docs: update .all-contributorsrc [skip ci]

* docs: update README.md [skip ci]

* docs: update .all-contributorsrc [skip ci]

Co-authored-by: allcontributors[bot] <46447321+allcontributors[bot]@users.noreply.github.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"
       ]
     },
     {
@@ -1045,7 +1047,8 @@
       "avatar_url": "https://avatars1.githubusercontent.com/u/6307904?v=4",
       "profile": "https://github.com/charlesfinley",
       "contributions": [
-        "code"
+        "code",
+        "review"
       ]
     },
     {
@@ -1065,6 +1068,252 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "raja-peeyush-kumar-singh",
+      "name": "Peeyush",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/5496024?v=4",
+      "profile": "https://github.com/raja-peeyush-kumar-singh",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "ravening",
+      "name": "Rakesh",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/10645273?v=4",
+      "profile": "https://github.com/ravening",
+      "contributions": [
+        "code",
+        "review"
+      ]
+    },
+    {
+      "login": "vINCENT8888801",
+      "name": "Wei Seng",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/8037883?v=4",
+      "profile": "https://github.com/vINCENT8888801",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "ashishtrivedi16",
+      "name": "Ashish Trivedi",
+      "avatar_url": "https://avatars3.githubusercontent.com/u/23194128?v=4",
+      "profile": "https://www.linkedin.com/in/ashish-trivedi-218379135/",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "RayYH",
+      "name": "洪月阳",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/41055099?v=4",
+      "profile": "https://rayyounghong.com",
+      "contributions": [
+        "code"
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+      "profile": "https://xdvrx1.github.io/",
+      "contributions": [
+        "review",
+        "ideas"
+      ]
+    },
+    {
+      "login": "ohbus",
+      "name": "Subhrodip Mohanta",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/13291222?v=4",
+      "profile": "http://subho.xyz",
+      "contributions": [
+        "code",
+        "review"
+      ]
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+    {
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+      "name": "Bethan Palmer",
+      "avatar_url": "https://avatars3.githubusercontent.com/u/13121570?v=4",
+      "profile": "https://github.com/nahteb",
+      "contributions": [
+        "code"
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+    {
+      "login": "ToxicDreamz",
+      "name": "Toxic Dreamz",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/45225562?v=4",
+      "profile": "https://github.com/ToxicDreamz",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "edycutjong",
+      "name": "Edy Cu Tjong",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/1098102?v=4",
+      "profile": "http://www.edycutjong.com",
+      "contributions": [
+        "doc"
+      ]
+    },
+    {
+      "login": "mkrzywanski",
+      "name": "Michał Krzywański",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/15279585?v=4",
+      "profile": "https://github.com/mkrzywanski",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "stefanbirkner",
+      "name": "Stefan Birkner",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/711349?v=4",
+      "profile": "https://www.stefan-birkner.de",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "fedorskvorcov",
+      "name": "Fedor Skvorcov",
+      "avatar_url": "https://avatars3.githubusercontent.com/u/43882212?v=4",
+      "profile": "https://github.com/fedorskvorcov",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "samilAyoub",
+      "name": "samilAyoub",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/61546990?v=4",
+      "profile": "https://github.com/samilAyoub",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "vdlald",
+      "name": "Vladislav Golubinov",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/29997701?v=4",
+      "profile": "https://github.com/vdlald",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "swarajsaaj",
+      "name": "Swaraj",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/6285049?v=4",
+      "profile": "https://github.com/swarajsaaj",
+      "contributions": [
+        "code"
+      ]
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+    {
+      "login": "ChFlick",
+      "name": "Christoph Flick",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/4465376?v=4",
+      "profile": "http://christophflick.de",
+      "contributions": [
+        "doc"
+      ]
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+    {
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+      "name": "Ascênio",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/7662016?v=4",
+      "profile": "https://github.com/Ascenio",
+      "contributions": [
+        "review"
+      ]
+    },
+    {
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+      "name": "Domenico Sibilio",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/24280982?v=4",
+      "profile": "https://www.linkedin.com/in/domenico-sibilio/",
+      "contributions": [
+        "doc"
+      ]
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+    {
+      "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"
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+      "avatar_url": "https://avatars0.githubusercontent.com/u/20307301?v=4",
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+        "code"
+      ]
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+      "profile": "https://github.com/OrangePants-R",
+      "contributions": [
+        "doc"
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+      "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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.github/workflows/maven-ci.yml

@@ -0,0 +1,80 @@
+#
+# 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.
+#
+
+# This workflow will build a Java project with Maven
+# For more information see: https://help.github.com/actions/language-and-framework-guides/building-and-testing-java-with-maven
+
+name: Java CI
+
+on:
+  push:
+    branches: [ master ]
+
+
+jobs:
+
+  build:
+
+    runs-on: ubuntu-20.04
+
+    steps:
+
+    - name: Checkout Code
+      uses: actions/checkout@v2
+      with:
+        # Disabling shallow clone for improving relevancy of SonarQube reporting
+        fetch-depth: 0
+
+    - name: Set up JDK 11
+      uses: actions/setup-java@v1
+      with:
+        java-version: 11
+
+    - name: Cache SonarCloud packages
+      uses: actions/cache@v2
+      with:
+        path: ~/.sonar/cache
+        key: ${{ runner.os }}-sonar
+        restore-keys: ${{ runner.os }}-sonar
+
+    - name: Cache Maven dependencies
+      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
+
+    # The SonarQube analysis is only for the master branch of the main repository.
+    # SonarQube scan does not work for forked repositories try changing it to xvfb-run mvn clean verify
+    # See https://jira.sonarsource.com/browse/MMF-1371
+    - name: Build with Maven and run SonarQube analysis
+      run: xvfb-run mvn clean verify org.sonarsource.scanner.maven:sonar-maven-plugin:sonar
+      env:
+        # These two env variables are needed for sonar analysis
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}

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

@@ -24,37 +24,41 @@
 # This workflow will build a Java project with Maven
 # For more information see: https://help.github.com/actions/language-and-framework-guides/building-and-testing-java-with-maven
 
-name: Java CI with Maven
+name: Java PR Builder
 
 on:
-  push:
-    branches: [ master ]
   pull_request:
     branches: [ master ]
+    types: [ opened, reopened, synchronize, labeled, unlabeled ]
 
 jobs:
   build:
 
-    runs-on: ubuntu-18.04
+    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
+    
+    - 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 xvfb
-    # SonarQube scan does not work for forked repositories
+      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
     - name: Build with Maven
-      if: github.ref != 'refs/heads/master'
       run: xvfb-run mvn clean verify
-    - name: Build with Maven and run SonarQube analysis
-      if: github.ref == 'refs/heads/master'
-      run: xvfb-run mvn clean verify org.sonarsource.scanner.maven:sonar-maven-plugin:sonar
-      env:
-        # These two env variables are needed for sonar analysis
-        GITHUB_TOKEN: ${{ secrets.REPOSITORY_ACCESS_TOKEN }}
-        SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}

README.md

@@ -6,10 +6,11 @@
 
 ![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)
-[![Sonarcloud Status](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=alert_status)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns) 
 <!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
-[![All Contributors](https://img.shields.io/badge/all_contributors-117-orange.svg?style=flat-square)](#contributors-)
+[![All Contributors](https://img.shields.io/badge/all_contributors-144-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
 # Introduction
@@ -30,7 +31,7 @@ This site showcases Java Design Patterns. The solutions have been developed by
 experienced programmers and architects from the open source community. The 
 patterns can be browsed by their high level descriptions or by looking at their
 source code. The source code examples are well commented and can be thought as
-programming tutorials how to implement a specific pattern. We use the most
+programming tutorials on how to implement a specific pattern. We use the most
 popular battle-proven open source Java technologies.
 
 Before you dive into the material, you should be familiar with various
@@ -118,7 +119,7 @@ This project is licensed under the terms of the MIT license.
   </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="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> <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" 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>
   </tr>
@@ -222,7 +223,7 @@ This project is licensed under the terms of the MIT license.
     <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="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> <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>
@@ -239,11 +240,50 @@ This project is licensed under the terms of the MIT license.
   <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></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>
   </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>
+  </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>
+  </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>
+  </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>
+  </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>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/akashchandwani"><img src="https://avatars2.githubusercontent.com/u/3483277?v=4" 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" 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" 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" 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" 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" 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" 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" 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>
 </table>
 

abstract-document/pom.xml

@@ -21,7 +21,7 @@
 	<parent>
 		<artifactId>java-design-patterns</artifactId>
 		<groupId>com.iluwatar</groupId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<artifactId>abstract-document</artifactId>
 	<dependencies>

abstract-document/src/main/java/com/iluwatar/abstractdocument/App.java

@@ -43,9 +43,11 @@ public class App {
   private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
 
   /**
-   * Executes the App.
+   * Program entry point.
+   *
+   * @param args command line args
    */
-  public App() {
+  public static void main(String[] args) {
     LOGGER.info("Constructing parts and car");
 
     var wheelProperties = Map.of(
@@ -75,14 +77,4 @@ public class App {
         p.getPrice().orElse(null))
     );
   }
-
-  /**
-   * Program entry point.
-   *
-   * @param args command line args
-   */
-  public static void main(String[] args) {
-    new App();
-  }
-
 }

abstract-document/src/test/java/com/iluwatar/abstractdocument/AbstractDocumentTest.java

@@ -40,7 +40,7 @@ public class AbstractDocumentTest {
   private static final String KEY = "key";
   private static final String VALUE = "value";
 
-  private class DocumentImplementation extends AbstractDocument {
+  private static class DocumentImplementation extends AbstractDocument {
 
     DocumentImplementation(Map<String, Object> properties) {
       super(properties);

abstract-document/src/test/java/com/iluwatar/abstractdocument/AppTest.java

@@ -25,14 +25,23 @@ package com.iluwatar.abstractdocument;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Simple App test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
 
   @Test
-  public void shouldExecuteAppWithoutException() {
-    App.main(null);
+  void shouldExecuteAppWithoutException() {
+    assertDoesNotThrow(() -> App.main(null));
   }
 
 }

abstract-factory/README.md

@@ -9,16 +9,19 @@ tags:
 ---
 
 ## Also known as
+
 Kit
 
 ## Intent
+
 Provide an interface for creating families of related or dependent
 objects without specifying their concrete classes.
 
 ## Explanation
+
 Real world example
 
-> To create a kingdom we need objects with common theme. Elven kingdom needs an Elven king, Elven castle and Elven army whereas Orcish kingdom needs an Orcish king, Orcish castle and Orcish army. There is a dependency between the objects in the kingdom.
+> To create a kingdom we need objects with a common theme. Elven kingdom needs an Elven king, Elven castle and Elven army whereas Orcish kingdom needs an Orcish king, Orcish castle and Orcish army. There is a dependency between the objects in the kingdom.
 
