Merge pull request #1506 from iluwatar/all-contributors/add-stefanbirkner

docs: add stefanbirkner as a contributor

.all-contributorsrc

@@ -1128,7 +1128,8 @@
       "avatar_url": "https://avatars0.githubusercontent.com/u/13291222?v=4",
       "profile": "http://subho.xyz",
       "contributions": [
-        "code"
+        "code",
+        "review"
       ]
     },
     {
@@ -1139,6 +1140,42 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "ToxicDreamz",
+      "name": "Toxic Dreamz",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/45225562?v=4",
+      "profile": "https://github.com/ToxicDreamz",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "edycutjong",
+      "name": "Edy Cu Tjong",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/1098102?v=4",
+      "profile": "http://www.edycutjong.com",
+      "contributions": [
+        "doc"
+      ]
+    },
+    {
+      "login": "mkrzywanski",
+      "name": "Michał Krzywański",
+      "avatar_url": "https://avatars0.githubusercontent.com/u/15279585?v=4",
+      "profile": "https://github.com/mkrzywanski",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "stefanbirkner",
+      "name": "Stefan Birkner",
+      "avatar_url": "https://avatars1.githubusercontent.com/u/711349?v=4",
+      "profile": "https://www.stefan-birkner.de",
+      "contributions": [
+        "code"
+      ]
     }
   ],
   "contributorsPerLine": 4,

.github/workflows/maven-ci.yml

@@ -60,5 +60,5 @@ jobs:
       run: xvfb-run mvn clean verify org.sonarsource.scanner.maven:sonar-maven-plugin:sonar
       env:
         # These two env variables are needed for sonar analysis
-        GITHUB_TOKEN: ${{ secrets.REPOSITORY_ACCESS_TOKEN }}
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}

README.md

@@ -6,10 +6,11 @@
 
 ![Java CI with Maven](https://github.com/iluwatar/java-design-patterns/workflows/Java%20CI%20with%20Maven/badge.svg)
 [![License MIT](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/iluwatar/java-design-patterns/master/LICENSE.md)
+[![Lines of Code](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=ncloc)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns) 
+[![Coverage](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=coverage)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns)
 [![Join the chat at https://gitter.im/iluwatar/java-design-patterns](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/iluwatar/java-design-patterns?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-[![Sonarcloud Status](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=alert_status)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns) 
 <!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
-[![All Contributors](https://img.shields.io/badge/all_contributors-125-orange.svg?style=flat-square)](#contributors-)
+[![All Contributors](https://img.shields.io/badge/all_contributors-129-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
 # Introduction
@@ -252,10 +253,16 @@ This project is licensed under the terms of the MIT license.
     <td align="center"><a href="https://www.linkedin.com/in/ashish-trivedi-218379135/"><img src="https://avatars3.githubusercontent.com/u/23194128?v=4" width="100px;" alt=""/><br /><sub><b>Ashish Trivedi</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ashishtrivedi16" title="Code">💻</a></td>
     <td align="center"><a href="https://rayyounghong.com"><img src="https://avatars1.githubusercontent.com/u/41055099?v=4" width="100px;" alt=""/><br /><sub><b>洪月阳</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=RayYH" title="Code">💻</a></td>
     <td align="center"><a href="https://xdvrx1.github.io/"><img src="https://avatars0.githubusercontent.com/u/47092464?v=4" width="100px;" alt=""/><br /><sub><b>xdvrx1</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Axdvrx1" title="Reviewed Pull Requests">👀</a> <a href="#ideas-xdvrx1" title="Ideas, Planning, & Feedback">🤔</a></td>
-    <td align="center"><a href="http://subho.xyz"><img src="https://avatars0.githubusercontent.com/u/13291222?v=4" width="100px;" alt=""/><br /><sub><b>Subhrodip Mohanta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ohbus" title="Code">💻</a></td>
+    <td align="center"><a href="http://subho.xyz"><img src="https://avatars0.githubusercontent.com/u/13291222?v=4" width="100px;" alt=""/><br /><sub><b>Subhrodip Mohanta</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ohbus" title="Code">💻</a> <a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Aohbus" title="Reviewed Pull Requests">👀</a></td>
   </tr>
   <tr>
     <td align="center"><a href="https://github.com/nahteb"><img src="https://avatars3.githubusercontent.com/u/13121570?v=4" width="100px;" alt=""/><br /><sub><b>Bethan Palmer</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=nahteb" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/ToxicDreamz"><img src="https://avatars0.githubusercontent.com/u/45225562?v=4" width="100px;" alt=""/><br /><sub><b>Toxic Dreamz</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=ToxicDreamz" title="Code">💻</a></td>
+    <td align="center"><a href="http://www.edycutjong.com"><img src="https://avatars1.githubusercontent.com/u/1098102?v=4" width="100px;" alt=""/><br /><sub><b>Edy Cu Tjong</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=edycutjong" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://github.com/mkrzywanski"><img src="https://avatars0.githubusercontent.com/u/15279585?v=4" width="100px;" alt=""/><br /><sub><b>Michał Krzywański</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=mkrzywanski" title="Code">💻</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="https://www.stefan-birkner.de"><img src="https://avatars1.githubusercontent.com/u/711349?v=4" width="100px;" alt=""/><br /><sub><b>Stefan Birkner</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=stefanbirkner" title="Code">💻</a></td>
   </tr>
 </table>
 

