Fix compile error

Fix checkstyle errors
Fix flux tests
2020-07-30 19:38:52 +03:00 · 2020-07-30 18:24:55 +03:00 · 2020-07-30 18:07:34 +03:00 · 2020-07-30 17:25:32 +03:00 · 2020-07-29 21:03:29 +03:00 · 2020-07-29 20:42:45 +03:00
245 changed files with 2121 additions and 1170 deletions
--- a/.all-contributorsrc
+++ b/.all-contributorsrc
@ -218,9 +218,829 @@
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  "projectOwner": "iluwatar",
  "repoType": "github",
--- a/README.md
+++ b/README.md
@ -9,7 +9,7 @@
 [![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-23-orange.svg?style=flat-square)](#contributors-)
+[![All Contributors](https://img.shields.io/badge/all_contributors-114-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->

 # Introduction
@ -73,36 +73,175 @@ This project is licensed under the terms of the MIT license.
    <td align="center"><a href="https://github.com/amit1307"><img src="https://avatars0.githubusercontent.com/u/23420222?v=4" width="100px;" alt=""/><br /><sub><b>amit1307</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amit1307" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/npathai"><img src="https://avatars2.githubusercontent.com/u/1792515?v=4" width="100px;" alt=""/><br /><sub><b>Narendra Pathai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=npathai" title="Code">💻</a> <a href="#ideas-npathai" title="Ideas, Planning, & Feedback">🤔</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Anpathai" title="Reviewed Pull Requests">👀</a></td>
    <td align="center"><a href="https://github.com/fluxw42"><img src="https://avatars1.githubusercontent.com/u/1545460?v=4" width="100px;" alt=""/><br /><sub><b>Jeroen Meulemeester</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fluxw42" title="Code">💻</a></td>
+  </tr>
+  <tr>
    <td align="center"><a href="http://www.joemccarthy.co.uk"><img src="https://avatars0.githubusercontent.com/u/4526195?v=4" width="100px;" alt=""/><br /><sub><b>Joseph McCarthy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mikulucky" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/thomasoss"><img src="https://avatars1.githubusercontent.com/u/22516154?v=4" width="100px;" alt=""/><br /><sub><b>Thomas</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=thomasoss" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/anuragagarwal561994"><img src="https://avatars1.githubusercontent.com/u/6075379?v=4" width="100px;" alt=""/><br /><sub><b>Anurag Agarwal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=anuragagarwal561994" title="Code">💻</a></td>
+    <td align="center"><a href="https://markusmo3.github.io"><img src="https://avatars1.githubusercontent.com/u/3317416?v=4" width="100px;" alt=""/><br /><sub><b>Markus Moser</b></sub></a><br /><a href="#design-markusmo3" title="Design">🎨</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=markusmo3" title="Code">💻</a> <a href="#ideas-markusmo3" title="Ideas, Planning, & Feedback">🤔</a></td>
  </tr>
  <tr>
-    <td align="center"><a href="https://markusmo3.github.io"><img src="https://avatars1.githubusercontent.com/u/3317416?v=4" width="100px;" alt=""/><br /><sub><b>Markus Moser</b></sub></a><br /><a href="#design-markusmo3" title="Design">🎨</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=markusmo3" title="Code">💻</a> <a href="#ideas-markusmo3" title="Ideas, Planning, & Feedback">🤔</a></td>
    <td align="center"><a href="https://twitter.com/i_sabiq"><img src="https://avatars1.githubusercontent.com/u/19510920?v=4" width="100px;" alt=""/><br /><sub><b>Sabiq Ihab</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=isabiq" title="Code">💻</a></td>
    <td align="center"><a href="http://inbravo.github.io"><img src="https://avatars3.githubusercontent.com/u/5253764?v=4" width="100px;" alt=""/><br /><sub><b>Amit Dixit</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=inbravo" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/piyushchaudhari04"><img src="https://avatars3.githubusercontent.com/u/10268029?v=4" width="100px;" alt=""/><br /><sub><b>Piyush Kailash Chaudhari</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=piyushchaudhari04" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/joshzambales"><img src="https://avatars1.githubusercontent.com/u/8704552?v=4" width="100px;" alt=""/><br /><sub><b>joshzambales</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=joshzambales" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/Crossy147"><img src="https://avatars2.githubusercontent.com/u/7272996?v=4" width="100px;" alt=""/><br /><sub><b>Kamil Pietruszka</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Crossy147" title="Code">💻</a></td>
-    <td align="center"><a href="http://cs.joensuu.fi/~zkhayda"><img src="https://avatars2.githubusercontent.com/u/660742?v=4" width="100px;" alt=""/><br /><sub><b>Zafar Khaydarov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Documentation">📖</a></td>
  </tr>
  <tr>
+    <td align="center"><a href="https://github.com/Crossy147"><img src="https://avatars2.githubusercontent.com/u/7272996?v=4" width="100px;" alt=""/><br /><sub><b>Kamil Pietruszka</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Crossy147" title="Code">💻</a></td>
+    <td align="center"><a href="http://cs.joensuu.fi/~zkhayda"><img src="https://avatars2.githubusercontent.com/u/660742?v=4" width="100px;" alt=""/><br /><sub><b>Zafar Khaydarov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=zafarella" title="Documentation">📖</a></td>
    <td align="center"><a href="https://kemitix.github.io/"><img src="https://avatars1.githubusercontent.com/u/1147749?v=4" width="100px;" alt=""/><br /><sub><b>Paul Campbell</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kemitix" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/Argyro-Sioziou"><img src="https://avatars0.githubusercontent.com/u/22822639?v=4" width="100px;" alt=""/><br /><sub><b>Argyro Sioziou</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Argyro-Sioziou" title="Code">💻</a></td>
+  </tr>
+  <tr>
    <td align="center"><a href="https://github.com/TylerMcConville"><img src="https://avatars0.githubusercontent.com/u/4946449?v=4" width="100px;" alt=""/><br /><sub><b>TylerMcConville</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=TylerMcConville" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/saksham93"><img src="https://avatars1.githubusercontent.com/u/37399540?v=4" width="100px;" alt=""/><br /><sub><b>saksham93</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=saksham93" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/nikhilbarar"><img src="https://avatars2.githubusercontent.com/u/37332144?v=4" width="100px;" alt=""/><br /><sub><b>nikhilbarar</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nikhilbarar" title="Code">💻</a></td>
    <td align="center"><a href="http://colinbut.com"><img src="https://avatars2.githubusercontent.com/u/10725674?v=4" width="100px;" alt=""/><br /><sub><b>Colin But</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=colinbut" title="Code">💻</a></td>
-    <td align="center"><a href="https://github.com/ruslanpa"><img src="https://avatars2.githubusercontent.com/u/1503411?v=4" width="100px;" alt=""/><br /><sub><b>Ruslan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ruslanpa" title="Code">💻</a></td>
  </tr>
  <tr>
+    <td align="center"><a href="https://github.com/ruslanpa"><img src="https://avatars2.githubusercontent.com/u/1503411?v=4" width="100px;" alt=""/><br /><sub><b>Ruslan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ruslanpa" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/JuhoKang"><img src="https://avatars1.githubusercontent.com/u/4745294?v=4" width="100px;" alt=""/><br /><sub><b>Juho Kang</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=JuhoKang" title="Code">💻</a></td>
    <td align="center"><a href="https://github.com/dheeraj-mummareddy"><img src="https://avatars2.githubusercontent.com/u/7002230?v=4" width="100px;" alt=""/><br /><sub><b>Dheeraj Mummareddy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dheeraj-mummareddy" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.bernardosulzbach.com"><img src="https://avatars0.githubusercontent.com/u/8271090?v=4" width="100px;" alt=""/><br /><sub><b>Bernardo Sulzbach</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=bernardosulzbach" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/4lexis"><img src="https://avatars0.githubusercontent.com/u/19871727?v=4" width="100px;" alt=""/><br /><sub><b>Aleksandar Dudukovic</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=4lexis" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.yusufaytas.com"><img src="https://avatars2.githubusercontent.com/u/1049483?v=4" width="100px;" alt=""/><br /><sub><b>Yusuf Aytaş</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yusufaytas" title="Code">💻</a></td>
+    <td align="center"><a href="http://futurehomes.hu"><img src="https://avatars2.githubusercontent.com/u/1001491?v=4" width="100px;" alt=""/><br /><sub><b>Mihály Kuprivecz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=qpi" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/kapinuss"><img src="https://avatars0.githubusercontent.com/u/17639945?v=4" width="100px;" alt=""/><br /><sub><b>Stanislav Kapinus</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kapinuss" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/gvsharma"><img src="https://avatars1.githubusercontent.com/u/6648152?v=4" width="100px;" alt=""/><br /><sub><b>GVSharma</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=gvsharma" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/SrdjanPaunovic"><img src="https://avatars1.githubusercontent.com/u/22815104?v=4" width="100px;" alt=""/><br /><sub><b>Srđan Paunović</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=SrdjanPaunovic" title="Code">💻</a></td>
+    <td align="center"><a href="https://sideris.xyz/"><img src="https://avatars3.githubusercontent.com/u/5484694?v=4" width="100px;" alt=""/><br /><sub><b>Petros G. Sideris</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=sideris" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/pramodgupta3/"><img src="https://avatars1.githubusercontent.com/u/2184241?v=4" width="100px;" alt=""/><br /><sub><b>Pramod Gupta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AIAmPramod" title="Reviewed Pull Requests">👀</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://amarnath510.github.io/portfolio"><img src="https://avatars0.githubusercontent.com/u/4599623?v=4" width="100px;" alt=""/><br /><sub><b>Amarnath Chandana</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Amarnath510" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Anurag870"><img src="https://avatars1.githubusercontent.com/u/6295975?v=4" width="100px;" alt=""/><br /><sub><b>Anurag870</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Anurag870" title="Code">💻</a></td>
+    <td align="center"><a href="http://theerroris.me"><img src="https://avatars0.githubusercontent.com/u/1685953?v=4" width="100px;" alt=""/><br /><sub><b>Wes Gilleland</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Deathnerd" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Harshrajsinh"><img src="https://avatars2.githubusercontent.com/u/22811531?v=4" width="100px;" alt=""/><br /><sub><b>Harshraj Thakor</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Harshrajsinh" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/MaVdbussche"><img src="https://avatars1.githubusercontent.com/u/26136934?v=4" width="100px;" alt=""/><br /><sub><b>Martin Vandenbussche</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=MaVdbussche" title="Code">💻</a></td>
+    <td align="center"><a href="https://alexsomai.com"><img src="https://avatars1.githubusercontent.com/u/5720977?v=4" width="100px;" alt=""/><br /><sub><b>Alexandru Somai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=alexsomai" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/amogozov"><img src="https://avatars3.githubusercontent.com/u/7372215?v=4" width="100px;" alt=""/><br /><sub><b>Artur Mogozov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amogozov" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/anthonycampbell"><img src="https://avatars3.githubusercontent.com/u/10249255?v=4" width="100px;" alt=""/><br /><sub><b>anthony</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=anthonycampbell" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://ccygnus.com/"><img src="https://avatars1.githubusercontent.com/u/9342724?v=4" width="100px;" alt=""/><br /><sub><b>Christian Cygnus</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=christophercolumbusdog" title="Code">💻</a></td>
+    <td align="center"><a href="https://about.me/dzmitryh"><img src="https://avatars2.githubusercontent.com/u/5390492?v=4" width="100px;" alt=""/><br /><sub><b>Dima Gubin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dzmitryh" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/jjjimenez100"><img src="https://avatars3.githubusercontent.com/u/22243493?v=4" width="100px;" alt=""/><br /><sub><b>Joshua Jimenez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=jjjimenez100" title="Code">💻</a></td>
+    <td align="center"><a href="http://about.me/kaiwinter"><img src="https://avatars0.githubusercontent.com/u/110982?v=4" width="100px;" alt=""/><br /><sub><b>Kai Winter</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kaiwinter" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/lbroman"><img src="https://avatars1.githubusercontent.com/u/86007?v=4" width="100px;" alt=""/><br /><sub><b>lbroman</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=lbroman" title="Code">💻</a></td>
+    <td align="center"><a href="https://przemeknowak.com"><img src="https://avatars1.githubusercontent.com/u/3254609?v=4" width="100px;" alt=""/><br /><sub><b>Przemek</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=pnowy" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/prafful1"><img src="https://avatars0.githubusercontent.com/u/14350274?v=4" width="100px;" alt=""/><br /><sub><b>Prafful Agarwal</b></sub></a><br /><a href="#content-prafful1" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/sankypanhale"><img src="https://avatars1.githubusercontent.com/u/6478783?v=4" width="100px;" alt=""/><br /><sub><b>Sanket Panhale</b></sub></a><br /><a href="#content-sankypanhale" title="Content">🖋</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/staillebois"><img src="https://avatars0.githubusercontent.com/u/23701200?v=4" width="100px;" alt=""/><br /><sub><b>staillebois</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=staillebois" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/valdar-hu"><img src="https://avatars3.githubusercontent.com/u/17962817?v=4" width="100px;" alt=""/><br /><sub><b>Krisztián Nagy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=valdar-hu" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.vanogrid.com"><img src="https://avatars0.githubusercontent.com/u/4307918?v=4" width="100px;" alt=""/><br /><sub><b>Alexander Ivanov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vanogrid" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/yosfik"><img src="https://avatars3.githubusercontent.com/u/4850270?v=4" width="100px;" alt=""/><br /><sub><b>Yosfik Alqadri</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yosfik" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/7agustibm"><img src="https://avatars0.githubusercontent.com/u/8149332?v=4" width="100px;" alt=""/><br /><sub><b>Agustí Becerra Milà</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=7agustibm" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Juaanma"><img src="https://avatars3.githubusercontent.com/u/7390500?v=4" width="100px;" alt=""/><br /><sub><b>Juan Manuel Suárez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Juaanma" title="Code">💻</a></td>