 In plain words
 
@@ -30,15 +33,18 @@ Wikipedia says
 
 **Programmatic Example**
 
-Translating the kingdom example above. First of all we have some interfaces and implementation for the objects in the kingdom
+Translating the kingdom example above. First of all we have some interfaces and implementation for the objects in the 
+kingdom.
 
 ```java
 public interface Castle {
   String getDescription();
 }
+
 public interface King {
   String getDescription();
 }
+
 public interface Army {
   String getDescription();
 }
@@ -66,7 +72,7 @@ public class ElfArmy implements Army {
   }
 }
 
-// Orcish implementations similarly...
+// Orcish implementations similarly -> ...
 
 ```
 
@@ -112,9 +118,17 @@ var castle = factory.createCastle();
 var king = factory.createKing();
 var army = factory.createArmy();
 
-castle.getDescription();  // Output: This is the Elven castle!
-king.getDescription(); // Output: This is the Elven king!
-army.getDescription(); // Output: This is the Elven Army!
+castle.getDescription();
+king.getDescription();
+army.getDescription();
+```
+
+Program output:
+
+```java
+This is the Elven castle!
+This is the Elven king!
+This is the Elven Army!
 ```
 
 Now, we can design a factory for our different kingdom factories. In this example, we created FactoryMaker, responsible for returning an instance of either ElfKingdomFactory or OrcKingdomFactory.  
@@ -156,46 +170,52 @@ public static void main(String[] args) {
 ```
 
 ## Class diagram
+
 ![alt text](./etc/abstract-factory.urm.png "Abstract Factory class diagram")
 
 
 ## Applicability
+
 Use the Abstract Factory pattern when
 
-* a system should be independent of how its products are created, composed and represented
-* a system should be configured with one of multiple families of products
-* a family of related product objects is designed to be used together, and you need to enforce this constraint
-* you want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
-* the lifetime of the dependency is conceptually shorter than the lifetime of the consumer.
-*	you need a run-time value to construct a particular dependency
-*	you want to decide which product to call from a family at runtime.
-*	you need to supply one or more parameters only known at run-time before you can resolve a dependency.
-* when you need consistency among products
-* you don’t want to change existing code when adding new products or families of products to the program.
+* The system should be independent of how its products are created, composed and represented
+* The system should be configured with one of multiple families of products
+* The family of related product objects is designed to be used together, and you need to enforce this constraint
+* You want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
+* The lifetime of the dependency is conceptually shorter than the lifetime of the consumer.
+* You need a run-time value to construct a particular dependency
+* You want to decide which product to call from a family at runtime.
+* You need to supply one or more parameters only known at run-time before you can resolve a dependency.
+* When you need consistency among products
+* You don’t want to change existing code when adding new products or families of products to the program.
 
-## Use Cases:	
+Example use cases	
 
-*	Selecting to call the appropriate implementation of FileSystemAcmeService or DatabaseAcmeService or NetworkAcmeService at runtime.
-*	Unit test case writing becomes much easier
+* Selecting to call to the appropriate implementation of FileSystemAcmeService or DatabaseAcmeService or NetworkAcmeService at runtime.
+* Unit test case writing becomes much easier
 * UI tools for different OS
 
 ## Consequences:
 
-*	Dependency injection in java hides the service class dependencies that can lead to runtime errors that would have been caught at compile time.
+* Dependency injection in java hides the service class dependencies that can lead to runtime errors that would have been caught at compile time.
 * While the pattern is great when creating predefined objects, adding the new ones might be challenging.
-* The code may become more complicated than it should be, since a lot of new interfaces and classes are introduced along with the pattern.
-
+* The code becomes more complicated than it should be, since a lot of new interfaces and classes are introduced along with the pattern.
 
 ## Tutorial
+
 * [Abstract Factory Pattern Tutorial](https://www.journaldev.com/1418/abstract-factory-design-pattern-in-java) 
 
-
-## Real world examples
+## Known uses
 
 * [javax.xml.parsers.DocumentBuilderFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/parsers/DocumentBuilderFactory.html)
 * [javax.xml.transform.TransformerFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/transform/TransformerFactory.html#newInstance--)
 * [javax.xml.xpath.XPathFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/xpath/XPathFactory.html#newInstance--)
 
+## Related patterns
+
+[Factory Method](https://java-design-patterns.com/patterns/factory-method/)
+[Factory Kit](https://java-design-patterns.com/patterns/factory-kit/)
+
 ## Credits
 
 * [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)

abstract-factory/pom.xml

@@ -22,7 +22,7 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<artifactId>abstract-factory</artifactId>
 	<dependencies>

abstract-factory/src/main/java/com/iluwatar/abstractfactory/App.java

@@ -23,7 +23,6 @@
 
 package com.iluwatar.abstractfactory;
 
-import com.iluwatar.abstractfactory.App.FactoryMaker.KingdomType;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -41,84 +40,14 @@ import org.slf4j.LoggerFactory;
  * and its implementations ( {@link ElfKingdomFactory}, {@link OrcKingdomFactory}). The example uses
  * both concrete implementations to create a king, a castle and an army.
  */
-public class App {
+public class App implements Runnable {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+  private static Logger log = LoggerFactory.getLogger(App.class);
 
-  private King king;
-  private Castle castle;
-  private Army army;
+  private final Kingdom kingdom = new Kingdom();
 
-  /**
-   * Creates kingdom.
-   */
-  public void createKingdom(final KingdomFactory factory) {
-    setKing(factory.createKing());
-    setCastle(factory.createCastle());
-    setArmy(factory.createArmy());
-  }
-
-  King getKing(final KingdomFactory factory) {
-    return factory.createKing();
-  }
-
-  public King getKing() {
-    return king;
-  }
-
-  private void setKing(final King king) {
-    this.king = king;
-  }
-
-  Castle getCastle(final KingdomFactory factory) {
-    return factory.createCastle();
-  }
-
-  public Castle getCastle() {
-    return castle;
-  }
-
-  private void setCastle(final Castle castle) {
-    this.castle = castle;
-  }
-
-  Army getArmy(final KingdomFactory factory) {
-    return factory.createArmy();
-  }
-
-  public Army getArmy() {
-    return army;
-  }
-
-  private void setArmy(final Army army) {
-    this.army = army;
-  }
-
-  /**
-   * The factory of kingdom factories.
-   */
-  public static class FactoryMaker {
-
-    /**
-     * Enumeration for the different types of Kingdoms.
-     */
-    public enum KingdomType {
-      ELF, ORC
-    }
-
-    /**
-     * The factory method to create KingdomFactory concrete objects.
-     */
-    public static KingdomFactory makeFactory(KingdomType type) {
-      switch (type) {
-        case ELF:
-          return new ElfKingdomFactory();
-        case ORC:
-          return new OrcKingdomFactory();
-        default:
-          throw new IllegalArgumentException("KingdomType not supported.");
-      }
-    }
+  public Kingdom getKingdom() {
+    return kingdom;
   }
 
   /**
@@ -127,19 +56,33 @@ public class App {
    * @param args command line args
    */
   public static void main(String[] args) {
-
     var app = new App();
+    app.run();
+  }
 
-    LOGGER.info("Elf Kingdom");
-    app.createKingdom(FactoryMaker.makeFactory(KingdomType.ELF));
-    LOGGER.info(app.getArmy().getDescription());
-    LOGGER.info(app.getCastle().getDescription());
-    LOGGER.info(app.getKing().getDescription());
+  @Override
+  public void run() {
+    log.info("Elf Kingdom");
+    createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    log.info(kingdom.getArmy().getDescription());
+    log.info(kingdom.getCastle().getDescription());
+    log.info(kingdom.getKing().getDescription());
 
-    LOGGER.info("Orc Kingdom");
-    app.createKingdom(FactoryMaker.makeFactory(KingdomType.ORC));
-    LOGGER.info(app.getArmy().getDescription());
-    LOGGER.info(app.getCastle().getDescription());
-    LOGGER.info(app.getKing().getDescription());
+    log.info("Orc Kingdom");
+    createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    log.info(kingdom.getArmy().getDescription());
+    log.info(kingdom.getCastle().getDescription());
+    log.info(kingdom.getKing().getDescription());
+  }
+
+  /**
+   * Creates kingdom.
+   * @param kingdomType type of Kingdom
+   */
+  public void createKingdom(final Kingdom.FactoryMaker.KingdomType kingdomType) {
+    final KingdomFactory kingdomFactory = Kingdom.FactoryMaker.makeFactory(kingdomType);
+    kingdom.setKing(kingdomFactory.createKing());
+    kingdom.setCastle(kingdomFactory.createCastle());
+    kingdom.setArmy(kingdomFactory.createArmy());
   }
 }

abstract-factory/src/main/java/com/iluwatar/abstractfactory/Kingdom.java

@@ -0,0 +1,82 @@
+/*
+ * 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.abstractfactory;
+
+public class Kingdom {
+
+  private King king;
+  private Castle castle;
+  private Army army;
+
+  public King getKing() {
+    return king;
+  }
+
+  public Castle getCastle() {
+    return castle;
+  }
+
+  public Army getArmy() {
+    return army;
+  }
+
+  public void setKing(King king) {
+    this.king = king;
+  }
+
+  public void setCastle(Castle castle) {
+    this.castle = castle;
+  }
+
+  public void setArmy(Army army) {
+    this.army = army;
+  }
+
+  /**
+   * The factory of kingdom factories.
+   */
+  public static class FactoryMaker {
+
+    /**
+     * Enumeration for the different types of Kingdoms.
+     */
+    public enum KingdomType {
+      ELF, ORC
+    }
+
+    /**
+     * The factory method to create KingdomFactory concrete objects.
+     */
+    public static KingdomFactory makeFactory(KingdomType type) {
+      switch (type) {
+        case ELF:
+          return new ElfKingdomFactory();
+        case ORC:
+          return new OrcKingdomFactory();
+        default:
+          throw new IllegalArgumentException("KingdomType not supported.");
+      }
+    }
+  }
+}

abstract-factory/src/test/java/com/iluwatar/abstractfactory/AbstractFactoryTest.java

@@ -23,65 +23,71 @@
 
 package com.iluwatar.abstractfactory;
 
+import org.junit.jupiter.api.Test;
+
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
-import com.iluwatar.abstractfactory.App.FactoryMaker;
-import com.iluwatar.abstractfactory.App.FactoryMaker.KingdomType;
-import org.junit.jupiter.api.BeforeEach;
-import org.junit.jupiter.api.Test;
-
 /**
  * Test for abstract factory.
  */
 public class AbstractFactoryTest {
 
   private final App app = new App();
-  private KingdomFactory elfFactory;
-  private KingdomFactory orcFactory;
-
-  @BeforeEach
-  public void setUp() {
-    elfFactory = FactoryMaker.makeFactory(KingdomType.ELF);
-    orcFactory = FactoryMaker.makeFactory(KingdomType.ORC);
-  }
 