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

@@ -25,14 +25,23 @@ package com.iluwatar.abstractdocument;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Simple App test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
 
   @Test
-  public void shouldExecuteAppWithoutException() {
-    App.main(null);
+  void shouldExecuteAppWithoutException() {
+    assertDoesNotThrow(() -> App.main(null));
   }
 
 }

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

@@ -25,12 +25,23 @@ package com.iluwatar.abstractfactory;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Abstract Factory example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -25,13 +25,23 @@ package com.iluwatar.acyclicvisitor;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that the Acyclic Visitor example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -25,12 +25,23 @@ package com.iluwatar.adapter;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Adapter example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

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

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

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

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

@@ -0,0 +1,48 @@
+/*
+ * 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.ambassador;
+
+/**
+ * Holds information regarding the status of the Remote Service.
+ *
+ * <p> This Enum replaces the integer value previously
+ * stored in {@link RemoteServiceInterface} as SonarCloud was identifying
+ * it as an issue. All test cases have been checked after changes,
+ * without failures. </p>
+ */
+
+public enum RemoteServiceStatus {
+  FAILURE(-1)
+  ;
+
+  private final long remoteServiceStatusValue;
+
+  RemoteServiceStatus(long remoteServiceStatusValue) {
+    this.remoteServiceStatusValue = remoteServiceStatusValue;
+  }
+
+  public long getRemoteServiceStatusValue() {
+    return remoteServiceStatusValue;
+  }
+}

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

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

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

@@ -25,13 +25,23 @@ package com.iluwatar.ambassador;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
 
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

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

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

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

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

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

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

@@ -25,12 +25,24 @@ package com.iluwatar.async.method.invocation;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  void test() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+
   }
 }

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

@@ -24,15 +24,26 @@
 package com.iluwatar.balking;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.function.Executable;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 
 /**
  * Application test
  */
 class AppTest {
 
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  void main() {
-    App.main();
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow((Executable) App::main);
   }
 
 }

bridge/README.md

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

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

@@ -25,12 +25,22 @@ package com.iluwatar.bridge;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -25,12 +25,23 @@ package com.iluwatar.builder;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
 class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -27,13 +27,23 @@ import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Business Delegate example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  public void test() throws IOException {
-    String[] args = {};
-    App.main(args);
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

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

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

@@ -25,13 +25,22 @@ package com.iluwatar.bytecode;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -27,12 +27,23 @@ import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Caching example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -25,12 +25,23 @@ package com.iluwatar.callback;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Callback example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -25,13 +25,23 @@ package com.iluwatar.chain;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

combinator/pom.xml

@@ -39,6 +39,12 @@
       <artifactId>junit</artifactId>
       <scope>test</scope>
     </dependency>
+
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
 </project>