+    <td align="center"><a href="http://www.devsedge.net/"><img src="https://avatars0.githubusercontent.com/u/9956006?v=4" width="100px;" alt=""/><br /><sub><b>Luigi Cortese</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=LuigiCortese" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Rzeposlaw"><img src="https://avatars2.githubusercontent.com/u/18425745?v=4" width="100px;" alt=""/><br /><sub><b>Katarzyna Rzepecka</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Rzeposlaw" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://adamski.pro"><img src="https://avatars1.githubusercontent.com/u/6537430?v=4" width="100px;" alt=""/><br /><sub><b>adamski.pro</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=akrystian" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/baislsl"><img src="https://avatars0.githubusercontent.com/u/17060584?v=4" width="100px;" alt=""/><br /><sub><b>Shengli Bai</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=baislsl" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/besok"><img src="https://avatars2.githubusercontent.com/u/29834592?v=4" width="100px;" alt=""/><br /><sub><b>Boris</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=besok" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/dmitraver"><img src="https://avatars3.githubusercontent.com/u/1798156?v=4" width="100px;" alt=""/><br /><sub><b>Dmitry Avershin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dmitraver" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/fanofxiaofeng"><img src="https://avatars0.githubusercontent.com/u/3983683?v=4" width="100px;" alt=""/><br /><sub><b>靳阳</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=fanofxiaofeng" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/hoangnam2261"><img src="https://avatars2.githubusercontent.com/u/31692990?v=4" width="100px;" alt=""/><br /><sub><b>hoangnam2261</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hoangnam2261" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/jarpit96"><img src="https://avatars2.githubusercontent.com/u/10098713?v=4" width="100px;" alt=""/><br /><sub><b>Arpit Jain</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=jarpit96" title="Code">💻</a></td>
+    <td align="center"><a href="http://joningi.net"><img src="https://avatars2.githubusercontent.com/u/6115148?v=4" width="100px;" alt=""/><br /><sub><b>Jón Ingi Sveinbjörnsson</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=joningiwork" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/kirill-vlasov"><img src="https://avatars3.githubusercontent.com/u/16112495?v=4" width="100px;" alt=""/><br /><sub><b>Kirill Vlasov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kirill-vlasov" title="Code">💻</a></td>
+    <td align="center"><a href="http://mitchell-irvin.com"><img src="https://avatars0.githubusercontent.com/u/16233245?v=4" width="100px;" alt=""/><br /><sub><b>Mitchell Irvin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mitchellirvin" title="Code">💻</a></td>
+    <td align="center"><a href="https://ranjeet-floyd.github.io"><img src="https://avatars0.githubusercontent.com/u/1992972?v=4" width="100px;" alt=""/><br /><sub><b>Ranjeet</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ranjeet-floyd" title="Code">💻</a></td>
+    <td align="center"><a href="https://alwayswithme.github.io"><img src="https://avatars3.githubusercontent.com/u/3234786?v=4" width="100px;" alt=""/><br /><sub><b>PhoenixYip</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Alwayswithme" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/MSaifAsif"><img src="https://avatars1.githubusercontent.com/u/6280554?v=4" width="100px;" alt=""/><br /><sub><b>M Saif Asif</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=MSaifAsif" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/kanwarpreet25"><img src="https://avatars0.githubusercontent.com/u/39183641?v=4" width="100px;" alt=""/><br /><sub><b>kanwarpreet25</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=kanwarpreet25" title="Code">💻</a></td>
+    <td align="center"><a href="http://leonmak.me"><img src="https://avatars3.githubusercontent.com/u/13071508?v=4" width="100px;" alt=""/><br /><sub><b>Leon Mak</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=leonmak" title="Code">💻</a></td>
+    <td align="center"><a href="http://www.wramdemark.se"><img src="https://avatars2.githubusercontent.com/u/7052193?v=4" width="100px;" alt=""/><br /><sub><b>Per Wramdemark</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=perwramdemark" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/waisuan"><img src="https://avatars2.githubusercontent.com/u/10975700?v=4" width="100px;" alt=""/><br /><sub><b>Evan Sia Wai Suan</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=waisuan" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/AnaghaSasikumar"><img src="https://avatars2.githubusercontent.com/u/42939261?v=4" width="100px;" alt=""/><br /><sub><b>AnaghaSasikumar</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=AnaghaSasikumar" title="Code">💻</a></td>
+    <td align="center"><a href="https://christofferh.com"><img src="https://avatars1.githubusercontent.com/u/767643?v=4" width="100px;" alt=""/><br /><sub><b>Christoffer Hamberg</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=christofferh" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/dgruntz"><img src="https://avatars0.githubusercontent.com/u/1516800?v=4" width="100px;" alt=""/><br /><sub><b>Dominik Gruntz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=dgruntz" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://bitbucket.org/hannespernpeintner/"><img src="https://avatars3.githubusercontent.com/u/1679437?v=4" width="100px;" alt=""/><br /><sub><b>Hannes</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hannespernpeintner" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/leogtzr"><img src="https://avatars0.githubusercontent.com/u/1211969?v=4" width="100px;" alt=""/><br /><sub><b>Leo Gutiérrez Ramírez</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=leogtzr" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/npczwh"><img src="https://avatars0.githubusercontent.com/u/14066422?v=4" width="100px;" alt=""/><br /><sub><b>Zhang WH</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=npczwh" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/oconnelc"><img src="https://avatars0.githubusercontent.com/u/1112973?v=4" width="100px;" alt=""/><br /><sub><b>Christopher O'Connell</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=oconnelc" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/giorgosmav21"><img src="https://avatars2.githubusercontent.com/u/22855493?v=4" width="100px;" alt=""/><br /><sub><b>George Mavroeidis</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=giorgosmav21" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/hbothra15"><img src="https://avatars1.githubusercontent.com/u/7418012?v=4" width="100px;" alt=""/><br /><sub><b>Hemant Bothra</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hbothra15" title="Code">💻</a> <a href="#design-hbothra15" title="Design">🎨</a></td>
+    <td align="center"><a href="https://www.kevinpeters.net/about/"><img src="https://avatars1.githubusercontent.com/u/12736734?v=4" width="100px;" alt=""/><br /><sub><b>Kevin Peters</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=igeligel" title="Code">💻</a></td>
+    <td align="center"><a href="https://llorllale.github.io/"><img src="https://avatars1.githubusercontent.com/u/2019896?v=4" width="100px;" alt=""/><br /><sub><b>George Aristy</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=llorllale" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/mookkiah"><img src="https://avatars1.githubusercontent.com/u/8975264?v=4" width="100px;" alt=""/><br /><sub><b>Mahendran Mookkiah</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mookkiah" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Azureyjt"><img src="https://avatars2.githubusercontent.com/u/18476317?v=4" width="100px;" alt=""/><br /><sub><b>Azureyjt</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Azureyjt" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/vehpsr"><img src="https://avatars2.githubusercontent.com/u/3133265?v=4" width="100px;" alt=""/><br /><sub><b>gans</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vehpsr" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ThatGuyWithTheHat"><img src="https://avatars0.githubusercontent.com/u/24470582?v=4" width="100px;" alt=""/><br /><sub><b>Matt</b></sub></a><br /><a href="#content-ThatGuyWithTheHat" title="Content">🖋</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://www.linkedin.com/in/gopinathlangote/"><img src="https://avatars2.githubusercontent.com/u/10210778?v=4" width="100px;" alt=""/><br /><sub><b>Gopinath Langote</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=gopinath-langote" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/hoswey"><img src="https://avatars3.githubusercontent.com/u/3689445?v=4" width="100px;" alt=""/><br /><sub><b>Hoswey</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=hoswey" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/amit2103"><img src="https://avatars3.githubusercontent.com/u/7566692?v=4" width="100px;" alt=""/><br /><sub><b>Amit Pandey</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=amit2103" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/gwildor28"><img src="https://avatars0.githubusercontent.com/u/16000365?v=4" width="100px;" alt=""/><br /><sub><b>gwildor28</b></sub></a><br /><a href="#content-gwildor28" title="Content">🖋</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://t.me/paul_docker"><img src="https://avatars1.githubusercontent.com/u/2404785?v=4" width="100px;" alt=""/><br /><sub><b>田浩</b></sub></a><br /><a href="#content-llitfkitfk" title="Content">🖋</a></td>
+    <td align="center"><a href="https://twitter.com/StPitsios"><img src="https://avatars1.githubusercontent.com/u/6773603?v=4" width="100px;" alt=""/><br /><sub><b>Stamatis Pitsios</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=pitsios-s" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/qza"><img src="https://avatars3.githubusercontent.com/u/233149?v=4" width="100px;" alt=""/><br /><sub><b>qza</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=qza" title="Code">💻</a></td>
+    <td align="center"><a href="http://tschis.github.io"><img src="https://avatars1.githubusercontent.com/u/20662669?v=4" width="100px;" alt=""/><br /><sub><b>Rodolfo Forte</b></sub></a><br /><a href="#content-Tschis" title="Content">🖋</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/ankurkaushal"><img src="https://avatars2.githubusercontent.com/u/2236616?v=4" width="100px;" alt=""/><br /><sub><b>Ankur Kaushal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ankurkaushal" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/ovidijus-okinskas/"><img src="https://avatars0.githubusercontent.com/u/20372387?v=4" width="100px;" alt=""/><br /><sub><b>Ovidijus Okinskas</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=okinskas" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/robertt240"><img src="https://avatars1.githubusercontent.com/u/9137432?v=4" width="100px;" alt=""/><br /><sub><b>Robert Kasperczyk</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=robertt240" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/trautonen"><img src="https://avatars3.githubusercontent.com/u/1641063?v=4" width="100px;" alt=""/><br /><sub><b>Tapio Rautonen</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=trautonen" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="http://vk.com/yuri.orlov"><img src="https://avatars0.githubusercontent.com/u/1595733?v=4" width="100px;" alt=""/><br /><sub><b>Yuri Orlov</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yorlov" title="Code">💻</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/varunu28/"><img src="https://avatars0.githubusercontent.com/u/7676016?v=4" width="100px;" alt=""/><br /><sub><b>Varun Upadhyay</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=varunu28" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/PalAditya"><img src="https://avatars2.githubusercontent.com/u/25523604?v=4" width="100px;" alt=""/><br /><sub><b>Aditya Pal</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=PalAditya" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/grzesiekkedzior"><img src="https://avatars3.githubusercontent.com/u/23739158?v=4" width="100px;" alt=""/><br /><sub><b>grzesiekkedzior</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=grzesiekkedzior" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/sivasubramanim"><img src="https://avatars2.githubusercontent.com/u/51107434?v=4" width="100px;" alt=""/><br /><sub><b>Sivasubramani M</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=sivasubramanim" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/d4gg4d"><img src="https://avatars2.githubusercontent.com/u/99457?v=4" width="100px;" alt=""/><br /><sub><b>Sami Airaksinen</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=d4gg4d" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/vertti"><img src="https://avatars0.githubusercontent.com/u/557751?v=4" width="100px;" alt=""/><br /><sub><b>Janne Sinivirta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=vertti" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Bobo1239"><img src="https://avatars1.githubusercontent.com/u/2302947?v=4" width="100px;" alt=""/><br /><sub><b>Boris-Chengbiao Zhou</b></sub></a><br /><a href="#content-Bobo1239" title="Content">🖋</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://jahhein.github.io"><img src="https://avatars2.githubusercontent.com/u/10779515?v=4" width="100px;" alt=""/><br /><sub><b>Jacob Hein</b></sub></a><br /><a href="#content-Jahhein" title="Content">🖋</a></td>
+    <td align="center"><a href="https://github.com/iamrichardjones"><img src="https://avatars3.githubusercontent.com/u/14842151?v=4" width="100px;" alt=""/><br /><sub><b>Richard Jones</b></sub></a><br /><a href="#content-iamrichardjones" title="Content">🖋</a></td>
+    <td align="center"><a href="https://rachelcarmena.github.io"><img src="https://avatars0.githubusercontent.com/u/22792183?v=4" width="100px;" alt=""/><br /><sub><b>Rachel M. Carmena</b></sub></a><br /><a href="#content-rachelcarmena" title="Content">🖋</a></td>
+    <td align="center"><a href="https://zd-zero.github.io"><img src="https://avatars0.githubusercontent.com/u/21978370?v=4" width="100px;" alt=""/><br /><sub><b>Zaerald Denze Lungos</b></sub></a><br /><a href="#content-zd-zero" title="Content">🖋</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://webpro.nl"><img src="https://avatars1.githubusercontent.com/u/456426?v=4" width="100px;" alt=""/><br /><sub><b>Lars Kappert</b></sub></a><br /><a href="#content-webpro" title="Content">🖋</a></td>
+    <td align="center"><a href="https://xiaod.info"><img src="https://avatars2.githubusercontent.com/u/21277644?v=4" width="100px;" alt=""/><br /><sub><b>Mike Liu</b></sub></a><br /><a href="#translation-xiaod-dev" title="Translation">🌍</a></td>
  </tr>
 </table>