   @Test
   public void king() {
-    final var elfKing = app.getKing(elfFactory);
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var elfKing = kingdom.getKing();
     assertTrue(elfKing instanceof ElfKing);
     assertEquals(ElfKing.DESCRIPTION, elfKing.getDescription());
-    final var orcKing = app.getKing(orcFactory);
+
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var orcKing = kingdom.getKing();
     assertTrue(orcKing instanceof OrcKing);
     assertEquals(OrcKing.DESCRIPTION, orcKing.getDescription());
   }
 
   @Test
   public void castle() {
-    final var elfCastle = app.getCastle(elfFactory);
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var elfCastle = kingdom.getCastle();
     assertTrue(elfCastle instanceof ElfCastle);
     assertEquals(ElfCastle.DESCRIPTION, elfCastle.getDescription());
-    final var orcCastle = app.getCastle(orcFactory);
+
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var orcCastle = kingdom.getCastle();
     assertTrue(orcCastle instanceof OrcCastle);
     assertEquals(OrcCastle.DESCRIPTION, orcCastle.getDescription());
   }
 
   @Test
   public void army() {
-    final var elfArmy = app.getArmy(elfFactory);
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var elfArmy = kingdom.getArmy();
     assertTrue(elfArmy instanceof ElfArmy);
     assertEquals(ElfArmy.DESCRIPTION, elfArmy.getDescription());
-    final var orcArmy = app.getArmy(orcFactory);
+
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var orcArmy = kingdom.getArmy();
     assertTrue(orcArmy instanceof OrcArmy);
     assertEquals(OrcArmy.DESCRIPTION, orcArmy.getDescription());
   }
 
   @Test
   public void createElfKingdom() {
-    app.createKingdom(elfFactory);
-    final var king = app.getKing();
-    final var castle = app.getCastle();
-    final var army = app.getArmy();
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ELF);
+    final var kingdom = app.getKingdom();
+
+    final var king = kingdom.getKing();
+    final var castle = kingdom.getCastle();
+    final var army = kingdom.getArmy();
     assertTrue(king instanceof ElfKing);
     assertEquals(ElfKing.DESCRIPTION, king.getDescription());
     assertTrue(castle instanceof ElfCastle);
@@ -92,10 +98,12 @@ public class AbstractFactoryTest {
 
   @Test
   public void createOrcKingdom() {
-    app.createKingdom(orcFactory);
-    final var king = app.getKing();
-    final var castle = app.getCastle();
-    final var army = app.getArmy();
+    app.createKingdom(Kingdom.FactoryMaker.KingdomType.ORC);
+    final var kingdom = app.getKingdom();
+
+    final var king = kingdom.getKing();
+    final var castle = kingdom.getCastle();
+    final var army = kingdom.getArmy();
     assertTrue(king instanceof OrcKing);
     assertEquals(OrcKing.DESCRIPTION, king.getDescription());
     assertTrue(castle instanceof OrcCastle);

abstract-factory/src/test/java/com/iluwatar/abstractfactory/AppTest.java

@@ -25,12 +25,23 @@ package com.iluwatar.abstractfactory;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Abstract Factory example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

acyclic-visitor/README.md

@@ -9,12 +9,126 @@ tags:
 ---
 
 ## Intent
-Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GOF VISITOR Pattern.
+
+Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating 
+the troublesome dependency cycles that are inherent to the GoF Visitor Pattern.
+
+## Explanation
+
+Real world example
+
+> We have a hierarchy of modem classes. The modems in this hierarchy need to be visited by an external algorithm based 
+> on filtering criteria (is it Unix or DOS compatible modem). 
+
+In plain words
+
+> Acyclic Visitor allows functions to be added to existing class hierarchies without modifying the hierarchies.
+
+[WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor) says
+
+> The Acyclic Visitor pattern allows new functions to be added to existing class hierarchies without affecting those 
+> hierarchies, and without creating the dependency cycles that are inherent to the GangOfFour VisitorPattern.
+
+**Programmatic Example**
+
+Here's the `Modem` hierarchy.
+
+```java
+public abstract class Modem {
+  public abstract void accept(ModemVisitor modemVisitor);
+}
+
+public class Zoom extends Modem {
+  ...
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof ZoomVisitor) {
+      ((ZoomVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only ZoomVisitor is allowed to visit Zoom modem");
+    }
+  }
+}
+
+public class Hayes extends Modem {
+  ...
+  @Override
+  public void accept(ModemVisitor modemVisitor) {
+    if (modemVisitor instanceof HayesVisitor) {
+      ((HayesVisitor) modemVisitor).visit(this);
+    } else {
+      LOGGER.info("Only HayesVisitor is allowed to visit Hayes modem");
+    }
+  }
+}
+```
+
+Next we introduce the `ModemVisitor` hierarchy.
+
+```java
+public interface ModemVisitor {
+}
+
+public interface HayesVisitor extends ModemVisitor {
+  void visit(Hayes hayes);
+}
+
+public interface ZoomVisitor extends ModemVisitor {
+  void visit(Zoom zoom);
+}
+
+public interface AllModemVisitor extends ZoomVisitor, HayesVisitor {
+}
+
+public class ConfigureForDosVisitor implements AllModemVisitor {
+  ...
+  @Override
+  public void visit(Hayes hayes) {
+    LOGGER.info(hayes + " used with Dos configurator.");
+  }
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Dos configurator.");
+  }
+}
+
+public class ConfigureForUnixVisitor implements ZoomVisitor {
+  ...
+  @Override
+  public void visit(Zoom zoom) {
+    LOGGER.info(zoom + " used with Unix configurator.");
+  }
+}
+```
+
+Finally, here are the visitors in action.
+
+```java
+    var conUnix = new ConfigureForUnixVisitor();
+    var conDos = new ConfigureForDosVisitor();
+    var zoom = new Zoom();
+    var hayes = new Hayes();
+    hayes.accept(conDos);
+    zoom.accept(conDos);
+    hayes.accept(conUnix);
+    zoom.accept(conUnix);   
+```
+
+Program output:
+
+```
+    // Hayes modem used with Dos configurator.
+    // Zoom modem used with Dos configurator.
+    // Only HayesVisitor is allowed to visit Hayes modem
+    // Zoom modem used with Unix configurator.
+```
 
 ## Class diagram
+
 ![alt text](./etc/acyclic-visitor.png "Acyclic Visitor")
 
 ## Applicability
+
 This pattern can be used:
 
 * When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy.
@@ -24,6 +138,7 @@ This pattern can be used:
 * When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive.
 
 ## Consequences
+
 The good:
 
 * No dependency cycles between class hierarchies.
@@ -32,11 +147,14 @@ The good:
 
 The bad:
 
-* Violates the principle of least surprise or Liskov's Substitution principle by showing that it can accept all visitors but actually only being interested in particular visitors.
+* Violates [Liskov's Substitution Principle](https://java-design-patterns.com/principles/#liskov-substitution-principle) by showing that it can accept all visitors but actually only being interested in particular visitors.
 * Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy.
 
 ## Related patterns
-* [Visitor Pattern](../visitor/)
+
+* [Visitor Pattern](https://java-design-patterns.com/patterns/visitor/)
 
 ## Credits
-* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
+
+* [Acyclic Visitor by Robert C. Martin](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
+* [Acyclic Visitor in WikiWikiWeb](https://wiki.c2.com/?AcyclicVisitor)

acyclic-visitor/etc/Acyclic Visitor.ucls

@@ -1,115 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<class-diagram version="1.2.2" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
-  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
-  <interface id="1" language="java" name="com.iluwatar.acyclicvisitor.ModemVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ModemVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="860" y="67"/>    
-    <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>  
-  </interface>  
-  <class id="2" language="java" name="com.iluwatar.acyclicvisitor.Modem" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Modem.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="327" y="77"/>    
-    <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.acyclicvisitor.ConfigureForUnixVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForUnixVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="124" width="196" x="647" y="225"/>    
-    <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.acyclicvisitor.Zoom" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Zoom.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="203" y="305"/>    
-    <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>  
-  <interface id="5" language="java" name="com.iluwatar.acyclicvisitor.HayesVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/HayesVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="1019" y="468"/>    
-    <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>  
-  </interface>  
-  <interface id="6" language="java" name="com.iluwatar.acyclicvisitor.ZoomVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ZoomVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="758" y="467"/>    
-    <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>  
-  </interface>  
-  <class id="7" language="java" name="com.iluwatar.acyclicvisitor.Hayes" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/Hayes.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="479" y="307"/>    
-    <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="8" language="java" name="com.iluwatar.acyclicvisitor.ConfigureForDosVisitor" project="acyclic-visitor" 
-    file="/acyclic-visitor/src/main/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitor.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="142" width="192" x="883" y="225"/>    
-    <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>  
-  <generalization id="9">    
-    <end type="SOURCE" refId="7"/>    
-    <end type="TARGET" refId="2"/>  
-  </generalization>  
-  <realization id="10">    
-    <end type="SOURCE" refId="8"/>    
-    <end type="TARGET" refId="6"/>  
-  </realization>  
-  <realization id="11">    
-    <end type="SOURCE" refId="8"/>    
-    <end type="TARGET" refId="5"/>  
-  </realization>  
-  <realization id="12">    
-    <end type="SOURCE" refId="3"/>    
-    <end type="TARGET" refId="6"/>  
-  </realization>  
-  <realization id="13">    
-    <end type="SOURCE" refId="3"/>    
-    <end type="TARGET" refId="1"/>  
-  </realization>  
-  <realization id="14">    
-    <end type="SOURCE" refId="8"/>    
-    <end type="TARGET" refId="1"/>  
-  </realization>  
-  <generalization id="15">    
-    <end type="SOURCE" refId="4"/>    
-    <end type="TARGET" refId="2"/>  
-  </generalization>  
-  <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>

acyclic-visitor/pom.xml

@@ -21,7 +21,7 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 
 	<artifactId>acyclic-visitor</artifactId>

acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/AppTest.java

@@ -25,13 +25,23 @@ package com.iluwatar.acyclicvisitor;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that the Acyclic Visitor example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

adapter/README.md

@@ -12,9 +12,8 @@ tags:
 Wrapper
 
 ## Intent
-Convert the interface of a class into another interface the clients
-expect. Adapter lets classes work together that couldn't otherwise because of
-incompatible interfaces.
+Convert the interface of a class into another interface the clients expect. Adapter lets classes work together that 
+couldn't otherwise because of incompatible interfaces.
 