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

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

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

@@ -25,12 +25,21 @@ package com.iluwatar.command;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Command example runs without errors.
  */
-public class AppTest {
+class AppTest {
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

commander/src/main/java/com/iluwatar/commander/Commander.java

@@ -36,9 +36,11 @@ import com.iluwatar.commander.queue.QueueDatabase;
 import com.iluwatar.commander.queue.QueueTask;
 import com.iluwatar.commander.queue.QueueTask.TaskType;
 import com.iluwatar.commander.shippingservice.ShippingService;
+import java.util.List;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+
 /**
  * <p>Commander pattern is used to handle all issues that can come up while making a
  * distributed transaction. The idea is to have a commander, which coordinates the execution of all
@@ -159,8 +161,8 @@ public class Commander {
 
   private void sendPaymentRequest(Order order) {
     if (System.currentTimeMillis() - order.createdTime >= this.paymentTime) {
-      if (order.paid.equals(PaymentStatus.Trying)) {
-        order.paid = PaymentStatus.NotDone;
+      if (order.paid.equals(PaymentStatus.TRYING)) {
+        order.paid = PaymentStatus.NOT_DONE;
         sendPaymentFailureMessage(order);
         LOG.error("Order " + order.id + ": Payment time for order over, failed and returning..");
       } //if succeeded or failed, would have been dequeued, no attempt to make payment     
@@ -172,15 +174,15 @@ public class Commander {
         if (!l.isEmpty()) {
           if (DatabaseUnavailableException.class.isAssignableFrom(l.get(0).getClass())) {
             LOG.debug("Order " + order.id + ": Error in connecting to payment service,"
-                + " trying again..");
+                    + " trying again..");
           } else {
             LOG.debug("Order " + order.id + ": Error in creating payment request..");
           }
           throw l.remove(0);
         }
-        if (order.paid.equals(PaymentStatus.Trying)) {
+        if (order.paid.equals(PaymentStatus.TRYING)) {
           var transactionId = paymentService.receiveRequest(order.price);
-          order.paid = PaymentStatus.Done;
+          order.paid = PaymentStatus.DONE;
           LOG.info("Order " + order.id + ": Payment successful, transaction Id: " + transactionId);
           if (!finalSiteMsgShown) {
             LOG.info("Payment made successfully, thank you for shopping with us!!");
@@ -193,26 +195,26 @@ public class Commander {
         if (PaymentDetailsErrorException.class.isAssignableFrom(err.getClass())) {
           if (!finalSiteMsgShown) {
             LOG.info("There was an error in payment. Your account/card details "
-                + "may have been incorrect. "
-                + "Meanwhile, your order has been converted to COD and will be shipped.");
+                    + "may have been incorrect. "
+                    + "Meanwhile, your order has been converted to COD and will be shipped.");
             finalSiteMsgShown = true;
           }
           LOG.error("Order " + order.id + ": Payment details incorrect, failed..");
-          o.paid = PaymentStatus.NotDone;
+          o.paid = PaymentStatus.NOT_DONE;
           sendPaymentFailureMessage(o);
         } else {
-          if (o.messageSent.equals(MessageSent.NoneSent)) {
+          if (o.messageSent.equals(MessageSent.NONE_SENT)) {
             if (!finalSiteMsgShown) {
               LOG.info("There was an error in payment. We are on it, and will get back to you "
-                  + "asap. Don't worry, your order has been placed and will be shipped.");
+                      + "asap. Don't worry, your order has been placed and will be shipped.");
               finalSiteMsgShown = true;
             }
             LOG.warn("Order " + order.id + ": Payment error, going to queue..");
             sendPaymentPossibleErrorMsg(o);
           }
-          if (o.paid.equals(PaymentStatus.Trying) && System
-              .currentTimeMillis() - o.createdTime < paymentTime) {
-            var qt = new QueueTask(o, TaskType.Payment, -1);
+          if (o.paid.equals(PaymentStatus.TRYING) && System
+                  .currentTimeMillis() - o.createdTime < paymentTime) {
+            var qt = new QueueTask(o, TaskType.PAYMENT, -1);
             updateQueue(qt);
           }
         }
@@ -234,12 +236,12 @@ public class Commander {
       // additional check not needed
       LOG.trace("Order " + qt.order.id + ": Queue time for order over, failed..");
       return;
-    } else if (qt.taskType.equals(TaskType.Payment) && !qt.order.paid.equals(PaymentStatus.Trying)
-        || qt.taskType.equals(TaskType.Messaging) && (qt.messageType == 1
-        && !qt.order.messageSent.equals(MessageSent.NoneSent)
-        || qt.order.messageSent.equals(MessageSent.PaymentFail)
-        || qt.order.messageSent.equals(MessageSent.PaymentSuccessful))
-        || qt.taskType.equals(TaskType.EmployeeDb) && qt.order.addedToEmployeeHandle) {
+    } else if (qt.taskType.equals(TaskType.PAYMENT) && !qt.order.paid.equals(PaymentStatus.TRYING)
+        || qt.taskType.equals(TaskType.MESSAGING) && (qt.messageType == 1
+        && !qt.order.messageSent.equals(MessageSent.NONE_SENT)
+        || qt.order.messageSent.equals(MessageSent.PAYMENT_FAIL)
+        || qt.order.messageSent.equals(MessageSent.PAYMENT_SUCCESSFUL))
+        || qt.taskType.equals(TaskType.EMPLOYEE_DB) && qt.order.addedToEmployeeHandle) {
       LOG.trace("Order " + qt.order.id + ": Not queueing task since task already done..");
       return;
     }
@@ -256,8 +258,8 @@ public class Commander {
         tryDoingTasksInQueue();
       };
       Retry.HandleErrorIssue<QueueTask> handleError = (qt1, err) -> {
-        if (qt1.taskType.equals(TaskType.Payment)) {
-          qt1.order.paid = PaymentStatus.NotDone;
+        if (qt1.taskType.equals(TaskType.PAYMENT)) {
+          qt1.order.paid = PaymentStatus.NOT_DONE;
           sendPaymentFailureMessage(qt1.order);
           LOG.error("Order " + qt1.order.id + ": Unable to enqueue payment task,"
               + " payment failed..");
@@ -331,35 +333,9 @@ public class Commander {
     }
     var list = messagingService.exceptionsList;
     Thread t = new Thread(() -> {
-      Retry.Operation op = (l) -> {
-        if (!l.isEmpty()) {
-          if (DatabaseUnavailableException.class.isAssignableFrom(l.get(0).getClass())) {
-            LOG.debug("Order " + order.id + ": Error in connecting to messaging service "
-                + "(Payment Success msg), trying again..");
-          } else {
-            LOG.debug("Order " + order.id + ": Error in creating Payment Success"
-                + " messaging request..");
-          }
-          throw l.remove(0);
-        }
-        if (!order.messageSent.equals(MessageSent.PaymentFail)
-            && !order.messageSent.equals(MessageSent.PaymentSuccessful)) {
-          var requestId = messagingService.receiveRequest(2);
-          order.messageSent = MessageSent.PaymentSuccessful;
-          LOG.info("Order " + order.id + ": Payment Success message sent,"
-              + " request Id: " + requestId);
-        }
-      };
+      Retry.Operation op = handleSuccessMessageRetryOperation(order);
       Retry.HandleErrorIssue<Order> handleError = (o, err) -> {
-        if ((o.messageSent.equals(MessageSent.NoneSent) || o.messageSent
-            .equals(MessageSent.PaymentTrying))
-            && System.currentTimeMillis() - o.createdTime < messageTime) {
-          var qt = new QueueTask(order, TaskType.Messaging, 2);
-          updateQueue(qt);
-          LOG.info("Order " + order.id + ": Error in sending Payment Success message, trying to"
-              + " queue task and add to employee handle..");
-          employeeHandleIssue(order);
-        }
+        handleSuccessMessageErrorIssue(order, o);
       };
       var r = new Retry<>(op, handleError, numOfRetries, retryDuration,
           e -> DatabaseUnavailableException.class.isAssignableFrom(e.getClass()));
@@ -372,6 +348,40 @@ public class Commander {
     t.start();
   }
 