 <!-- markdownlint-enable -->
 <!-- prettier-ignore-end -->
 <!-- ALL-CONTRIBUTORS-LIST:END -->
-
-This project follows the [all-contributors](https://github.com/all-contributors/all-contributors) specification. Contributions of any kind welcome!
--- a/abstract-document/src/test/java/com/iluwatar/abstractdocument/AbstractDocumentTest.java
+++ b/abstract-document/src/test/java/com/iluwatar/abstractdocument/AbstractDocumentTest.java
@ -47,7 +47,7 @@ public class AbstractDocumentTest {
    }
  }

-  private DocumentImplementation document = new DocumentImplementation(new HashMap<>());
+  private final DocumentImplementation document = new DocumentImplementation(new HashMap<>());

  @Test
  public void shouldPutAndGetValue() {
--- a/abstract-factory/src/test/java/com/iluwatar/abstractfactory/AbstractFactoryTest.java
+++ b/abstract-factory/src/test/java/com/iluwatar/abstractfactory/AbstractFactoryTest.java
@ -36,7 +36,7 @@ import org.junit.jupiter.api.Test;
 */
 public class AbstractFactoryTest {

-  private App app = new App();
+  private final App app = new App();
  private KingdomFactory elfFactory;
  private KingdomFactory orcFactory;

--- a/acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitorTest.java
+++ b/acyclic-visitor/src/test/java/com/iluwatar/acyclicvisitor/ConfigureForDosVisitorTest.java
@ -37,7 +37,7 @@ import uk.org.lidalia.slf4jtest.TestLoggerFactory;
 */
 public class ConfigureForDosVisitorTest {

-  private TestLogger logger = TestLoggerFactory.getTestLogger(ConfigureForDosVisitor.class);
+  private final TestLogger logger = TestLoggerFactory.getTestLogger(ConfigureForDosVisitor.class);
  