 ## Explanation
 

adapter/pom.xml

@@ -22,7 +22,7 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<artifactId>adapter</artifactId>
 	<dependencies>

adapter/src/test/java/com/iluwatar/adapter/AppTest.java

@@ -25,12 +25,23 @@ package com.iluwatar.adapter;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Adapter example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

aggregator-microservices/README.md

@@ -6,20 +6,97 @@ permalink: /patterns/aggregator-microservices/
 categories: Architectural
 tags:
 - Cloud distributed
+- Decoupling
+- Microservices
 ---
 
 ## Intent
 
-The user makes a single call to the Aggregator, and the aggregator then calls each relevant microservice and collects
-the data, apply business logic to it, and further publish is as a REST Endpoint.
-More variations of the aggregator are: 
-- Proxy Microservice Design Pattern: A different microservice is called upon the business need. 
-- Chained Microservice Design Pattern: In this case each microservice is dependent/ chained to a series 
-of other microservices.
+The user makes a single call to the aggregator service, and the aggregator then calls each relevant microservice.
+
+## Explanation
+
+Real world example
+
+> Our web marketplace needs information about products and their current inventory. It makes a call to an aggregator
+> service which in turn calls the product information microservice and product inventory microservice returning the
+> combined information. 
+
+In plain words
+
+> Aggregator Microservice collects pieces of data from various microservices and returns an aggregate for processing. 
+
+Stack Overflow says
+
+> Aggregator Microservice invokes multiple services to achieve the functionality required by the application.
+
+**Programmatic Example**
+
+Let's start from the data model. Here's our `Product`.
+
+```java
+public class Product {
+  private String title;
+  private int productInventories;
+  // getters and setters ->
+  ...
+}
+```
+
+Next we can introduce our `Aggregator` microservice. It contains clients `ProductInformationClient` and
+`ProductInventoryClient` for calling respective microservices.
+
+```java
+@RestController
+public class Aggregator {
+
+  @Resource
+  private ProductInformationClient informationClient;
+
+  @Resource
+  private ProductInventoryClient inventoryClient;
+
+  @RequestMapping(path = "/product", method = RequestMethod.GET)
+  public Product getProduct() {
+
+    var product = new Product();
+    var productTitle = informationClient.getProductTitle();
+    var productInventory = inventoryClient.getProductInventories();
+
+    //Fallback to error message
+    product.setTitle(requireNonNullElse(productTitle, "Error: Fetching Product Title Failed"));
+
+    //Fallback to default error inventory
+    product.setProductInventories(requireNonNullElse(productInventory, -1));
+
+    return product;
+  }
+}
+```
+
+Here's the essence of information microservice implementation. Inventory microservice is similar, it just returns
+inventory counts.
+
+```java
+@RestController
+public class InformationController {
+  @RequestMapping(value = "/information", method = RequestMethod.GET)
+  public String getProductTitle() {
+    return "The Product Title.";
+  }
+}
+```
+
+Now calling our `Aggregator` REST API returns the product information.
+
+```bash
+curl http://localhost:50004/product
+{"title":"The Product Title.","productInventories":5}
+```
 
 ## Class diagram
 
-![alt text](./etc/aggregator-microservice.png "Aggregator Microservice")
+![alt text](./aggregator-service/etc/aggregator-service.png "Aggregator Microservice")
 
 ## Applicability
 
@@ -28,3 +105,5 @@ Use the Aggregator Microservices pattern when you need a unified API for various
 ## Credits
 
 * [Microservice Design Patterns](http://web.archive.org/web/20190705163602/http://blog.arungupta.me/microservice-design-patterns/)
+* [Microservices Patterns: With examples in Java](https://www.amazon.com/gp/product/1617294543/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617294543&linkId=8b4e570267bc5fb8b8189917b461dc60)
+* [Architectural Patterns: Uncover essential patterns in the most indispensable realm of enterprise architecture](https://www.amazon.com/gp/product/B077T7V8RC/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=B077T7V8RC&linkId=c34d204bfe1b277914b420189f09c1a4)

aggregator-microservices/aggregator-service/pom.xml

@@ -20,7 +20,7 @@
 	<parent>
 		<artifactId>aggregator-microservices</artifactId>
 		<groupId>com.iluwatar</groupId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<modelVersion>4.0.0</modelVersion>
 	<artifactId>aggregator-service</artifactId>

aggregator-microservices/information-microservice/pom.xml

@@ -20,7 +20,7 @@
 	<parent>
 		<artifactId>aggregator-microservices</artifactId>
 		<groupId>com.iluwatar</groupId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<modelVersion>4.0.0</modelVersion>
 

aggregator-microservices/inventory-microservice/pom.xml

@@ -20,7 +20,7 @@
 	<parent>
 		<artifactId>aggregator-microservices</artifactId>
 		<groupId>com.iluwatar</groupId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<modelVersion>4.0.0</modelVersion>
 	<artifactId>inventory-microservice</artifactId>

aggregator-microservices/pom.xml

@@ -29,7 +29,7 @@
     <parent>
         <artifactId>java-design-patterns</artifactId>
         <groupId>com.iluwatar</groupId>
-        <version>1.23.0-SNAPSHOT</version>
+        <version>1.24.0-SNAPSHOT</version>
     </parent>
     <modelVersion>4.0.0</modelVersion>
     <artifactId>aggregator-microservices</artifactId>

ambassador/README.md

@@ -10,28 +10,37 @@ tags:
 ---
 
 ## Intent
+
 Provide a helper service instance on a client and offload common functionality away from a shared resource.
 
 ## Explanation
+
 Real world example
 
-> A remote service has many clients accessing a function it provides. The service is a legacy application and is impossible to update. Large numbers of requests from users are causing connectivity issues. New rules for request frequency should be implemented along with latency checks and client-side logging.
+> A remote service has many clients accessing a function it provides. The service is a legacy application and is 
+> impossible to update. Large numbers of requests from users are causing connectivity issues. New rules for request 
+> frequency should be implemented along with latency checks and client-side logging.
 
 In plain words
 
-> Using the ambassador pattern, we can implement less-frequent polling from clients along with latency checks and logging.
+> With the Ambassador pattern, we can implement less-frequent polling from clients along with latency checks and 
+> logging.
 
 Microsoft documentation states
 
-> An ambassador service can be thought of as an out-of-process proxy that is co-located with the client. This pattern can be useful for offloading common client connectivity tasks such as monitoring, logging, routing, security (such as TLS), and resiliency patterns in a language agnostic way. It is often used with legacy applications, or other applications that are difficult to modify, in order to extend their networking capabilities. It can also enable a specialized team to implement those features.
+> An ambassador service can be thought of as an out-of-process proxy which is co-located with the client. This pattern 
+> can be useful for offloading common client connectivity tasks such as monitoring, logging, routing, 
+> security (such as TLS), and resiliency patterns in a language agnostic way. It is often used with legacy applications, 
+> or other applications that are difficult to modify, in order to extend their networking capabilities. It can also 
+> enable a specialized team to implement those features.
 
 **Programmatic Example**
 
-With the above example in mind we will imitate the functionality in a simple manner. We have an interface implemented by the remote service as well as the ambassador service:
+With the above introduction in mind we will imitate the functionality in this example. We have an interface implemented 
+by the remote service as well as the ambassador service:
 
 ```java
 interface RemoteServiceInterface {
-
     long doRemoteFunction(int value) throws Exception;
 }
 ```
@@ -136,7 +145,7 @@ public class Client {
 }
 ```
 
-And here are two clients using the service.
+Here are two clients using the service.
 