+  private void handleSuccessMessageErrorIssue(Order order, Order o) {
+    if ((o.messageSent.equals(MessageSent.NONE_SENT) || o.messageSent
+        .equals(MessageSent.PAYMENT_TRYING))
+        && System.currentTimeMillis() - o.createdTime < messageTime) {
+      var qt = new QueueTask(order, TaskType.MESSAGING, 2);
+      updateQueue(qt);
+      LOG.info("Order " + order.id + ": Error in sending Payment Success message, trying to"
+          + " queue task and add to employee handle..");
+      employeeHandleIssue(order);
+    }
+  }
+
+  private Retry.Operation handleSuccessMessageRetryOperation(Order order) {
+    return (l) -> {
+      if (!l.isEmpty()) {
+        if (DatabaseUnavailableException.class.isAssignableFrom(l.get(0).getClass())) {
+          LOG.debug("Order " + order.id + ": Error in connecting to messaging service "
+              + "(Payment Success msg), trying again..");
+        } else {
+          LOG.debug("Order " + order.id + ": Error in creating Payment Success"
+              + " messaging request..");
+        }
+        throw l.remove(0);
+      }
+      if (!order.messageSent.equals(MessageSent.PAYMENT_FAIL)
+          && !order.messageSent.equals(MessageSent.PAYMENT_SUCCESSFUL)) {
+        var requestId = messagingService.receiveRequest(2);
+        order.messageSent = MessageSent.PAYMENT_SUCCESSFUL;
+        LOG.info("Order " + order.id + ": Payment Success message sent,"
+            + " request Id: " + requestId);
+      }
+    };
+  }
+
   private void sendPaymentFailureMessage(Order order) {
     if (System.currentTimeMillis() - order.createdTime >= this.messageTime) {
       LOG.trace("Order " + order.id + ": Message time for order over, returning..");
@@ -380,34 +390,10 @@ public class Commander {
     var list = messagingService.exceptionsList;
     var t = new Thread(() -> {
       Retry.Operation op = (l) -> {
-        if (!l.isEmpty()) {
-          if (DatabaseUnavailableException.class.isAssignableFrom(l.get(0).getClass())) {
-            LOG.debug("Order " + order.id + ": Error in connecting to messaging service "
-                + "(Payment Failure msg), trying again..");
-          } else {
-            LOG.debug("Order " + order.id + ": Error in creating Payment Failure"
-                + " message request..");
-          }
-          throw l.remove(0);
-        }
-        if (!order.messageSent.equals(MessageSent.PaymentFail)
-            && !order.messageSent.equals(MessageSent.PaymentSuccessful)) {
-          var requestId = messagingService.receiveRequest(0);
-          order.messageSent = MessageSent.PaymentFail;
-          LOG.info("Order " + order.id + ": Payment Failure message sent successfully,"
-              + " request Id: " + requestId);
-        }
+        handlePaymentFailureRetryOperation(order, l);
       };
       Retry.HandleErrorIssue<Order> handleError = (o, err) -> {
-        if ((o.messageSent.equals(MessageSent.NoneSent) || o.messageSent
-            .equals(MessageSent.PaymentTrying))
-            && System.currentTimeMillis() - o.createdTime < messageTime) {
-          var qt = new QueueTask(order, TaskType.Messaging, 0);
-          updateQueue(qt);
-          LOG.warn("Order " + order.id + ": Error in sending Payment Failure message, "
-              + "trying to queue task and add to employee handle..");
-          employeeHandleIssue(o);
-        }
+        handlePaymentErrorIssue(order, o);
       };
       var r = new Retry<>(op, handleError, numOfRetries, retryDuration,
           e -> DatabaseUnavailableException.class.isAssignableFrom(e.getClass()));
@@ -420,6 +406,38 @@ public class Commander {
     t.start();
   }
 