  @Test
  public void testVisitForZoom() {    
--- a/adapter/README.md
+++ b/adapter/README.md
@ -56,7 +56,7 @@ And captain expects an implementation of `RowingBoat` interface to be able to mo
 ```java
 public class Captain {

-  private RowingBoat rowingBoat;
+  private final RowingBoat rowingBoat;
  // default constructor and setter for rowingBoat
  public Captain(RowingBoat rowingBoat) {
    this.rowingBoat = rowingBoat;
@ -75,7 +75,7 @@ public class FishingBoatAdapter implements RowingBoat {

  private static final Logger LOGGER = LoggerFactory.getLogger(FishingBoatAdapter.class);

-  private FishingBoat boat;
+  private final FishingBoat boat;

  public FishingBoatAdapter() {
    boat = new FishingBoat();
--- a/adapter/src/main/java/com/iluwatar/adapter/FishingBoatAdapter.java
+++ b/adapter/src/main/java/com/iluwatar/adapter/FishingBoatAdapter.java
@ -29,7 +29,7 @@ package com.iluwatar.adapter;
 */
 public class FishingBoatAdapter implements RowingBoat {

-  private FishingBoat boat;
+  private final FishingBoat boat;

  public FishingBoatAdapter() {
    boat = new FishingBoat();
--- a/ambassador/src/test/java/com/iluwatar/ambassador/RemoteServiceTest.java
+++ b/ambassador/src/test/java/com/iluwatar/ambassador/RemoteServiceTest.java
@ -48,7 +48,7 @@ class RemoteServiceTest {
  }

  private static class StaticRandomProvider implements RandomProvider {
-    private double value;
+    private final double value;

    StaticRandomProvider(double value) {
      this.value = value;
--- a/async-method-invocation/src/main/java/com/iluwatar/async/method/invocation/ThreadAsyncExecutor.java
+++ b/async-method-invocation/src/main/java/com/iluwatar/async/method/invocation/ThreadAsyncExecutor.java
@ -100,7 +100,7 @@ public class ThreadAsyncExecutor implements AsyncExecutor {
    void setValue(T value) {
      this.value = value;
      this.state = COMPLETED;
-      this.callback.ifPresent(ac -> ac.onComplete(value, Optional.<Exception>empty()));
+      this.callback.ifPresent(ac -> ac.onComplete(value, Optional.empty()));
      synchronized (lock) {
        lock.notifyAll();
      }
--- a/business-delegate/src/main/java/com/iluwatar/business/delegate/Client.java
+++ b/business-delegate/src/main/java/com/iluwatar/business/delegate/Client.java
@ -28,7 +28,7 @@ package com.iluwatar.business.delegate;
 */
 public class Client {

-  private BusinessDelegate businessDelegate;
+  private final BusinessDelegate businessDelegate;

  public Client(BusinessDelegate businessDelegate) {
    this.businessDelegate = businessDelegate;
--- a/business-delegate/src/main/java/com/iluwatar/business/delegate/ServiceType.java
+++ b/business-delegate/src/main/java/com/iluwatar/business/delegate/ServiceType.java
@ -28,5 +28,5 @@ package com.iluwatar.business.delegate;
 */
 public enum ServiceType {

-  EJB, JMS;
+  EJB, JMS
 }
--- a/bytecode/src/main/java/com/iluwatar/bytecode/VirtualMachine.java
+++ b/bytecode/src/main/java/com/iluwatar/bytecode/VirtualMachine.java
@ -30,9 +30,9 @@ import java.util.Stack;
 */
 public class VirtualMachine {

-  private Stack<Integer> stack = new Stack<>();
+  private final Stack<Integer> stack = new Stack<>();

-  private Wizard[] wizards = new Wizard[2];
+  private final Wizard[] wizards = new Wizard[2];

  /**
   * Constructor.
--- a/bytecode/src/test/java/com/iluwatar/bytecode/VirtualMachineTest.java
+++ b/bytecode/src/test/java/com/iluwatar/bytecode/VirtualMachineTest.java
@ -104,7 +104,7 @@ public class VirtualMachineTest {
    bytecode[2] = LITERAL.getIntValue();
    bytecode[3] = 50;                        // health amount
    bytecode[4] = SET_HEALTH.getIntValue();
-    bytecode[5] = LITERAL.getIntValue();;
+    bytecode[5] = LITERAL.getIntValue();
    bytecode[6] = wizardNumber;
    bytecode[7] = GET_HEALTH.getIntValue();

--- a/caching/src/main/java/com/iluwatar/caching/CachingPolicy.java
+++ b/caching/src/main/java/com/iluwatar/caching/CachingPolicy.java
@ -29,7 +29,7 @@ package com.iluwatar.caching;
 public enum CachingPolicy {
  THROUGH("through"), AROUND("around"), BEHIND("behind"), ASIDE("aside");

-  private String policy;
+  private final String policy;

  CachingPolicy(String policy) {
    this.policy = policy;
--- a/chain/README.md
+++ b/chain/README.md
@ -65,7 +65,7 @@ Then the request handler hierarchy
 ```java
 public abstract class RequestHandler {
  private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
-  private RequestHandler next;
+  private final RequestHandler next;

  public RequestHandler(RequestHandler next) {
    this.next = next;
--- a/chain/src/main/java/com/iluwatar/chain/RequestHandler.java
+++ b/chain/src/main/java/com/iluwatar/chain/RequestHandler.java
@ -33,7 +33,7 @@ public abstract class RequestHandler {

  private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);

-  private RequestHandler next;
+  private final RequestHandler next;

  public RequestHandler(RequestHandler next) {
    this.next = next;
--- a/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Car.java
+++ b/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Car.java
@ -87,10 +87,7 @@ public class Car {
    } else if (!model.equals(other.model)) {
      return false;
    }
-    if (year != other.year) {
-      return false;
-    }
-    return true;
+    return year == other.year;
  }

  public String getMake() {
--- a/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Person.java
+++ b/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/Person.java
@ -29,7 +29,7 @@ import java.util.List;
 * A Person class that has the list of cars that the person owns and use.
 */
 public class Person {
-  private List<Car> cars;
+  private final List<Car> cars;

  /**
   * Constructor to create an instance of person.
--- a/collection-pipeline/src/test/java/com/iluwatar/collectionpipeline/AppTest.java
+++ b/collection-pipeline/src/test/java/com/iluwatar/collectionpipeline/AppTest.java
@ -37,7 +37,7 @@ import org.slf4j.LoggerFactory;
 public class AppTest {
  private static final Logger LOGGER = LoggerFactory.getLogger(AppTest.class);

-  private List<Car> cars = CarFactory.createCars();
+  private final List<Car> cars = CarFactory.createCars();

  @Test
  public void testGetModelsAfter2000UsingFor() {
--- a/command/README.md
+++ b/command/README.md
@ -36,8 +36,8 @@ public class Wizard {

  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);

-  private Deque<Command> undoStack = new LinkedList<>();
-  private Deque<Command> redoStack = new LinkedList<>();
+  private final Deque<Command> undoStack = new LinkedList<>();
+  private final Deque<Command> redoStack = new LinkedList<>();

  public Wizard() {}

--- a/command/src/main/java/com/iluwatar/command/Size.java
+++ b/command/src/main/java/com/iluwatar/command/Size.java
@ -1,43 +1,43 @@
-/*
- * 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;
-
-/**
- * Enumeration for target size.
- */
-public enum Size {
-
-  SMALL("small"), NORMAL("normal");
-
-  private String title;
-
-  Size(String title) {
-    this.title = title;
-  }
-
-  @Override
-  public String toString() {
-    return title;
-  }
-}
+/*
+ * 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;
+
+/**
+ * Enumeration for target size.
+ */
+public enum Size {
+
+  SMALL("small"), NORMAL("normal");
+
+  private final String title;
+
+  Size(String title) {
+    this.title = title;
+  }
+
+  @Override
+  public String toString() {
+    return title;
+  }
+}
--- a/command/src/main/java/com/iluwatar/command/Visibility.java
+++ b/command/src/main/java/com/iluwatar/command/Visibility.java
@ -1,43 +1,43 @@
-/*
- * 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;
-
-/**
- * Enumeration for target visibility.
- */
-public enum Visibility {
-
-  VISIBLE("visible"), INVISIBLE("invisible");
-
-  private String title;
-
-  Visibility(String title) {
-    this.title = title;
-  }
-
-  @Override
-  public String toString() {
-    return title;
-  }
-}
+/*
+ * 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;
+
+/**
+ * Enumeration for target visibility.
+ */
+public enum Visibility {
+
+  VISIBLE("visible"), INVISIBLE("invisible");
+
+  private final String title;
+
+  Visibility(String title) {
+    this.title = title;
+  }
+
+  @Override
+  public String toString() {
+    return title;
+  }
+}
--- a/command/src/main/java/com/iluwatar/command/Wizard.java
+++ b/command/src/main/java/com/iluwatar/command/Wizard.java
@ -35,8 +35,8 @@ public class Wizard {

  private static final Logger LOGGER = LoggerFactory.getLogger(Wizard.class);