 ```java
 public class App {
@@ -149,13 +158,29 @@ public class App {
 }
 ```
 
+Here's the output for running the example:
+
+```java
+Time taken (ms): 111
+Service result: 120
+Time taken (ms): 931
+Failed to reach remote: (1)
+Time taken (ms): 665
+Failed to reach remote: (2)
+Time taken (ms): 538
+Failed to reach remote: (3)
+Service result: -1
+```
+
 ## Class diagram
+
 ![alt text](./etc/ambassador.urm.png "Ambassador class diagram")
 
 ## Applicability
-Ambassador is applicable when working with a legacy remote service that cannot
-be modified or would be extremely difficult to modify. Connectivity features can
-be implemented on the client avoiding the need for changes on the remote service.
+
+Ambassador is applicable when working with a legacy remote service which cannot be modified or would be extremely 
+difficult to modify. Connectivity features can be implemented on the client avoiding the need for changes on the remote 
+service.
 
 * Ambassador provides a local interface for a remote service.
 * Ambassador provides logging, circuit breaking, retries and security on the client.
@@ -168,10 +193,14 @@ be implemented on the client avoiding the need for changes on the remote service
 * Offload remote service tasks
 * Facilitate network connection
 
-## Real world examples
+## Known uses
 
 * [Kubernetes-native API gateway for microservices](https://github.com/datawire/ambassador)
 
+## Related patterns
+
+* [Proxy](https://java-design-patterns.com/patterns/proxy/)
+
 ## Credits
 
 * [Ambassador pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/ambassador)

ambassador/pom.xml

@@ -20,7 +20,7 @@
 	<parent>
 		<artifactId>java-design-patterns</artifactId>
 		<groupId>com.iluwatar</groupId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<modelVersion>4.0.0</modelVersion>
 	<artifactId>ambassador</artifactId>

ambassador/src/main/java/com/iluwatar/ambassador/RemoteService.java

@@ -62,7 +62,7 @@ public class RemoteService implements RemoteServiceInterface {
    *
    * @param value integer value to be multiplied.
    * @return if waitTime is less than {@link RemoteService#THRESHOLD}, it returns value * 10,
-   *     otherwise {@link RemoteServiceInterface#FAILURE}.
+   *     otherwise {@link RemoteServiceStatus#FAILURE}.
    */
   @Override
   public long doRemoteFunction(int value) {
@@ -74,6 +74,7 @@ public class RemoteService implements RemoteServiceInterface {
     } catch (InterruptedException e) {
       LOGGER.error("Thread sleep state interrupted", e);
     }
-    return waitTime <= THRESHOLD ? value * 10 : FAILURE;
+    return waitTime <= THRESHOLD ? value * 10
+            : RemoteServiceStatus.FAILURE.getRemoteServiceStatusValue();
   }
 }

ambassador/src/main/java/com/iluwatar/ambassador/RemoteServiceInterface.java

@@ -27,7 +27,6 @@ package com.iluwatar.ambassador;
  * Interface shared by ({@link RemoteService}) and ({@link ServiceAmbassador}).
  */
 interface RemoteServiceInterface {
-  int FAILURE = -1;
 
-  long doRemoteFunction(int value) throws Exception;
+  long doRemoteFunction(int value);
 }

ambassador/src/main/java/com/iluwatar/ambassador/RemoteServiceStatus.java

@@ -21,37 +21,28 @@
  * THE SOFTWARE.
  */
 
-package com.iluwatar.command;
+package com.iluwatar.ambassador;
 
 /**
- * InvisibilitySpell is a concrete command.
+ * Holds information regarding the status of the Remote Service.
+ *
+ * <p> This Enum replaces the integer value previously
+ * stored in {@link RemoteServiceInterface} as SonarCloud was identifying
+ * it as an issue. All test cases have been checked after changes,
+ * without failures. </p>
  */
-public class InvisibilitySpell extends Command {
 
-  private Target target;
+public enum RemoteServiceStatus {
+  FAILURE(-1)
+  ;
 
-  @Override
-  public void execute(Target target) {
-    target.setVisibility(Visibility.INVISIBLE);
-    this.target = target;
+  private final long remoteServiceStatusValue;
+
+  RemoteServiceStatus(long remoteServiceStatusValue) {
+    this.remoteServiceStatusValue = remoteServiceStatusValue;
   }
 
-  @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 long getRemoteServiceStatusValue() {
+    return remoteServiceStatusValue;
   }
 }

ambassador/src/main/java/com/iluwatar/ambassador/ServiceAmbassador.java

@@ -23,6 +23,7 @@
 
 package com.iluwatar.ambassador;
 
+import static com.iluwatar.ambassador.RemoteServiceStatus.FAILURE;
 import static java.lang.Thread.sleep;
 
 import org.slf4j.Logger;
@@ -58,14 +59,14 @@ public class ServiceAmbassador implements RemoteServiceInterface {
 
   private long safeCall(int value) {
     var retries = 0;
-    var result = (long) FAILURE;
+    var result = FAILURE.getRemoteServiceStatusValue();
 
     for (int i = 0; i < RETRIES; i++) {
       if (retries >= RETRIES) {
-        return FAILURE;
+        return FAILURE.getRemoteServiceStatusValue();
       }
 
-      if ((result = checkLatency(value)) == FAILURE) {
+      if ((result = checkLatency(value)) == FAILURE.getRemoteServiceStatusValue()) {
         LOGGER.info("Failed to reach remote: (" + (i + 1) + ")");
         retries++;
         try {

ambassador/src/test/java/com/iluwatar/ambassador/AppTest.java

@@ -25,13 +25,23 @@ package com.iluwatar.ambassador;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
 
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

ambassador/src/test/java/com/iluwatar/ambassador/ClientTest.java

@@ -37,6 +37,6 @@ class ClientTest {
     Client client = new Client();
     var result = client.useService(10);
 
-    assertTrue(result == 100 || result == RemoteService.FAILURE);
+    assertTrue(result == 100 || result == RemoteServiceStatus.FAILURE.getRemoteServiceStatusValue());
   }
 }

ambassador/src/test/java/com/iluwatar/ambassador/RemoteServiceTest.java

@@ -37,7 +37,7 @@ class RemoteServiceTest {
   void testFailedCall() {
     var remoteService = new RemoteService(new StaticRandomProvider(0.21));
     var result = remoteService.doRemoteFunction(10);
-    assertEquals(RemoteServiceInterface.FAILURE, result);
+    assertEquals(RemoteServiceStatus.FAILURE.getRemoteServiceStatusValue(), result);
   }
 
   @Test

ambassador/src/test/java/com/iluwatar/ambassador/ServiceAmbassadorTest.java

@@ -35,6 +35,6 @@ class ServiceAmbassadorTest {
   @Test
   void test() {
     long result = new ServiceAmbassador().doRemoteFunction(10);
-    assertTrue(result == 100 || result == RemoteServiceInterface.FAILURE);
+    assertTrue(result == 100 || result == RemoteServiceStatus.FAILURE.getRemoteServiceStatusValue());
   }
 }

api-gateway/README.md

@@ -12,49 +12,53 @@ tags:
 
 ## Intent
 
-Aggregate calls to microservices in a single location: the API Gateway. The user makes a single call to the API Gateway, 
-and the API Gateway then calls each relevant microservice.
+Aggregate calls to microservices in a single location, the API Gateway. The user makes a single call 
+to the API Gateway, and the API Gateway then calls each relevant microservice.
 
 ## Explanation
 
-With the Microservices pattern, a client may need data from multiple different microservices. If the client called each 
-microservice directly, that could contribute to longer load times, since the client would have to make a network request 
-for each microservice called. Moreover, having the client call each microservice directly ties the client to that 
-microservice - if the internal implementations of the microservices change (for example, if two microservices are 
-combined sometime in the future) or if the location (host and port) of a microservice changes, then every client that 
+With the Microservices pattern, a client may need data from multiple different microservices. If the 
+client called each microservice directly, that could contribute to longer load times, since the 
+client would have to make a network request for each microservice called. Moreover, having the 
+client call each microservice directly ties the client to that microservice - if the internal 
+implementations of the microservices change (for example, if two microservices are combined sometime 
+in the future) or if the location (host and port) of a microservice changes, then every client that 
 makes use of those microservices must be updated.
 
-The intent of the API Gateway pattern is to alleviate some of these issues. In the API Gateway pattern, an additional 
-entity (the API Gateway) is placed between the client and the microservices. The job of the API Gateway is to aggregate 
-the calls to the microservices. Rather than the client calling each microservice individually, the client calls the 
-API Gateway a single time. The API Gateway then calls each of the microservices that the client needs.
+The intent of the API Gateway pattern is to alleviate some of these issues. In the API Gateway 
+pattern, an additional entity (the API Gateway) is placed between the client and the microservices. 
+The job of the API Gateway is to aggregate the calls to the microservices. Rather than the client 
+calling each microservice individually, the client calls the API Gateway a single time. The API 
+Gateway then calls each of the microservices that the client needs.
 
 Real world example
 
-> We are implementing microservices and API Gateway pattern for an e-commerce site. In this system the API Gateway makes 
-calls to the Image and Price microservices.
+> We are implementing microservices and API Gateway pattern for an e-commerce site. In this system 
+> the API Gateway makes calls to the Image and Price microservices.
 
 In plain words
 
-> For a system implemented using microservices architecture, API Gateway is the single entry point that aggregates the 
-calls to the individual microservices. 
+> For a system implemented using microservices architecture, API Gateway is the single entry point 
+> that aggregates the calls to the individual microservices. 
 
 Wikipedia says
 
-> API Gateway is a server that acts as an API front-end, receives API requests, enforces throttling and security 
-policies, passes requests to the back-end service and then passes the response back to the requester. A gateway often 
-includes a transformation engine to orchestrate and modify the requests and responses on the fly. A gateway can also 
-provide functionality such as collecting analytics data and providing caching. The gateway can provide functionality to 
-support authentication, authorization, security, audit and regulatory compliance.
+> API Gateway is a server that acts as an API front-end, receives API requests, enforces throttling 
+> and security policies, passes requests to the back-end service and then passes the response back 
+> to the requester. A gateway often includes a transformation engine to orchestrate and modify the 
+> requests and responses on the fly. A gateway can also provide functionality such as collecting 
+> analytics data and providing caching. The gateway can provide functionality to support 
+> authentication, authorization, security, audit and regulatory compliance.
 
 **Programmatic Example**
 
-This implementation shows what the API Gateway pattern could look like for an e-commerce site. The `ApiGateway` makes 
-calls to the Image and Price microservices using the `ImageClientImpl` and `PriceClientImpl` respectively. Customers 
-viewing the site on a desktop device can see both price information and an image of a product, so the `ApiGateway` calls 
-both of the microservices and aggregates the data in the `DesktopProduct` model. However, mobile users only see price 
-information; they do not see a product image. For mobile users, the `ApiGateway` only retrieves price information, which 
-it uses to populate the `MobileProduct`.
+This implementation shows what the API Gateway pattern could look like for an e-commerce site. The 
+`ApiGateway` makes calls to the Image and Price microservices using the `ImageClientImpl` and 
+`PriceClientImpl` respectively. Customers viewing the site on a desktop device can see both price 
+information and an image of a product, so the `ApiGateway` calls both of the microservices and 
+aggregates the data in the `DesktopProduct` model. However, mobile users only see price information; 
+they do not see a product image. For mobile users, the `ApiGateway` only retrieves price 
+information, which it uses to populate the `MobileProduct`.
 
 Here's the Image microservice implementation.
 
@@ -64,7 +68,6 @@ public interface ImageClient {
 }
 
 public class ImageClientImpl implements ImageClient {
-
   @Override
   public String getImagePath() {
     var httpClient = HttpClient.newHttpClient();
@@ -114,7 +117,7 @@ public class PriceClientImpl implements PriceClient {
 }
 ```
 