+  private void handlePaymentErrorIssue(Order order, Order o) {
+    if ((o.messageSent.equals(MessageSent.NONE_SENT) || o.messageSent
+        .equals(MessageSent.PAYMENT_TRYING))
+        && System.currentTimeMillis() - o.createdTime < messageTime) {
+      var qt = new QueueTask(order, TaskType.MESSAGING, 0);
+      updateQueue(qt);
+      LOG.warn("Order " + order.id + ": Error in sending Payment Failure message, "
+          + "trying to queue task and add to employee handle..");
+      employeeHandleIssue(o);
+    }
+  }
+
+  private void handlePaymentFailureRetryOperation(Order order, List<Exception> l) throws Exception {
+    if (!l.isEmpty()) {
+      if (DatabaseUnavailableException.class.isAssignableFrom(l.get(0).getClass())) {
+        LOG.debug("Order " + order.id + ": Error in connecting to messaging service "
+            + "(Payment Failure msg), trying again..");
+      } else {
+        LOG.debug("Order " + order.id + ": Error in creating Payment Failure"
+            + " message request..");
+      }
+      throw l.remove(0);
+    }
+    if (!order.messageSent.equals(MessageSent.PAYMENT_FAIL)
+        && !order.messageSent.equals(MessageSent.PAYMENT_SUCCESSFUL)) {
+      var requestId = messagingService.receiveRequest(0);
+      order.messageSent = MessageSent.PAYMENT_FAIL;
+      LOG.info("Order " + order.id + ": Payment Failure message sent successfully,"
+          + " request Id: " + requestId);
+    }
+  }
+
   private void sendPaymentPossibleErrorMsg(Order order) {
     if (System.currentTimeMillis() - order.createdTime >= this.messageTime) {
       LOG.trace("Message time for order over, returning..");
@@ -428,34 +446,10 @@ public class Commander {
     var list = messagingService.exceptionsList;
     var t = new Thread(() -> {
       Retry.Operation op = (l) -> {
-        if (!l.isEmpty()) {
-          if (DatabaseUnavailableException.class.isAssignableFrom(l.get(0).getClass())) {
-            LOG.debug("Order " + order.id + ": Error in connecting to messaging service "
-                + "(Payment Error msg), trying again..");
-          } else {
-            LOG.debug("Order " + order.id + ": Error in creating Payment Error"
-                + " messaging request..");
-          }
-          throw l.remove(0);
-        }
-        if (order.paid.equals(PaymentStatus.Trying) && order.messageSent
-            .equals(MessageSent.NoneSent)) {
-          var requestId = messagingService.receiveRequest(1);
-          order.messageSent = MessageSent.PaymentTrying;
-          LOG.info("Order " + order.id + ": Payment Error message sent successfully,"
-              + " request Id: " + requestId);
-        }
+        handlePaymentPossibleErrorMsgRetryOperation(order, l);
       };
       Retry.HandleErrorIssue<Order> handleError = (o, err) -> {
-        if (o.messageSent.equals(MessageSent.NoneSent) && order.paid
-            .equals(PaymentStatus.Trying)
-            && System.currentTimeMillis() - o.createdTime < messageTime) {
-          var qt = new QueueTask(order, TaskType.Messaging, 1);
-          updateQueue(qt);
-          LOG.warn("Order " + order.id + ": Error in sending Payment Error message, "
-              + "trying to queue task and add to employee handle..");
-          employeeHandleIssue(o);
-        }
+        handlePaymentPossibleErrorMsgErrorIssue(order, o);
       };
       var r = new Retry<>(op, handleError, numOfRetries, retryDuration,
           e -> DatabaseUnavailableException.class.isAssignableFrom(e.getClass()));
@@ -468,6 +462,39 @@ public class Commander {
     t.start();
   }
 