-  private Deque<Command> undoStack = new LinkedList<>();
-  private Deque<Command> redoStack = new LinkedList<>();
+  private final Deque<Command> undoStack = new LinkedList<>();
+  private final Deque<Command> redoStack = new LinkedList<>();

  public Wizard() {
    // comment to ignore sonar issue: LEVEL critical
--- a/commander/src/main/java/com/iluwatar/commander/employeehandle/EmployeeDatabase.java
+++ b/commander/src/main/java/com/iluwatar/commander/employeehandle/EmployeeDatabase.java
@ -33,7 +33,7 @@ import java.util.Hashtable;
 */

 public class EmployeeDatabase extends Database<Order> {
-  private Hashtable<String, Order> data;
+  private final Hashtable<String, Order> data;

  public EmployeeDatabase() {
    this.data = new Hashtable<>();
--- a/commander/src/main/java/com/iluwatar/commander/messagingservice/MessagingDatabase.java
+++ b/commander/src/main/java/com/iluwatar/commander/messagingservice/MessagingDatabase.java
@ -33,7 +33,7 @@ import java.util.Hashtable;
 */

 public class MessagingDatabase extends Database<MessageRequest> {
-  private Hashtable<String, MessageRequest> data;
+  private final Hashtable<String, MessageRequest> data;

  public MessagingDatabase() {
    this.data = new Hashtable<>();
--- a/commander/src/main/java/com/iluwatar/commander/paymentservice/PaymentDatabase.java
+++ b/commander/src/main/java/com/iluwatar/commander/paymentservice/PaymentDatabase.java
@ -34,7 +34,7 @@ import java.util.Hashtable;

 public class PaymentDatabase extends Database<PaymentRequest> {

-  private Hashtable<String, PaymentRequest> data;
+  private final Hashtable<String, PaymentRequest> data;

  public PaymentDatabase() {
    this.data = new Hashtable<>();
--- a/commander/src/main/java/com/iluwatar/commander/queue/QueueDatabase.java
+++ b/commander/src/main/java/com/iluwatar/commander/queue/QueueDatabase.java
@ -35,7 +35,7 @@ import java.util.List;

 public class QueueDatabase extends Database<QueueTask> {

-  private Queue<QueueTask> data;
+  private final Queue<QueueTask> data;
  public List<Exception> exceptionsList;

  public QueueDatabase(Exception... exc) {
--- a/commander/src/main/java/com/iluwatar/commander/shippingservice/ShippingDatabase.java
+++ b/commander/src/main/java/com/iluwatar/commander/shippingservice/ShippingDatabase.java
@ -34,7 +34,7 @@ import java.util.Hashtable;

 public class ShippingDatabase extends Database<ShippingRequest> {

-  private Hashtable<String, ShippingRequest> data;
+  private final Hashtable<String, ShippingRequest> data;

  public ShippingDatabase() {
    this.data = new Hashtable<>();
--- a/composite/README.md
+++ b/composite/README.md
@ -34,7 +34,7 @@ Taking our sentence example from above. Here we have the base class and differen
 ```java
 public abstract class LetterComposite {

-  private List<LetterComposite> children = new ArrayList<>();
+  private final List<LetterComposite> children = new ArrayList<>();

  public void add(LetterComposite letter) {
    children.add(letter);
@ -59,7 +59,7 @@ public abstract class LetterComposite {

 public class Letter extends LetterComposite {

-  private char character;
+  private final char character;

  public Letter(char c) {
    this.character = c;
--- a/composite/src/main/java/com/iluwatar/composite/Letter.java
+++ b/composite/src/main/java/com/iluwatar/composite/Letter.java
@ -28,7 +28,7 @@ package com.iluwatar.composite;
 */
 public class Letter extends LetterComposite {

-  private char character;
+  private final char character;

  public Letter(char c) {
    this.character = c;
--- a/composite/src/main/java/com/iluwatar/composite/LetterComposite.java
+++ b/composite/src/main/java/com/iluwatar/composite/LetterComposite.java
@ -1,58 +1,58 @@
-/*
- * 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.composite;
-
-import java.util.ArrayList;
-import java.util.List;
-
-/**
- * Composite interface.
- */
-public abstract class LetterComposite {
-
-  private List<LetterComposite> children = new ArrayList<>();
-
-  public void add(LetterComposite letter) {
-    children.add(letter);
-  }
-
-  public int count() {
-    return children.size();
-  }
-
-  protected void printThisBefore() {
-  }
-
-  protected void printThisAfter() {
-  }
-
-  /**
-   * Print.
-   */
-  public void print() {
-    printThisBefore();
-    children.forEach(LetterComposite::print);
-    printThisAfter();
-  }
-}
+/*
+ * 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.composite;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Composite interface.
+ */
+public abstract class LetterComposite {
+
+  private final List<LetterComposite> children = new ArrayList<>();
+
+  public void add(LetterComposite letter) {
+    children.add(letter);
+  }
+
+  public int count() {
+    return children.size();
+  }
+
+  protected void printThisBefore() {
+  }
+
+  protected void printThisAfter() {
+  }
+
+  /**
+   * Print.
+   */
+  public void print() {
+    printThisBefore();
+    children.forEach(LetterComposite::print);
+    printThisAfter();
+  }
+}
--- a/converter/src/main/java/com/iluwatar/converter/User.java
+++ b/converter/src/main/java/com/iluwatar/converter/User.java
@ -29,10 +29,10 @@ import java.util.Objects;
 * User class.
 */
 public class User {
-  private String firstName;
-  private String lastName;
-  private boolean isActive;
-  private String userId;
+  private final String firstName;
+  private final String lastName;
+  private final boolean isActive;
+  private final String userId;

  /**
   * Constructor.
--- a/converter/src/main/java/com/iluwatar/converter/UserDto.java
+++ b/converter/src/main/java/com/iluwatar/converter/UserDto.java
@ -30,10 +30,10 @@ import java.util.Objects;
 */
 public class UserDto {

-  private String firstName;
-  private String lastName;
-  private boolean isActive;
-  private String email;
+  private final String firstName;
+  private final String lastName;
+  private final boolean isActive;
+  private final String email;

  /**
   * Constructor.
--- a/converter/src/test/java/com/iluwatar/converter/ConverterTest.java
+++ b/converter/src/test/java/com/iluwatar/converter/ConverterTest.java
@ -34,7 +34,7 @@ import org.junit.jupiter.api.Test;
 */
 public class ConverterTest {

-  private UserConverter userConverter = new UserConverter();
+  private final UserConverter userConverter = new UserConverter();

  /**
   * Tests whether a converter created of opposite functions holds equality as a bijection.
--- a/cqrs/src/main/java/com/iluwatar/cqrs/commandes/CommandServiceImpl.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/commandes/CommandServiceImpl.java
@ -34,7 +34,7 @@ import org.hibernate.SessionFactory;
 */
 public class CommandServiceImpl implements ICommandService {

-  private SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+  private final SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

  private Author getAuthorByUsername(String username) {
    Author author;
--- a/cqrs/src/main/java/com/iluwatar/cqrs/queries/QueryServiceImpl.java
+++ b/cqrs/src/main/java/com/iluwatar/cqrs/queries/QueryServiceImpl.java
@ -38,7 +38,7 @@ import org.hibernate.transform.Transformers;
 */
 public class QueryServiceImpl implements IQueryService {

-  private SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
+  private final SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

  @Override
  public Author getAuthorByUsername(String username) {
--- a/dao/README.md
+++ b/dao/README.md
@ -112,7 +112,7 @@ public interface CustomerDao {

 public class InMemoryCustomerDao implements CustomerDao {

-  private Map<Integer, Customer> idToCustomer = new HashMap<>();
+  private final Map<Integer, Customer> idToCustomer = new HashMap<>();

  @Override
  public Stream<Customer> getAll() {
--- a/dao/src/main/java/com/iluwatar/dao/App.java
+++ b/dao/src/main/java/com/iluwatar/dao/App.java
@ -44,7 +44,7 @@ import org.slf4j.LoggerFactory;
 */
 public class App {
  private static final String DB_URL = "jdbc:h2:~/dao";
-  private static Logger log = LoggerFactory.getLogger(App.class);
+  private static final Logger log = LoggerFactory.getLogger(App.class);
  private static final String ALL_CUSTOMERS = "customerDao.getAllCustomers(): ";

  /**
--- a/dao/src/main/java/com/iluwatar/dao/InMemoryCustomerDao.java
+++ b/dao/src/main/java/com/iluwatar/dao/InMemoryCustomerDao.java
@ -36,7 +36,7 @@ import java.util.stream.Stream;
 */
 public class InMemoryCustomerDao implements CustomerDao {

-  private Map<Integer, Customer> idToCustomer = new HashMap<>();
+  private final Map<Integer, Customer> idToCustomer = new HashMap<>();

  /**
   * An eagerly evaluated stream of customers stored in memory.
--- a/dao/src/test/java/com/iluwatar/dao/DbCustomerDaoTest.java
+++ b/dao/src/test/java/com/iluwatar/dao/DbCustomerDaoTest.java
@ -50,7 +50,7 @@ public class DbCustomerDaoTest {

  private static final String DB_URL = "jdbc:h2:~/dao";
  private DbCustomerDao dao;
-  private Customer existingCustomer = new Customer(1, "Freddy", "Krueger");
+  private final Customer existingCustomer = new Customer(1, "Freddy", "Krueger");

  /**
   * Creates customers schema.
--- a/data-bus/src/main/java/com/iluwatar/databus/members/MessageCollectorMember.java
+++ b/data-bus/src/main/java/com/iluwatar/databus/members/MessageCollectorMember.java
@ -41,7 +41,7 @@ public class MessageCollectorMember implements Member {

  private final String name;

-  private List<String> messages = new ArrayList<>();
+  private final List<String> messages = new ArrayList<>();

  public MessageCollectorMember(String name) {
    this.name = name;
--- a/data-mapper/src/main/java/com/iluwatar/datamapper/App.java
+++ b/data-mapper/src/main/java/com/iluwatar/datamapper/App.java
@ -1,83 +1,83 @@
-/*
- * 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.datamapper;
-
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
-/**
- * The Data Mapper (DM) is a layer of software that separates the in-memory objects from the
- * database. Its responsibility is to transfer data between the two and also to isolate them from
- * each other. With Data Mapper the in-memory objects needn't know even that there's a database
- * present; they need no SQL interface code, and certainly no knowledge of the database schema. (The
- * database schema is always ignorant of the objects that use it.) Since it's a form of Mapper ,
- * Data Mapper itself is even unknown to the domain layer.
- *
- * <p>The below example demonstrates basic CRUD operations: Create, Read, Update, and Delete.
- */
-public final class App {
-
-  private static Logger log = LoggerFactory.getLogger(App.class);
-  private static final String STUDENT_STRING = "App.main(), student : ";
-
-
-  /**
-   * Program entry point.
-   *
-   * @param args command line args.
-   */
-  public static void main(final String... args) {
-
-    /* Create new data mapper for type 'first' */
-    final var mapper = new StudentDataMapperImpl();
-
-    /* Create new student */
-    var student = new Student(1, "Adam", 'A');
-
-    /* Add student in respectibe store */
-    mapper.insert(student);
-
-    log.debug(STUDENT_STRING + student + ", is inserted");
-
-    /* Find this student */
-    final var studentToBeFound = mapper.find(student.getStudentId());
-
-    log.debug(STUDENT_STRING + studentToBeFound + ", is searched");
-
-    /* Update existing student object */
-    student = new Student(student.getStudentId(), "AdamUpdated", 'A');
-
-    /* Update student in respectibe db */
-    mapper.update(student);
-
-    log.debug(STUDENT_STRING + student + ", is updated");
-    log.debug(STUDENT_STRING + student + ", is going to be deleted");
-
-    /* Delete student in db */
-    mapper.delete(student);
-  }
-
-  private App() {
-  }
-}
+/*
+ * 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.datamapper;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The Data Mapper (DM) is a layer of software that separates the in-memory objects from the
+ * database. Its responsibility is to transfer data between the two and also to isolate them from
+ * each other. With Data Mapper the in-memory objects needn't know even that there's a database
+ * present; they need no SQL interface code, and certainly no knowledge of the database schema. (The
+ * database schema is always ignorant of the objects that use it.) Since it's a form of Mapper ,
+ * Data Mapper itself is even unknown to the domain layer.
+ *
+ * <p>The below example demonstrates basic CRUD operations: Create, Read, Update, and Delete.
+ */
+public final class App {
+
+  private static final Logger log = LoggerFactory.getLogger(App.class);
+  private static final String STUDENT_STRING = "App.main(), student : ";
+
+
+  /**
+   * Program entry point.
+   *
+   * @param args command line args.
+   */
+  public static void main(final String... args) {
+
+    /* Create new data mapper for type 'first' */
+    final var mapper = new StudentDataMapperImpl();
+
+    /* Create new student */
+    var student = new Student(1, "Adam", 'A');
+
+    /* Add student in respectibe store */
+    mapper.insert(student);
+
+    log.debug(STUDENT_STRING + student + ", is inserted");
+
+    /* Find this student */
+    final var studentToBeFound = mapper.find(student.getStudentId());
+
+    log.debug(STUDENT_STRING + studentToBeFound + ", is searched");
+
+    /* Update existing student object */
+    student = new Student(student.getStudentId(), "AdamUpdated", 'A');
+
+    /* Update student in respectibe db */
+    mapper.update(student);
+
+    log.debug(STUDENT_STRING + student + ", is updated");
+    log.debug(STUDENT_STRING + student + ", is going to be deleted");
+
+    /* Delete student in db */
+    mapper.delete(student);
+  }
+
+  private App() {
+  }
+}
--- a/data-mapper/src/main/java/com/iluwatar/datamapper/StudentDataMapperImpl.java
+++ b/data-mapper/src/main/java/com/iluwatar/datamapper/StudentDataMapperImpl.java
@ -33,7 +33,7 @@ import java.util.Optional;
 public final class StudentDataMapperImpl implements StudentDataMapper {

  /* Note: Normally this would be in the form of an actual database */
-  private List<Student> students = new ArrayList<>();
+  private final List<Student> students = new ArrayList<>();

  @Override
  public Optional<Student> find(int studentId) {
--- a/data-transfer-object/README.md
+++ b/data-transfer-object/README.md
@ -64,7 +64,7 @@ Customer resource class acts as the server for customer information.

