ifPresent(Callback::call);
+  }
+
+  public abstract void execute();
+}
+
+public final class SimpleTask extends Task {
+
+  private static final Logger LOGGER = getLogger(SimpleTask.class);
+
+  @Override
+  public void execute() {
+    LOGGER.info("Perform some important activity and after call the callback method.");
+  }
+}
+```
+
+最后这里是我们如何执行一个任务然后接收一个回调当它完成时。
+
+```java
+    var task = new SimpleTask();
+    task.executeWith(() -> LOGGER.info("I'm done now."));
+```
+## 类图
+![alt text](../../callback/etc/callback.png "Callback")
+
+## 适用性
+使用回调模式当
+* 当一些同步或异步架构动作必须在一些定义好的活动执行后执行时。
+
+## Java例子
+
+* [CyclicBarrier](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CyclicBarrier.html#CyclicBarrier%28int,%20java.lang.Runnable%29) 构造函数可以接受回调，该回调将在每次障碍被触发时触发。

zh/command/README.md

@@ -0,0 +1,253 @@
+---
+layout: pattern
+title: Command
+folder: command
+permalink: /patterns/command/
+categories: Behavioral
+tags:
+ - Gang of Four
+---
+
+## 或称
+行动, 事务模式
+
+## 目的
+将请求封装为对象，从而使你可以将具有不同请求的客户端参数化，队列或记录请求，并且支持可撤销操作。
+
+## 解释
+真实世界例子
+
+> 有一个巫师在地精上施放咒语。咒语在地精上一一执行。第一个咒语使地精缩小，第二个使他不可见。然后巫师将咒语一个个的反转。这里的每一个咒语都是一个可撤销的命令对象。
+
+用通俗的话说
+
+> 用命令对象的方式存储请求以在将来时可以执行它或撤销它。
+
+维基百科说
+
+> 在面向对象编程中，命令模式是一种行为型设计模式，它把在稍后执行的一个动作或触发的一个事件所需要的所有信息封装到一个对象中。
+
+**编程示例**
+
+这是巫师和地精的示例代码。让我们从巫师类开始。
+
+```java
+public class Wizard {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);
+
+  private final Deque<Command> undoStack = new LinkedList<>();
+  private final Deque<Command> redoStack = new LinkedList<>();
+
+  public Wizard() {}
+
+  public void castSpell(Command command, Target target) {
+    LOGGER.info("{} casts {} at {}", this, command, target);
+    command.execute(target);
+    undoStack.offerLast(command);
+  }
+
+  public void undoLastSpell() {
+    if (!undoStack.isEmpty()) {
+      var previousSpell = undoStack.pollLast();
+      redoStack.offerLast(previousSpell);
+      LOGGER.info("{} undoes {}", this, previousSpell);
+      previousSpell.undo();
+    }
+  }
+
+  public void redoLastSpell() {
+    if (!redoStack.isEmpty()) {
+      var previousSpell = redoStack.pollLast();
+      undoStack.offerLast(previousSpell);
+      LOGGER.info("{} redoes {}", this, previousSpell);
+      previousSpell.redo();
+    }
+  }
+
+  @Override
+  public String toString() {
+    return "Wizard";
+  }
+}
+```
+
+接下来我们介绍咒语层级
+
+```java
+public interface Command {
+
+  void execute(Target target);
+
+  void undo();
+
+  void redo();
+
+  String toString();
+}
+
+public class InvisibilitySpell implements Command {
+
+  private Target target;
+
+  @Override
+  public void execute(Target target) {
+    target.setVisibility(Visibility.INVISIBLE);
+    this.target = target;
+  }
+
+  @Override
+  public void undo() {
+    if (target != null) {
+      target.setVisibility(Visibility.VISIBLE);
+    }
+  }
+
+  @Override
+  public void redo() {
+    if (target != null) {
+      target.setVisibility(Visibility.INVISIBLE);
+    }
+  }
+
+  @Override
+  public String toString() {
+    return "Invisibility spell";
+  }
+}
+
+public class ShrinkSpell implements Command {
+
+  private Size oldSize;
+  private Target target;
+
+  @Override
+  public void execute(Target target) {