-And here we can see how API Gateway maps the requests to the microservices.
+Here we can see how API Gateway maps the requests to the microservices.
 
 ```java
 public class ApiGateway {

api-gateway/api-gateway-service/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>api-gateway</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <modelVersion>4.0.0</modelVersion>
   <artifactId>api-gateway-service</artifactId>

api-gateway/api-gateway-service/src/main/java/com/iluwatar/api/gateway/ImageClientImpl.java

@@ -23,11 +23,16 @@
 
 package com.iluwatar.api.gateway;
 
+import static org.slf4j.LoggerFactory.getLogger;
+
 import java.io.IOException;
 import java.net.URI;
 import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
 import java.net.http.HttpResponse.BodyHandlers;
+
+import org.slf4j.Logger;
 import org.springframework.stereotype.Component;
 
 /**
@@ -35,6 +40,8 @@ import org.springframework.stereotype.Component;
  */
 @Component
 public class ImageClientImpl implements ImageClient {
+  private static final Logger LOGGER = getLogger(ImageClientImpl.class);
+
   /**
    * Makes a simple HTTP Get request to the Image microservice.
    *
@@ -49,12 +56,26 @@ public class ImageClientImpl implements ImageClient {
         .build();
 
     try {
+      LOGGER.info("Sending request to fetch image path");
       var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
+      logResponse(httpResponse);
       return httpResponse.body();
     } catch (IOException | InterruptedException e) {
-      e.printStackTrace();
+      LOGGER.error("Failure occurred while getting image path", e);
     }
 
     return null;
   }
+
+  private void logResponse(HttpResponse<String> httpResponse) {
+    if (isSuccessResponse(httpResponse.statusCode())) {
+      LOGGER.info("Image path received successfully");
+    } else {
+      LOGGER.warn("Image path request failed");
+    }
+  }
+
+  private boolean isSuccessResponse(int responseCode) {
+    return responseCode >= 200 && responseCode <= 299;
+  }
 }

api-gateway/api-gateway-service/src/main/java/com/iluwatar/api/gateway/PriceClientImpl.java

@@ -23,18 +23,26 @@
 
 package com.iluwatar.api.gateway;
 
+import static org.slf4j.LoggerFactory.getLogger;
+
 import java.io.IOException;
 import java.net.URI;
 import java.net.http.HttpClient;
 import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
 import java.net.http.HttpResponse.BodyHandlers;
+
+import org.slf4j.Logger;
 import org.springframework.stereotype.Component;
 
+
 /**
  * An adapter to communicate with the Price microservice.
  */
 @Component
 public class PriceClientImpl implements PriceClient {
+  private static final Logger LOGGER = getLogger(PriceClientImpl.class);
+
   /**
    * Makes a simple HTTP Get request to the Price microservice.
    *
@@ -49,12 +57,26 @@ public class PriceClientImpl implements PriceClient {
         .build();
 
     try {
+      LOGGER.info("Sending request to fetch price info");
       var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
+      logResponse(httpResponse);
       return httpResponse.body();
     } catch (IOException | InterruptedException e) {
-      e.printStackTrace();
+      LOGGER.error("Failure occurred while getting price info", e);
     }
 
     return null;
   }
+
+  private void logResponse(HttpResponse<String> httpResponse) {
+    if (isSuccessResponse(httpResponse.statusCode())) {
+      LOGGER.info("Price info received successfully");
+    } else {
+      LOGGER.warn("Price info request failed");
+    }
+  }
+
+  private boolean isSuccessResponse(int responseCode) {
+    return responseCode >= 200 && responseCode <= 299;
+  }
 }

api-gateway/image-microservice/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>api-gateway</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <modelVersion>4.0.0</modelVersion>
   <artifactId>image-microservice</artifactId>

api-gateway/image-microservice/src/main/java/com/iluwatar/image/microservice/ImageController.java

@@ -23,15 +23,20 @@
 
 package com.iluwatar.image.microservice;
 
+import static org.slf4j.LoggerFactory.getLogger;
+
+import org.slf4j.Logger;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestMethod;
 import org.springframework.web.bind.annotation.RestController;
 
+
 /**
  * Exposes the Image microservice's endpoints.
  */
 @RestController
 public class ImageController {
+  private static final Logger LOGGER = getLogger(ImageController.class);
 
   /**
    * An endpoint for a user to retrieve an image path.
@@ -40,6 +45,7 @@ public class ImageController {
    */
   @RequestMapping(value = "/image-path", method = RequestMethod.GET)
   public String getImagePath() {
+    LOGGER.info("Successfully found image path");
     return "/product-image.png";
   }
 }

api-gateway/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>java-design-patterns</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <modelVersion>4.0.0</modelVersion>
   <artifactId>api-gateway</artifactId>

api-gateway/price-microservice/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>api-gateway</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
 
   <modelVersion>4.0.0</modelVersion>

api-gateway/price-microservice/src/main/java/com/iluwatar/price/microservice/PriceController.java

@@ -23,15 +23,20 @@
 
 package com.iluwatar.price.microservice;
 
+import static org.slf4j.LoggerFactory.getLogger;
+
+import org.slf4j.Logger;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestMethod;
 import org.springframework.web.bind.annotation.RestController;
 
+
 /**
  * Exposes the Price microservice's endpoints.
  */
 @RestController
 public class PriceController {
+  private static final Logger LOGGER = getLogger(PriceController.class);
 
   /**
    * An endpoint for a user to retrieve a product's price.
@@ -40,6 +45,7 @@ public class PriceController {
    */
   @RequestMapping(value = "/price", method = RequestMethod.GET)
   public String getPrice() {
+    LOGGER.info("Successfully found price info");
     return "20";
   }
 }

arrange-act-assert/README.md

@@ -9,22 +9,137 @@ tags:
 ---
 
 ## Also known as
+
 Given/When/Then
 
 ## Intent
-The Arrange/Act/Assert (AAA) is a pattern for organizing unit tests.
+
+Arrange/Act/Assert (AAA) is a pattern for organizing unit tests.
 It breaks tests down into three clear and distinct steps:
+
 1. Arrange: Perform the setup and initialization required for the test.
 2. Act: Take action(s) required for the test.
 3. Assert: Verify the outcome(s) of the test.
 
+## Explanation
+
+This pattern has several significant benefits. It creates a clear separation between a test's
+setup, operations, and results. This structure makes the code easier to read and understand. If
+you place the steps in order and format your code to separate them, you can scan a test and
+quickly comprehend what it does.
+
+It also enforces a certain degree of discipline when you write your tests. You have to think
+clearly about the three steps your test will perform. It makes tests more natural to write at
+the same time since you already have an outline.
+
+Real world example
+
+> We need to write comprehensive and clear unit test suite for a class.
+
+In plain words
+
+> Arrange/Act/Assert is a testing pattern that organizes tests into three clear steps for easy
+> maintenance.
+
+WikiWikiWeb says
+
+> Arrange/Act/Assert is a pattern for arranging and formatting code in UnitTest methods. 
+
+**Programmatic Example**
+
+Let's first introduce our `Cash` class to be unit tested.
+
+```java
+public class Cash {
+
+  private int amount;
+
+  Cash(int amount) {
+    this.amount = amount;
+  }
+
+  void plus(int addend) {
+    amount += addend;
+  }
+
+  boolean minus(int subtrahend) {
+    if (amount >= subtrahend) {
+      amount -= subtrahend;
+      return true;
+    } else {
+      return false;
+    }
+  }
+
+  int count() {
+    return amount;
+  }
+}
+```
+
+Then we write our unit tests according to Arrange/Act/Assert pattern. Notice the clearly
+separated steps for each unit test.
+
+```java
+public class CashAAATest {
+
+  @Test
+  public void testPlus() {
+    //Arrange
+    var cash = new Cash(3);
+    //Act
+    cash.plus(4);
+    //Assert
+    assertEquals(7, cash.count());
+  }
+
+  @Test
+  public void testMinus() {
+    //Arrange
+    var cash = new Cash(8);
+    //Act
+    var result = cash.minus(5);
+    //Assert
+    assertTrue(result);
+    assertEquals(3, cash.count());
+  }
+
+  @Test
+  public void testInsufficientMinus() {
+    //Arrange
+    var cash = new Cash(1);
+    //Act
+    var result = cash.minus(6);
+    //Assert
+    assertFalse(result);
+    assertEquals(1, cash.count());
+  }
+
+  @Test
+  public void testUpdate() {
+    //Arrange
+    var cash = new Cash(5);
+    //Act
+    cash.plus(6);
+    var result = cash.minus(3);
+    //Assert
+    assertTrue(result);
+    assertEquals(8, cash.count());
+  }
+}
+```
+
 ## Applicability
+
 Use Arrange/Act/Assert pattern when
 
-* you need to structure your unit tests so they're easier to read, maintain, and enhance. 
+* You need to structure your unit tests so that they're easier to read, maintain, and enhance. 
 
 ## Credits
 
 * [Arrange, Act, Assert: What is AAA Testing?](https://blog.ncrunch.net/post/arrange-act-assert-aaa-testing.aspx)
 * [Bill Wake: 3A – Arrange, Act, Assert](https://xp123.com/articles/3a-arrange-act-assert/)
 * [Martin Fowler: GivenWhenThen](https://martinfowler.com/bliki/GivenWhenThen.html)
+* [xUnit Test Patterns: Refactoring Test Code](https://www.amazon.com/gp/product/0131495054/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0131495054&linkId=99701e8f4af2f7e8dd50d720c9b63dbf)
+* [Unit Testing Principles, Practices, and Patterns](https://www.amazon.com/gp/product/1617296279/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617296279&linkId=74c75cf22a63c3e4758ae08aa0a0cc35)
+* [Test Driven Development: By Example](https://www.amazon.com/gp/product/0321146530/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0321146530&linkId=5c63a93d8c1175b84ca5087472ef0e05)

arrange-act-assert/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <artifactId>java-design-patterns</artifactId>
     <groupId>com.iluwatar</groupId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <modelVersion>4.0.0</modelVersion>
 

async-method-invocation/pom.xml

@@ -22,7 +22,7 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<artifactId>async-method-invocation</artifactId>
 	<dependencies>

async-method-invocation/src/test/java/com/iluwatar/async/method/invocation/AppTest.java

@@ -25,12 +25,24 @@ package com.iluwatar.async.method.invocation;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  void test() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+
   }
 }

balking/pom.xml

@@ -20,7 +20,7 @@
 	<parent>
 		<artifactId>java-design-patterns</artifactId>
 		<groupId>com.iluwatar</groupId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<modelVersion>4.0.0</modelVersion>
 

balking/src/test/java/com/iluwatar/balking/AppTest.java

@@ -24,15 +24,26 @@
 package com.iluwatar.balking;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.function.Executable;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 
 /**
  * Application test
  */
 class AppTest {
 
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  void main() {
-    App.main();
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow((Executable) App::main);
   }
 
 }

bridge/README.md

@@ -9,20 +9,26 @@ tags:
 ---
 
 ## Also known as
+
 Handle/Body
 
 ## Intent
+
 Decouple an abstraction from its implementation so that the two can vary independently.
 
 ## Explanation
 
 Real world example
 
-> Consider you have a weapon with different enchantments and you are supposed to allow mixing different weapons with different enchantments. What would you do? Create multiple copies of each of the weapons for each of the enchantments or would you just create separate enchantment and set it for the weapon as needed? Bridge pattern allows you to do the second.
+> Consider you have a weapon with different enchantments, and you are supposed to allow mixing 
+> different weapons with different enchantments. What would you do? Create multiple copies of each 
+> of the weapons for each of the enchantments or would you just create separate enchantment and set 
+> it for the weapon as needed? Bridge pattern allows you to do the second.
 
 In Plain Words
 
-> Bridge pattern is about preferring composition over inheritance. Implementation details are pushed from a hierarchy to another object with a separate hierarchy.
+> Bridge pattern is about preferring composition over inheritance. Implementation details are pushed 
+> from a hierarchy to another object with a separate hierarchy.
 
 Wikipedia says
 
@@ -30,7 +36,7 @@ Wikipedia says
 
 **Programmatic Example**
 
-Translating our weapon example from above. Here we have the `Weapon` hierarchy
+Translating our weapon example from above. Here we have the `Weapon` hierarchy:
 
 ```java
 public interface Weapon {
@@ -105,7 +111,7 @@ public class Hammer implements Weapon {
 }
 ```
 
-And the separate enchantment hierarchy
+Here's the separate enchantment hierarchy:
 
 ```java
 public interface Enchantment {
@@ -151,7 +157,7 @@ public class SoulEatingEnchantment implements Enchantment {
 }
 ```
 
-And both the hierarchies in action
+Here are both hierarchies in action:
 