 ```java
 public class CustomerResource {
-  private List<CustomerDto> customers;
+  private final List<CustomerDto> customers;

  public CustomerResource(List<CustomerDto> customers) {
    this.customers = customers;
--- a/data-transfer-object/src/main/java/com/iluwatar/datatransfer/CustomerResource.java
+++ b/data-transfer-object/src/main/java/com/iluwatar/datatransfer/CustomerResource.java
@ -30,7 +30,7 @@ import java.util.List;
 * has all customer details.
 */
 public class CustomerResource {
-  private List<CustomerDto> customers;
+  private final List<CustomerDto> customers;

  /**
   * Initialise resource with existing customers.
--- a/decorator/README.md
+++ b/decorator/README.md
@ -70,7 +70,7 @@ public class ClubbedTroll implements Troll {

  private static final Logger LOGGER = LoggerFactory.getLogger(ClubbedTroll.class);

-  private Troll decorated;
+  private final Troll decorated;

  public ClubbedTroll(Troll decorated) {
    this.decorated = decorated;
--- a/decorator/src/main/java/com/iluwatar/decorator/ClubbedTroll.java
+++ b/decorator/src/main/java/com/iluwatar/decorator/ClubbedTroll.java
@ -33,7 +33,7 @@ public class ClubbedTroll implements Troll {

  private static final Logger LOGGER = LoggerFactory.getLogger(ClubbedTroll.class);

-  private Troll decorated;
+  private final Troll decorated;

  public ClubbedTroll(Troll decorated) {
    this.decorated = decorated;
--- a/decorator/src/test/java/com/iluwatar/decorator/SimpleTrollTest.java
+++ b/decorator/src/test/java/com/iluwatar/decorator/SimpleTrollTest.java
@ -68,7 +68,7 @@ public class SimpleTrollTest {

  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {

-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();

    public InMemoryAppender(Class clazz) {
      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
--- a/delegation/src/test/java/com/iluwatar/delegation/simple/DelegateTest.java
+++ b/delegation/src/test/java/com/iluwatar/delegation/simple/DelegateTest.java
@ -86,7 +86,7 @@ public class DelegateTest {
   */
  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {

-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();

    public InMemoryAppender() {
      ((Logger) LoggerFactory.getLogger("root")).addAppender(this);
--- a/dependency-injection/README.md
+++ b/dependency-injection/README.md
@ -62,7 +62,7 @@ public interface Wizard {

 public class AdvancedWizard implements Wizard {

-  private Tobacco tobacco;
+  private final Tobacco tobacco;

  public AdvancedWizard(Tobacco tobacco) {
    this.tobacco = tobacco;
--- a/dependency-injection/src/main/java/com/iluwatar/dependency/injection/AdvancedWizard.java
+++ b/dependency-injection/src/main/java/com/iluwatar/dependency/injection/AdvancedWizard.java
@ -29,7 +29,7 @@ package com.iluwatar.dependency.injection;
 */
 public class AdvancedWizard implements Wizard {

-  private Tobacco tobacco;
+  private final Tobacco tobacco;

  public AdvancedWizard(Tobacco tobacco) {
    this.tobacco = tobacco;
--- a/dependency-injection/src/main/java/com/iluwatar/dependency/injection/GuiceWizard.java
+++ b/dependency-injection/src/main/java/com/iluwatar/dependency/injection/GuiceWizard.java
@ -31,7 +31,7 @@ import javax.inject.Inject;
 */
 public class GuiceWizard implements Wizard {

-  private Tobacco tobacco;
+  private final Tobacco tobacco;

  @Inject
  public GuiceWizard(Tobacco tobacco) {
--- a/dependency-injection/src/main/java/com/iluwatar/dependency/injection/SimpleWizard.java
+++ b/dependency-injection/src/main/java/com/iluwatar/dependency/injection/SimpleWizard.java
@ -29,7 +29,7 @@ package com.iluwatar.dependency.injection;
 */
 public class SimpleWizard implements Wizard {

-  private OldTobyTobacco tobacco = new OldTobyTobacco();
+  private final OldTobyTobacco tobacco = new OldTobyTobacco();

  public void smoke() {
    tobacco.smoke(this);
--- a/dependency-injection/src/test/java/com/iluwatar/dependency/injection/utils/InMemoryAppender.java
+++ b/dependency-injection/src/test/java/com/iluwatar/dependency/injection/utils/InMemoryAppender.java
@ -37,7 +37,7 @@ import java.util.List;
 */
 public class InMemoryAppender extends AppenderBase<ILoggingEvent> {

-  private List<ILoggingEvent> log = new LinkedList<>();
+  private final List<ILoggingEvent> log = new LinkedList<>();

  public InMemoryAppender(Class clazz) {
    ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
--- a/dirty-flag/src/main/java/com/iluwatar/dirtyflag/World.java
+++ b/dirty-flag/src/main/java/com/iluwatar/dirtyflag/World.java
@ -34,7 +34,7 @@ import java.util.List;
 public class World {

  private List<String> countries;
-  private DataFetcher df;
+  private final DataFetcher df;

  public World() {
    this.countries = new ArrayList<String>();
--- a/double-buffer/src/main/java/com/iluwatar/doublebuffer/FrameBuffer.java
+++ b/double-buffer/src/main/java/com/iluwatar/doublebuffer/FrameBuffer.java
@ -33,7 +33,7 @@ public class FrameBuffer implements Buffer {
  public static final int WIDTH = 10;
  public static final int HEIGHT = 8;

-  private Pixel[] pixels = new Pixel[WIDTH * HEIGHT];
+  private final Pixel[] pixels = new Pixel[WIDTH * HEIGHT];

  public FrameBuffer() {
    clearAll();
--- a/double-buffer/src/main/java/com/iluwatar/doublebuffer/Pixel.java
+++ b/double-buffer/src/main/java/com/iluwatar/doublebuffer/Pixel.java
@ -31,7 +31,7 @@ public enum Pixel {
  WHITE(0),
  BLACK(1);

-  private int color;
+  private final int color;

  Pixel(int color) {
    this.color = color;
--- a/double-buffer/src/main/java/com/iluwatar/doublebuffer/Scene.java
+++ b/double-buffer/src/main/java/com/iluwatar/doublebuffer/Scene.java
@ -35,7 +35,7 @@ public class Scene {

  private static final Logger LOGGER = LoggerFactory.getLogger(Scene.class);