+    oldSize = target.getSize();
+    target.setSize(Size.SMALL);
+    this.target = target;
+  }
+
+  @Override
+  public void undo() {
+    if (oldSize != null && target != null) {
+      var temp = target.getSize();
+      target.setSize(oldSize);
+      oldSize = temp;
+    }
+  }
+
+  @Override
+  public void redo() {
+    undo();
+  }
+
+  @Override
+  public String toString() {
+    return "Shrink spell";
+  }
+}
+```
+
+最后我们有咒语的目标地精。
+
+```java
+public abstract class Target {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Target.class);
+
+  private Size size;
+
+  private Visibility visibility;
+
+  public Size getSize() {
+    return size;
+  }
+
+  public void setSize(Size size) {
+    this.size = size;
+  }
+
+  public Visibility getVisibility() {
+    return visibility;
+  }
+
+  public void setVisibility(Visibility visibility) {
+    this.visibility = visibility;
+  }
+
+  @Override
+  public abstract String toString();
+
+  public void printStatus() {
+    LOGGER.info("{}, [size={}] [visibility={}]", this, getSize(), getVisibility());
+  }
+}
+
+public class Goblin extends Target {
+
+  public Goblin() {
+    setSize(Size.NORMAL);
+    setVisibility(Visibility.VISIBLE);
+  }
+
+  @Override
+  public String toString() {
+    return "Goblin";
+  }
+
+}
+```
+
+最后是整个示例的实践。
+
+```java
+var wizard = new Wizard();
+var goblin = new Goblin();
+goblin.printStatus();
+// Goblin, [size=normal] [visibility=visible]
+wizard.castSpell(new ShrinkSpell(), goblin);
+// Wizard casts Shrink spell at Goblin
+goblin.printStatus();
+// Goblin, [size=small] [visibility=visible]
+wizard.castSpell(new InvisibilitySpell(), goblin);
+// Wizard casts Invisibility spell at Goblin
+goblin.printStatus();
+// Goblin, [size=small] [visibility=invisible]
+wizard.undoLastSpell();
+// Wizard undoes Invisibility spell
+goblin.printStatus();
+// Goblin, [size=small] [visibility=visible]
+```
+
+## 类图
+![alt text](../../command/etc/command.png "Command")
+
+## 适用性
+使用命令模式当你想
+
+* 通过操作将对象参数化。您可以使用回调函数（即，已在某处注册以便稍后调用的函数）以过程语言表示这种参数化。命令是回调的一种面向对象替代方案。
+* 在不同的时间指定，排队和执行请求。一个命令对象的生存期可以独立于原始请求。如果请求的接收方可以以地址空间无关的方式来表示，那么你可以将请求的命令对象传输到其他进程并在那里执行请求。
+* 支持撤销。命令的执行操作可以在命令本身中存储状态以反转其效果。命令接口必须有添加的反执行操作，该操作可以逆转上一次执行调用的效果。执行的命令存储在历史列表中。无限撤消和重做通过分别向后和向前遍历此列表来实现，分别调用unexecute和execute。
+* 支持日志记录更改，以便在系统崩溃时可以重新应用它们。通过使用加载和存储操作扩展命令接口，你可以保留更改的永久日志。从崩溃中恢复涉及从磁盘重新加载记录的命令，并通过执行操作重新执行它们。
+* 通过原始的操作来构建一个以高级操作围绕的系统。这种结构在支持事务的信息系统中很常见。事务封装了一组数据更改。命令模式提供了一种对事务进行建模的方法。命令具有公共接口，让你以相同的方式调用所有事务。该模式还可以通过新的事务来轻松扩展系统。
+
+## 典型用例
+
+* 保留请求历史
+* 实现回调功能
+* 实现撤销功能
+
+## Java世界例子
+
+* [java.lang.Runnable](http://docs.oracle.com/javase/8/docs/api/java/lang/Runnable.html)
+* [org.junit.runners.model.Statement](https://github.com/junit-team/junit4/blob/master/src/main/java/org/junit/runners/model/Statement.java)
+* [Netflix Hystrix](https://github.com/Netflix/Hystrix/wiki)
+* [javax.swing.Action](http://docs.oracle.com/javase/8/docs/api/javax/swing/Action.html)
+
+## 鸣谢
+
+* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
+* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
+* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)
+* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=f27d2644fbe5026ea448791a8ad09c94)