 ```java
 var enchantedSword = new Sword(new SoulEatingEnchantment());
@@ -178,18 +184,21 @@ hammer.unwield();
 ```
 
 ## Class diagram
+
 ![alt text](./etc/bridge.urm.png "Bridge class diagram")
 
 ## Applicability
+
 Use the Bridge pattern when
 
-* you want to avoid a permanent binding between an abstraction and its implementation. This might be the case, for example, when the implementation must be selected or switched at run-time.
-* both the abstractions and their implementations should be extensible by subclassing. In this case, the Bridge pattern lets you combine the different abstractions and implementations and extend them independently
-* changes in the implementation of an abstraction should have no impact on clients; that is, their code should not have to be recompiled.
-* you have a proliferation of classes. Such a class hierarchy indicates the need for splitting an object into two parts. Rumbaugh uses the term "nested generalizations" to refer to such class hierarchies
-* you want to share an implementation among multiple objects (perhaps using reference counting), and this fact should be hidden from the client. A simple example is Coplien's String class, in which multiple objects can share the same string representation.
+* You want to avoid a permanent binding between an abstraction and its implementation. This might be the case, for example, when the implementation must be selected or switched at run-time.
+* Both the abstractions and their implementations should be extensible by subclassing. In this case, the Bridge pattern lets you combine the different abstractions and implementations and extend them independently.
+* Changes in the implementation of an abstraction should have no impact on clients; that is, their code should not have to be recompiled.
+* You have a proliferation of classes. Such a class hierarchy indicates the need for splitting an object into two parts. Rumbaugh uses the term "nested generalizations" to refer to such class hierarchies.
+* You want to share an implementation among multiple objects (perhaps using reference counting), and this fact should be hidden from the client. A simple example is Coplien's String class, in which multiple objects can share the same string representation.
 
 ## Tutorial
+
 * [Bridge Pattern Tutorial](https://www.journaldev.com/1491/bridge-design-pattern-java)
 
 ## Credits

bridge/pom.xml

@@ -22,7 +22,7 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<artifactId>bridge</artifactId>
 	<dependencies>

bridge/src/test/java/com/iluwatar/bridge/AppTest.java

@@ -25,12 +25,22 @@ package com.iluwatar.bridge;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

builder/README.md

@@ -9,36 +9,47 @@ tags:
 ---
 
 ## Intent
-Separate the construction of a complex object from its
-representation so that the same construction process can create different
-representations.
+
+Separate the construction of a complex object from its representation so that the same construction 
+process can create different representations.
 
 ## Explanation
 
 Real world example
 
-> Imagine a character generator for a role playing game. The easiest option is to let computer create the character for you. But if you want to select the character details like profession, gender, hair color etc. the character generation becomes a step-by-step process that completes when all the selections are ready.
+> Imagine a character generator for a role-playing game. The easiest option is to let the computer 
+> create the character for you. If you want to manually select the character details like 
+> profession, gender, hair color etc. the character generation becomes a step-by-step process that 
+> completes when all the selections are ready.
 
 In plain words
 
-> Allows you to create different flavors of an object while avoiding constructor pollution. Useful when there could be several flavors of an object. Or when there are a lot of steps involved in creation of an object.
+> Allows you to create different flavors of an object while avoiding constructor pollution. Useful 
+> when there could be several flavors of an object. Or when there are a lot of steps involved in 
+> creation of an object.
 
 Wikipedia says
 
-> The builder pattern is an object creation software design pattern with the intentions of finding a solution to the telescoping constructor anti-pattern.
+> The builder pattern is an object creation software design pattern with the intentions of finding 
+> a solution to the telescoping constructor anti-pattern.
 
-Having said that let me add a bit about what telescoping constructor anti-pattern is. At one point or the other we have all seen a constructor like below:
+Having said that let me add a bit about what telescoping constructor anti-pattern is. At one point 
+or the other, we have all seen a constructor like below:
 
 ```java
 public Hero(Profession profession, String name, HairType hairType, HairColor hairColor, Armor armor, Weapon weapon) {
 }
 ```
 
-As you can see the number of constructor parameters can quickly get out of hand and it might become difficult to understand the arrangement of parameters. Plus this parameter list could keep on growing if you would want to add more options in future. This is called telescoping constructor anti-pattern.
+As you can see the number of constructor parameters can quickly get out of hand, and it may become 
+difficult to understand the arrangement of parameters. Plus this parameter list could keep on 
+growing if you would want to add more options in the future. This is called telescoping constructor 
+anti-pattern.
 
 **Programmatic Example**
 
-The sane alternative is to use the Builder pattern. First of all we have our hero that we want to create
+The sane alternative is to use the Builder pattern. First of all we have our hero that we want to 
+create:
 
 ```java
 public final class Hero {
@@ -60,7 +71,7 @@ public final class Hero {
 }
 ```
 
-And then we have the builder
+Then we have the builder:
 
 ```java
   public static class Builder {
@@ -105,20 +116,22 @@ And then we have the builder
   }
 ```
 
-And then it can be used as:
+Then it can be used as:
 
 ```java
 var mage = new Hero.Builder(Profession.MAGE, "Riobard").withHairColor(HairColor.BLACK).withWeapon(Weapon.DAGGER).build();
 ```
 
 ## Class diagram
+
 ![alt text](./etc/builder.urm.png "Builder class diagram")
 
 ## Applicability
+
 Use the Builder pattern when
 
-* the algorithm for creating a complex object should be independent of the parts that make up the object and how they're assembled
-* the construction process must allow different representations for the object that's constructed
+* The algorithm for creating a complex object should be independent of the parts that make up the object and how they're assembled
+* The construction process must allow different representations for the object that's constructed
 
 ## Real world examples
 

builder/pom.xml

@@ -22,7 +22,7 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<artifactId>builder</artifactId>
 	<dependencies>

builder/src/test/java/com/iluwatar/builder/AppTest.java

@@ -25,12 +25,23 @@ package com.iluwatar.builder;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

business-delegate/pom.xml

@@ -22,7 +22,7 @@
 	<parent>
 		<groupId>com.iluwatar</groupId>
 		<artifactId>java-design-patterns</artifactId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<artifactId>business-delegate</artifactId>
 	<dependencies>

business-delegate/src/test/java/com/iluwatar/business/delegate/AppTest.java

@@ -27,13 +27,23 @@ import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Business Delegate example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  public void test() throws IOException {
-    String[] args = {};
-    App.main(args);
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

bytecode/pom.xml

@@ -20,7 +20,7 @@
 	<parent>
 		<artifactId>java-design-patterns</artifactId>
 		<groupId>com.iluwatar</groupId>
-		<version>1.23.0-SNAPSHOT</version>
+		<version>1.24.0-SNAPSHOT</version>
 	</parent>
 	<modelVersion>4.0.0</modelVersion>
 

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

@@ -58,12 +58,14 @@ public class App {
     var vm = new VirtualMachine();
     vm.getWizards()[0] = wizard;
 
-    interpretInstruction("LITERAL 0", vm);
-    interpretInstruction("LITERAL 0", vm);
+    String literal = "LITERAL 0";
+
+    interpretInstruction(literal, vm);
+    interpretInstruction(literal, vm);
     interpretInstruction("GET_HEALTH", vm);
-    interpretInstruction("LITERAL 0", vm);
+    interpretInstruction(literal, vm);
     interpretInstruction("GET_AGILITY", vm);
-    interpretInstruction("LITERAL 0", vm);
+    interpretInstruction(literal, vm);
     interpretInstruction("GET_WISDOM ", vm);
     interpretInstruction("ADD", vm);
     interpretInstruction("LITERAL 2", vm);

bytecode/src/test/java/com/iluwatar/bytecode/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.bytecode;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

caching/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <artifactId>caching</artifactId>
   <dependencies>

caching/src/test/java/com/iluwatar/caching/AppTest.java

@@ -27,12 +27,23 @@ import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Caching example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

callback/README.md

@@ -9,14 +9,16 @@ tags:
 ---
 
 ## Intent
-Callback is a piece of executable code that is passed as an argument to other code, which is expected to call back 
-(execute) the argument at some convenient time.
+
+Callback is a piece of executable code that is passed as an argument to other code, which is 
+expected to call back (execute) the argument at some convenient time.
 
 ## Explanation
 
 Real world example
 
-> We need to be notified after executing task has finished. We pass a callback method for the executor and wait for it to call back on us.     
+> We need to be notified after executing task has finished. We pass a callback method for 
+> the executor and wait for it to call back on us.     
 
 In plain words
 
@@ -24,7 +26,9 @@ In plain words
 
 Wikipedia says
 
-> In computer programming, a callback, also known as a "call-after" function, is any executable code that is passed as an argument to other code; that other code is expected to call back (execute) the argument at a given time.
+> In computer programming, a callback, also known as a "call-after" function, is any executable 
+> code that is passed as an argument to other code; that other code is expected to call 
+> back (execute) the argument at a given time.
 
 **Programmatic Example**
 
@@ -61,7 +65,7 @@ public final class SimpleTask extends Task {
 }
 ```
 
-Finally here's how we execute a task and receive a callback when it's finished.
+Finally, here's how we execute a task and receive a callback when it's finished.
 
 ```java
     var task = new SimpleTask();
@@ -69,13 +73,15 @@ Finally here's how we execute a task and receive a callback when it's finished.
 ```
 
 ## Class diagram
+
 ![alt text](./etc/callback.png "Callback")
 
 ## Applicability
+
 Use the Callback pattern when
 
 * when some arbitrary synchronous or asynchronous action must be performed after execution of some defined activity.
 
 ## Real world examples
 
-* [CyclicBarrier](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CyclicBarrier.html#CyclicBarrier%28int,%20java.lang.Runnable%29) constructor can accept callback that will be triggered every time when barrier is tripped.
+* [CyclicBarrier](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CyclicBarrier.html#CyclicBarrier%28int,%20java.lang.Runnable%29) constructor can accept a callback that will be triggered every time a barrier is tripped.

callback/etc/callback.urm.puml

@@ -8,11 +8,6 @@ package com.iluwatar.callback {
   interface Callback {
     + call() {abstract}
   }
-  class LambdasApp {
-    - LOGGER : Logger {static}
-    - LambdasApp()
-    + main(args : String[]) {static}
-  }
   class SimpleTask {
     - LOGGER : Logger {static}
     + SimpleTask()

callback/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <artifactId>callback</artifactId>
   <dependencies>

callback/src/main/java/com/iluwatar/callback/module-info.java

@@ -1,26 +0,0 @@
-/*
- * 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.
- */
-
-module com.iluwatar.callback {
-	requires org.slf4j;
-}

callback/src/test/java/com/iluwatar/callback/AppTest.java

@@ -25,12 +25,23 @@ package com.iluwatar.callback;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Callback example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

chain/README.md

@@ -9,27 +9,32 @@ tags:
 ---
 
 ## Intent
-Avoid coupling the sender of a request to its receiver by giving
-more than one object a chance to handle the request. Chain the receiving
-objects and pass the request along the chain until an object handles it.
+Avoid coupling the sender of a request to its receiver by giving more than one object a chance to 
+handle the request. Chain the receiving objects and pass the request along the chain until an object 
+handles it.
 
 ## Explanation
 
 Real world example
 
-> The Orc King gives loud orders to his army. The closest one to react is the commander, then officer and then soldier. The commander, officer and soldier here form a chain of responsibility.
+> The Orc King gives loud orders to his army. The closest one to react is the commander, then 
+> officer and then soldier. The commander, officer and soldier here form a chain of responsibility.
 
 In plain words
 
-> It helps building a chain of objects. Request enters from one end and keeps going from object to object till it finds the suitable handler.
+> It helps to build a chain of objects. A request enters from one end and keeps going from an object 
+> to another until it finds a suitable handler.
 
 Wikipedia says
 
-> In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain.
+> In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of 
+> a source of command objects and a series of processing objects. Each processing object contains 
+> logic that defines the types of command objects that it can handle; the rest are passed to the 
+> next processing object in the chain.
 