-  private Buffer[] frameBuffers;
+  private final Buffer[] frameBuffers;

  private int current;

--- a/double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/InventoryTest.java
+++ b/double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/InventoryTest.java
@ -109,7 +109,7 @@ public class InventoryTest {


  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();

    public InMemoryAppender(Class clazz) {
      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
--- a/double-dispatch/src/main/java/com/iluwatar/doubledispatch/Rectangle.java
+++ b/double-dispatch/src/main/java/com/iluwatar/doubledispatch/Rectangle.java
@ -28,10 +28,10 @@ package com.iluwatar.doubledispatch;
 */
 public class Rectangle {

-  private int left;
-  private int top;
-  private int right;
-  private int bottom;
+  private final int left;
+  private final int top;
+  private final int right;
+  private final int bottom;

  /**
   * Constructor.
--- a/event-aggregator/src/main/java/com/iluwatar/event/aggregator/Event.java
+++ b/event-aggregator/src/main/java/com/iluwatar/event/aggregator/Event.java
@ -31,7 +31,7 @@ public enum Event {
  STARK_SIGHTED("Stark sighted"), WARSHIPS_APPROACHING("Warships approaching"), TRAITOR_DETECTED(
      "Traitor detected");

-  private String description;
+  private final String description;

  Event(String description) {
    this.description = description;
--- a/event-aggregator/src/main/java/com/iluwatar/event/aggregator/EventEmitter.java
+++ b/event-aggregator/src/main/java/com/iluwatar/event/aggregator/EventEmitter.java
@ -31,7 +31,7 @@ import java.util.List;
 */
 public abstract class EventEmitter {

-  private List<EventObserver> observers;
+  private final List<EventObserver> observers;

  public EventEmitter() {
    observers = new LinkedList<>();
--- a/event-aggregator/src/main/java/com/iluwatar/event/aggregator/Weekday.java
+++ b/event-aggregator/src/main/java/com/iluwatar/event/aggregator/Weekday.java
@ -36,7 +36,7 @@ public enum Weekday {
  SATURDAY("Saturday"),
  SUNDAY("Sunday");

-  private String description;
+  private final String description;

  Weekday(String description) {
    this.description = description;
--- a/event-aggregator/src/test/java/com/iluwatar/event/aggregator/KingJoffreyTest.java
+++ b/event-aggregator/src/test/java/com/iluwatar/event/aggregator/KingJoffreyTest.java
@ -74,7 +74,7 @@ public class KingJoffreyTest {
  }

  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();

    public InMemoryAppender(Class<?> clazz) {
      ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);
--- a/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/Event.java
+++ b/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/Event.java
@ -33,9 +33,9 @@ public class Event implements IEvent, Runnable {

  private static final Logger LOGGER = LoggerFactory.getLogger(Event.class);

-  private int eventId;
-  private int eventTime;
-  private boolean isSynchronous;
+  private final int eventId;
+  private final int eventTime;
+  private final boolean isSynchronous;
  private Thread thread;
  private boolean isComplete = false;
  private ThreadCompleteListener eventListener;
--- a/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/EventManager.java
+++ b/event-asynchronous/src/main/java/com/iluwatar/event/asynchronous/EventManager.java
@ -43,8 +43,8 @@ public class EventManager implements ThreadCompleteListener {
  public static final int MAX_ID = MAX_RUNNING_EVENTS;
  public static final int MAX_EVENT_TIME = 1800; // in seconds / 30 minutes.
  private int currentlyRunningSyncEvent = -1;
-  private Random rand;
-  private Map<Integer, Event> eventPool;
+  private final Random rand;
+  private final Map<Integer, Event> eventPool;

  private static final String DOES_NOT_EXIST = " does not exist.";

--- a/event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserCreatedEvent.java
+++ b/event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserCreatedEvent.java
@ -32,7 +32,7 @@ import com.iluwatar.eda.model.User;
 */
 public class UserCreatedEvent extends AbstractEvent {

-  private User user;
+  private final User user;

  public UserCreatedEvent(User user) {
    this.user = user;
--- a/event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserUpdatedEvent.java
+++ b/event-driven-architecture/src/main/java/com/iluwatar/eda/event/UserUpdatedEvent.java
@ -32,7 +32,7 @@ import com.iluwatar.eda.model.User;
 */
 public class UserUpdatedEvent extends AbstractEvent {

-  private User user;
+  private final User user;

  public UserUpdatedEvent(User user) {
    this.user = user;
--- a/event-driven-architecture/src/main/java/com/iluwatar/eda/framework/EventDispatcher.java
+++ b/event-driven-architecture/src/main/java/com/iluwatar/eda/framework/EventDispatcher.java
@ -32,7 +32,7 @@ import java.util.Map;
 */
 public class EventDispatcher {

-  private Map<Class<? extends Event>, Handler<? extends Event>> handlers;
+  private final Map<Class<? extends Event>, Handler<? extends Event>> handlers;

  public EventDispatcher() {
    handlers = new HashMap<>();
--- a/event-driven-architecture/src/main/java/com/iluwatar/eda/model/User.java
+++ b/event-driven-architecture/src/main/java/com/iluwatar/eda/model/User.java
@ -32,7 +32,7 @@ import com.iluwatar.eda.event.UserUpdatedEvent;
 */
 public class User {

-  private String username;
+  private final String username;

  public User(String username) {
    this.username = username;
--- a/event-queue/src/main/java/com/iluwatar/event/queue/Audio.java
+++ b/event-queue/src/main/java/com/iluwatar/event/queue/Audio.java
@ -49,7 +49,7 @@ public class Audio {

  private volatile Thread updateThread = null;

-  private PlayMessage[] pendingAudio = new PlayMessage[MAX_PENDING];
+  private final PlayMessage[] pendingAudio = new PlayMessage[MAX_PENDING];

  // Visible only for testing purposes
  Audio() {
--- a/extension-objects/src/main/java/concreteextensions/Commander.java
+++ b/extension-objects/src/main/java/concreteextensions/Commander.java
@ -35,7 +35,7 @@ public class Commander implements CommanderExtension {

  private static final Logger LOGGER = LoggerFactory.getLogger(Commander.class);

-  private CommanderUnit unit;
+  private final CommanderUnit unit;

  public Commander(CommanderUnit commanderUnit) {
    this.unit = commanderUnit;
--- a/extension-objects/src/main/java/concreteextensions/Sergeant.java
+++ b/extension-objects/src/main/java/concreteextensions/Sergeant.java
@ -35,7 +35,7 @@ public class Sergeant implements SergeantExtension {

  private static final Logger LOGGER = LoggerFactory.getLogger(Sergeant.class);

-  private SergeantUnit unit;
+  private final SergeantUnit unit;

  public Sergeant(SergeantUnit sergeantUnit) {
    this.unit = sergeantUnit;
--- a/extension-objects/src/main/java/concreteextensions/Soldier.java
+++ b/extension-objects/src/main/java/concreteextensions/Soldier.java
@ -34,7 +34,7 @@ import units.SoldierUnit;
 public class Soldier implements SoldierExtension {
  private static final Logger LOGGER = LoggerFactory.getLogger(Soldier.class);

-  private SoldierUnit unit;
+  private final SoldierUnit unit;

  public Soldier(SoldierUnit soldierUnit) {
    this.unit = soldierUnit;
--- a/facade/README.md
+++ b/facade/README.md
@ -83,7 +83,7 @@ public abstract class DwarvenMineWorker {

  public abstract String name();

-  static enum Action {
+  enum Action {
    GO_TO_SLEEP, WAKE_UP, GO_HOME, GO_TO_MINE, WORK
  }
 }
--- a/facade/src/test/java/com/iluwatar/facade/DwarvenGoldmineFacadeTest.java
+++ b/facade/src/test/java/com/iluwatar/facade/DwarvenGoldmineFacadeTest.java
@ -110,7 +110,7 @@ public class DwarvenGoldmineFacadeTest {

  private class InMemoryAppender extends AppenderBase<ILoggingEvent> {

-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();

    public InMemoryAppender() {
      ((Logger) LoggerFactory.getLogger("root")).addAppender(this);
--- a/factory-method/src/main/java/com/iluwatar/factory/method/ElfBlacksmith.java
+++ b/factory-method/src/main/java/com/iluwatar/factory/method/ElfBlacksmith.java
@ -32,7 +32,7 @@ import java.util.Map;
 */
 public class ElfBlacksmith implements Blacksmith {

-  private static Map<WeaponType, ElfWeapon> ELFARSENAL;
+  private static final Map<WeaponType, ElfWeapon> ELFARSENAL;

  static {
    ELFARSENAL = new HashMap<>(WeaponType.values().length);
--- a/factory-method/src/main/java/com/iluwatar/factory/method/ElfWeapon.java
+++ b/factory-method/src/main/java/com/iluwatar/factory/method/ElfWeapon.java
@ -28,7 +28,7 @@ package com.iluwatar.factory.method;
 */
 public class ElfWeapon implements Weapon {

-  private WeaponType weaponType;
+  private final WeaponType weaponType;

  public ElfWeapon(WeaponType weaponType) {
    this.weaponType = weaponType;
--- a/factory-method/src/main/java/com/iluwatar/factory/method/OrcBlacksmith.java
+++ b/factory-method/src/main/java/com/iluwatar/factory/method/OrcBlacksmith.java
@ -32,7 +32,7 @@ import java.util.Map;
 */
 public class OrcBlacksmith implements Blacksmith {

-  private static Map<WeaponType, OrcWeapon> ORCARSENAL;
+  private static final Map<WeaponType, OrcWeapon> ORCARSENAL;

  static {
    ORCARSENAL = new HashMap<>(WeaponType.values().length);
--- a/factory-method/src/main/java/com/iluwatar/factory/method/OrcWeapon.java
+++ b/factory-method/src/main/java/com/iluwatar/factory/method/OrcWeapon.java
@ -28,7 +28,7 @@ package com.iluwatar.factory.method;
 */
 public class OrcWeapon implements Weapon {