 **Programmatic Example**
 
-Translating our example with orcs from above. First we have the request class
+Translating our example with the orcs from above. First we have the `Request` class:
 
 ```java
 public class Request {
@@ -140,14 +145,16 @@ king.makeRequest(new Request(RequestType.COLLECT_TAX, "collect tax")); // Orc so
 ```
 
 ## Class diagram
+
 ![alt text](./etc/chain.urm.png "Chain of Responsibility class diagram")
 
 ## Applicability
+
 Use Chain of Responsibility when
 
-* more than one object may handle a request, and the handler isn't known a priori. The handler should be ascertained automatically
-* you want to issue a request to one of several objects without specifying the receiver explicitly
-* the set of objects that can handle a request should be specified dynamically
+* More than one object may handle a request, and the handler isn't known a priori. The handler should be ascertained automatically.
+* You want to issue a request to one of several objects without specifying the receiver explicitly.
+* The set of objects that can handle a request should be specified dynamically.
 
 ## Real world examples
 

chain/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <artifactId>chain</artifactId>
   <dependencies>

chain/src/main/java/com/iluwatar/chain/module-info.java

@@ -1,26 +0,0 @@
-/*
- * 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.
- */
-
-module com.iluwatar.chain {
-	requires org.slf4j;
-}

chain/src/test/java/com/iluwatar/chain/AppTest.java

@@ -25,13 +25,23 @@ package com.iluwatar.chain;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * 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}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

circuit-breaker/README.md

@@ -12,167 +12,304 @@ tags:
 
 ## Intent
 
-Handle costly remote *procedure/service* calls in such a way that the failure of a **single** service/component cannot bring the whole application down, and we can reconnect to the service as soon as possible.
+Handle costly remote service calls in such a way that the failure of a single service/component 
+cannot bring the whole application down, and we can reconnect to the service as soon as possible.
 
 ## Explanation
 
 Real world example
 
-> Imagine a Web App that has both local (example: files and images) and remote (example: 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 and 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 without any problem. 
+> 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
 
-> Allows us to save resources when we know a remote service failed. Useful when all parts of our application are highly decoupled from each other, and failure of one component doesn't mean the other parts will stop working.
+> Circuit Breaker allows graceful handling of failed remote services. It's especially useful when 
+> all parts of our application are highly decoupled from each other, and failure of one component 
+> doesn't mean the other parts will stop working.
 
 Wikipedia says
 
-> **Circuit breaker** is a design pattern used in modern software development. It is used to detect failures and encapsulates the logic of preventing a failure from constantly recurring, during maintenance, temporary external system failure or unexpected system difficulties.
-
-So, how does this all come together?
+> Circuit breaker is a design pattern used in modern software development. It is used to detect 
+> failures and encapsulates the logic of preventing a failure from constantly recurring, during 
+> maintenance, temporary external system failure or unexpected system difficulties.
 
 ## Programmatic Example
-With the above example in mind we will imitate the functionality in a simple manner. We have two services: A *monitoring service* which will mimic the web app and will make both **local** and **remote** calls.
+
+So, how does this all come together? With the above example in mind we will imitate the 
+functionality in a simple example. A monitoring service mimics the web app and makes both local and 
+remote calls.
 
 The service architecture is as follows:
 
 ![alt text](./etc/ServiceDiagram.PNG "Service Diagram")
 
-In terms of code, the End user application is:
+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 is: 
+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();
     }
   }
 }
 ```
-As it can be seen, it does the call to get local resources directly, but it wraps the call to remote (costly) service in a circuit breaker object, which prevents faults as follows:
+As it can be seen, it does the call to get local resources directly, but it wraps the call to 
+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;
       }
     }
   }
 }
 ```
 
-How does the above pattern prevent failures? Let's understand via this finite state machine implemented by it.
+How does the above pattern prevent failures? Let's understand via this finite state machine 
+implemented by it.
 
 ![alt text](./etc/StateDiagram.PNG "State Diagram")
 
-- We initialize the Circuit Breaker object with certain parameters: **timeout**, **failureThreshold** and **retryTimePeriod** which help determine how resilient the API is.
-- Initially, we are in the **closed** state and the remote call to API happens.
+- We initialize the Circuit Breaker object with certain parameters: `timeout`, `failureThreshold` and `retryTimePeriod` which help determine how resilient the API is.
+- Initially, we are in the `closed` state and nos remote calls to the API have occurred.
 - Every time the call succeeds, we reset the state to as it was in the beginning.
-- If the number of failures cross a certain threshold, we move to the **open** state, which acts just like an open circuit and prevents remote service calls from being made, thus saving resources. (Here, we return the response called ```stale response from API```)
-- Once we exceed the retry timeout period, we move to the **half-open** state and make another call to the remote service again to check if the service is working so that we can serve fresh content. A *failure* sets it back to **open** state and another attempt is made after retry timeout period, while a *success* sets it to **closed** state so that everything starts working normally again. 
+- If the number of failures cross a certain threshold, we move to the `open` state, which acts just like an open circuit and prevents remote service calls from being made, thus saving resources. (Here, we return the response called ```stale response from API```)
+- Once we exceed the retry timeout period, we move to the `half-open` state and make another call to the remote service again to check if the service is working so that we can serve fresh content. A failure sets it back to `open` state and another attempt is made after retry timeout period, while a success sets it to `closed` state so that everything starts working normally again. 
 
 ## Class diagram
+
 ![alt text](./etc/circuit-breaker.urm.png "Circuit Breaker class diagram")
 
 ## Applicability
+
 Use the Circuit Breaker pattern when
 
 - Building a fault-tolerant application where failure of some services shouldn't bring the entire application down.
-- Building an continuously incremental/continuous delivery application, as some of it's components can be upgraded without shutting it down entirely.
+- Building a continuously running (always-on) application, so that its components can be upgraded without shutting it down entirely.
 
 ## Related Patterns
 

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/pom.xml

@@ -27,7 +27,7 @@
     <parent>
         <groupId>com.iluwatar</groupId>
         <artifactId>java-design-patterns</artifactId>
-        <version>1.23.0-SNAPSHOT</version>
+        <version>1.24.0-SNAPSHOT</version>
     </parent>
     <artifactId>circuit-breaker</artifactId>
     <dependencies>

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

@@ -1,40 +1,35 @@
-/*
- * 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 abstract class Command {
-
-  public abstract void execute(Target target);
-
-  public abstract void undo();
-
-  public abstract void redo();
-
-  @Override
-  public abstract 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.circuitbreaker;
+
+/**
+ * A quick response remote service, that responds healthy without any delay or failure.
+ */
+public class QuickRemoteService implements RemoteService {
+
+  @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

@@ -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;
+
+/**
+ * Exception thrown when {@link RemoteService} does not respond successfully.
+ */
+public class RemoteServiceException extends Exception {
+
+  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");
   }
 }

collection-pipeline/pom.xml

@@ -27,7 +27,7 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
   <artifactId>collection-pipeline</artifactId>
   <dependencies>

collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/module-info.java

@@ -1,26 +0,0 @@
-/*
- * 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.
- */
-
-module com.iluwatar.collectionpipeline {
-	requires org.slf4j;
-}

combinator/pom.xml

@@ -29,7 +29,7 @@
   <parent>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.23.0-SNAPSHOT</version>
+    <version>1.24.0-SNAPSHOT</version>
   </parent>
 
   <artifactId>combinator</artifactId>
@@ -39,6 +39,12 @@
       <artifactId>junit</artifactId>
       <scope>test</scope>
     </dependency>
+
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
 </project>

combinator/src/test/java/com/iluwatar/combinator/CombinatorAppTest.java

@@ -25,12 +25,19 @@ package com.iluwatar.combinator;
 
 import org.junit.Test;
 
-import static org.junit.Assert.*;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 
 public class CombinatorAppTest {
 
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link CombinatorApp#main(String[])}
+   * throws an exception.
+   */
+
   @Test
-  public void main() {
-    CombinatorApp.main(new String[]{});
+  public void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> CombinatorApp.main(new String[]{}));
   }
 }

command/README.md

@@ -9,15 +9,20 @@ tags:
 ---
 
 ## Also known as
+
 Action, Transaction
 
 ## Intent
-Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
+
+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 the spells one by one. Each spell here is a command object that can be undone.
+> 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
 
@@ -25,34 +30,32 @@ 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 a later time.
+> 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**
 
-Here's the sample code with wizard and goblin. Let's start from the wizard class.
+Here's the sample code with wizard and goblin. Let's start from the `Wizard` class.
 
 ```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();
     }
   }
 
@@ -60,8 +63,7 @@ public class Wizard {
     if (!redoStack.isEmpty()) {
       var previousSpell = redoStack.pollLast();
       undoStack.offerLast(previousSpell);
-      LOGGER.info("{} redoes {}", this, previousSpell);
-      previousSpell.redo();
+      previousSpell.run();
     }
   }
 
@@ -72,85 +74,7 @@ public class Wizard {
 }
 ```
 
-Next we present the spell hierarchy.
-
-```java
-public abstract class Command {
-
-  public abstract void execute(Target target);
-
-  public abstract void undo();
-
-  public abstract void redo();
-
-  @Override
-  public abstract String toString();
-}
-
-public class InvisibilitySpell extends 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 extends 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";
-  }
-}
-```
-
-And last 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 {
@@ -197,47 +121,110 @@ 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 here's the whole example in action.
+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();
-// Goblin, [size=normal] [visibility=visible]
-wizard.castSpell(new ShrinkSpell(), goblin);
-// Wizard casts Shrink spell at Goblin
+wizard.castSpell(goblin::changeSize);
 goblin.printStatus();
-// Goblin, [size=small] [visibility=visible]
-wizard.castSpell(new InvisibilitySpell(), goblin);
-// Wizard casts Invisibility spell at Goblin
+
+wizard.castSpell(goblin::changeVisibility);
 goblin.printStatus();
-// Goblin, [size=small] [visibility=invisible]
+
 wizard.undoLastSpell();
-// Wizard undoes Invisibility spell
 goblin.printStatus();
-// Goblin, [size=small] [visibility=visible]
+
+wizard.undoLastSpell();
+goblin.printStatus();
+
+wizard.redoLastSpell();
+goblin.printStatus();
+
+wizard.redoLastSpell();
+goblin.printStatus();
+```
+
+Here's the program output:
+
+```java
+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
+
 ![alt text](./etc/command.png "Command")
 
 ## Applicability
-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 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 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 Unexecute operation that reverses the effects of a previous call to execute. Executed commands are stored in a history list. Unlimited-level undo and redo is achieved by traversing this list backwards and forwards calling unexecute 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 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 system with new transactions
+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
+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
+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
+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
+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
+system with new transactions.
 
 ## Typical Use Case
 
-* to keep a history of requests
-* implement callback functionality
-* implement the undo functionality
+* To keep a history of requests
+* Implement callback functionality
+* Implement the undo functionality
 
 ## Real world examples