-  private WeaponType weaponType;
+  private final WeaponType weaponType;

  public OrcWeapon(WeaponType weaponType) {
    this.weaponType = weaponType;
--- a/factory-method/src/main/java/com/iluwatar/factory/method/WeaponType.java
+++ b/factory-method/src/main/java/com/iluwatar/factory/method/WeaponType.java
@ -30,7 +30,7 @@ public enum WeaponType {

  SHORT_SWORD("short sword"), SPEAR("spear"), AXE("axe"), UNDEFINED("");

-  private String title;
+  private final String title;

  WeaponType(String title) {
    this.title = title;
--- a/feature-toggle/src/main/java/com/iluwatar/featuretoggle/pattern/propertiesversion/PropertiesFeatureToggleVersion.java
+++ b/feature-toggle/src/main/java/com/iluwatar/featuretoggle/pattern/propertiesversion/PropertiesFeatureToggleVersion.java
@ -42,7 +42,7 @@ import java.util.Properties;
 */
 public class PropertiesFeatureToggleVersion implements Service {

-  private boolean isEnhanced;
+  private final boolean isEnhanced;

  /**
   * Creates an instance of {@link PropertiesFeatureToggleVersion} using the passed {@link
--- a/feature-toggle/src/main/java/com/iluwatar/featuretoggle/user/User.java
+++ b/feature-toggle/src/main/java/com/iluwatar/featuretoggle/user/User.java
@ -29,7 +29,7 @@ package com.iluwatar.featuretoggle.user;
 */
 public class User {

-  private String name;
+  private final String name;

  /**
   * Default Constructor setting the username.
--- a/feature-toggle/src/main/java/com/iluwatar/featuretoggle/user/UserGroup.java
+++ b/feature-toggle/src/main/java/com/iluwatar/featuretoggle/user/UserGroup.java
@ -35,8 +35,8 @@ import java.util.List;
 */
 public class UserGroup {

-  private static List<User> freeGroup = new ArrayList<>();
-  private static List<User> paidGroup = new ArrayList<>();
+  private static final List<User> freeGroup = new ArrayList<>();
+  private static final List<User> paidGroup = new ArrayList<>();


  /**
--- a/fluentinterface/src/main/java/com/iluwatar/fluentinterface/app/App.java
+++ b/fluentinterface/src/main/java/com/iluwatar/fluentinterface/app/App.java
@ -94,7 +94,7 @@ public class App {
        .filter(positives())
        .first(4)
        .last(2)
-        .map(number -> "String[" + valueOf(number) + "]")
+        .map(number -> "String[" + number + "]")
        .asList();
    prettyPrint("The lazy list contains the last two of the first four positive numbers "
        + "mapped to Strings: ", lastTwoOfFirstFourStringMapped);
--- a/fluentinterface/src/main/java/com/iluwatar/fluentinterface/fluentiterable/lazy/LazyFluentIterable.java
+++ b/fluentinterface/src/main/java/com/iluwatar/fluentinterface/fluentiterable/lazy/LazyFluentIterable.java
@ -198,7 +198,7 @@ public class LazyFluentIterable<E> implements FluentIterable<E> {
      @Override
      public Iterator<T> iterator() {
        return new DecoratingIterator<T>(null) {
-          Iterator<E> oldTypeIterator = iterable.iterator();
+          final Iterator<E> oldTypeIterator = iterable.iterator();

          @Override
          public T computeNext() {
--- a/flux/src/main/java/com/iluwatar/flux/action/Action.java
+++ b/flux/src/main/java/com/iluwatar/flux/action/Action.java
@ -28,7 +28,7 @@ package com.iluwatar.flux.action;
 */
 public abstract class Action {

-  private ActionType type;
+  private final ActionType type;

  public Action(ActionType type) {
    this.type = type;
--- a/flux/src/main/java/com/iluwatar/flux/action/ActionType.java
+++ b/flux/src/main/java/com/iluwatar/flux/action/ActionType.java
@ -28,6 +28,6 @@ package com.iluwatar.flux.action;
 */
 public enum ActionType {

-  MENU_ITEM_SELECTED, CONTENT_CHANGED;
+  MENU_ITEM_SELECTED, CONTENT_CHANGED

 }
--- a/flux/src/main/java/com/iluwatar/flux/action/Content.java
+++ b/flux/src/main/java/com/iluwatar/flux/action/Content.java
@ -31,7 +31,7 @@ public enum Content {
  PRODUCTS("Products - This page lists the company's products."), COMPANY(
      "Company - This page displays information about the company.");

-  private String title;
+  private final String title;

  Content(String title) {
    this.title = title;
--- a/flux/src/main/java/com/iluwatar/flux/action/ContentAction.java
+++ b/flux/src/main/java/com/iluwatar/flux/action/ContentAction.java
@ -28,7 +28,7 @@ package com.iluwatar.flux.action;
 */
 public class ContentAction extends Action {

-  private Content content;
+  private final Content content;

  public ContentAction(Content content) {
    super(ActionType.CONTENT_CHANGED);
--- a/flux/src/main/java/com/iluwatar/flux/action/MenuAction.java
+++ b/flux/src/main/java/com/iluwatar/flux/action/MenuAction.java
@ -29,7 +29,7 @@ package com.iluwatar.flux.action;
 */
 public class MenuAction extends Action {

-  private MenuItem menuItem;
+  private final MenuItem menuItem;

  public MenuAction(MenuItem menuItem) {
    super(ActionType.MENU_ITEM_SELECTED);
--- a/flux/src/main/java/com/iluwatar/flux/action/MenuItem.java
+++ b/flux/src/main/java/com/iluwatar/flux/action/MenuItem.java
@ -30,7 +30,7 @@ public enum MenuItem {

  HOME("Home"), PRODUCTS("Products"), COMPANY("Company");

-  private String title;
+  private final String title;

  MenuItem(String title) {
    this.title = title;
--- a/flux/src/main/java/com/iluwatar/flux/dispatcher/Dispatcher.java
+++ b/flux/src/main/java/com/iluwatar/flux/dispatcher/Dispatcher.java
@ -39,7 +39,7 @@ public final class Dispatcher {

  private static Dispatcher instance = new Dispatcher();

-  private List<Store> stores = new LinkedList<>();
+  private final List<Store> stores = new LinkedList<>();

  private Dispatcher() {
  }
--- a/flux/src/main/java/com/iluwatar/flux/store/Store.java
+++ b/flux/src/main/java/com/iluwatar/flux/store/Store.java
@ -33,7 +33,7 @@ import java.util.List;
 */
 public abstract class Store {

-  private List<View> views = new LinkedList<>();
+  private final List<View> views = new LinkedList<>();

  public abstract void onAction(Action action);

--- a/flyweight/src/main/java/com/iluwatar/flyweight/AlchemistShop.java
+++ b/flyweight/src/main/java/com/iluwatar/flyweight/AlchemistShop.java
@ -34,8 +34,8 @@ public class AlchemistShop {

  private static final Logger LOGGER = LoggerFactory.getLogger(AlchemistShop.class);

-  private List<Potion> topShelf;
-  private List<Potion> bottomShelf;
+  private final List<Potion> topShelf;
+  private final List<Potion> bottomShelf;

  /**
   * Constructor.
--- a/front-controller/src/test/java/com/iluwatar/front/controller/utils/InMemoryAppender.java
+++ b/front-controller/src/test/java/com/iluwatar/front/controller/utils/InMemoryAppender.java
@ -36,7 +36,7 @@ import java.util.List;
 */
 public class InMemoryAppender extends AppenderBase<ILoggingEvent> {

-  private List<ILoggingEvent> log = new LinkedList<>();
+  private final List<ILoggingEvent> log = new LinkedList<>();

  public InMemoryAppender() {
    ((Logger) LoggerFactory.getLogger("root")).addAppender(this);
--- a/half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/App.java
+++ b/half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/App.java
@ -95,7 +95,7 @@ public class App {
   * ArithmeticSumTask.
   */
  static class ArithmeticSumTask implements AsyncTask<Long> {
-    private long numberOfElements;
+    private final long numberOfElements;

    public ArithmeticSumTask(long numberOfElements) {
      this.numberOfElements = numberOfElements;
--- a/half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsynchronousService.java
+++ b/half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsynchronousService.java
@ -48,7 +48,7 @@ public class AsynchronousService {
   * tasks should be performed in the background which does not affect the performance of main
   * thread.
   */
-  private ExecutorService service;
+  private final ExecutorService service;

  /**
   * Creates an asynchronous service using {@code workQueue} as communication channel between
--- a/hexagonal/src/main/java/com/iluwatar/hexagonal/banking/InMemoryBank.java
+++ b/hexagonal/src/main/java/com/iluwatar/hexagonal/banking/InMemoryBank.java
@ -32,7 +32,7 @@ import java.util.Map;
 */
 public class InMemoryBank implements WireTransfers {

-  private static Map<String, Integer> accounts = new HashMap<>();
+  private static final Map<String, Integer> accounts = new HashMap<>();

  static {
    accounts
--- a/hexagonal/src/main/java/com/iluwatar/hexagonal/database/InMemoryTicketRepository.java
+++ b/hexagonal/src/main/java/com/iluwatar/hexagonal/database/InMemoryTicketRepository.java
@ -34,7 +34,7 @@ import java.util.Optional;
 */
 public class InMemoryTicketRepository implements LotteryTicketRepository {

-  private static Map<LotteryTicketId, LotteryTicket> tickets = new HashMap<>();
+  private static final Map<LotteryTicketId, LotteryTicket> tickets = new HashMap<>();

  @Override
  public Optional<LotteryTicket> findById(LotteryTicketId id) {
--- a/Show More
+++ b/Show More