+  private void handlePaymentPossibleErrorMsgErrorIssue(Order order, Order o) {
+    if (o.messageSent.equals(MessageSent.NONE_SENT) && order.paid
+        .equals(PaymentStatus.TRYING)
+        && System.currentTimeMillis() - o.createdTime < messageTime) {
+      var qt = new QueueTask(order, TaskType.MESSAGING, 1);
+      updateQueue(qt);
+      LOG.warn("Order " + order.id + ": Error in sending Payment Error message, "
+          + "trying to queue task and add to employee handle..");
+      employeeHandleIssue(o);
+    }
+  }
+
+  private void handlePaymentPossibleErrorMsgRetryOperation(Order order, List<Exception> l)
+          throws Exception {
+    if (!l.isEmpty()) {
+      if (DatabaseUnavailableException.class.isAssignableFrom(l.get(0).getClass())) {
+        LOG.debug("Order " + order.id + ": Error in connecting to messaging service "
+            + "(Payment Error msg), trying again..");
+      } else {
+        LOG.debug("Order " + order.id + ": Error in creating Payment Error"
+            + " messaging request..");
+      }
+      throw l.remove(0);
+    }
+    if (order.paid.equals(PaymentStatus.TRYING) && order.messageSent
+        .equals(MessageSent.NONE_SENT)) {
+      var requestId = messagingService.receiveRequest(1);
+      order.messageSent = MessageSent.PAYMENT_TRYING;
+      LOG.info("Order " + order.id + ": Payment Error message sent successfully,"
+          + " request Id: " + requestId);
+    }
+  }
+
   private void employeeHandleIssue(Order order) {
     if (System.currentTimeMillis() - order.createdTime >= this.employeeTime) {
       LOG.trace("Order " + order.id + ": Employee handle time for order over, returning..");
@@ -490,7 +517,7 @@ public class Commander {
       Retry.HandleErrorIssue<Order> handleError = (o, err) -> {
         if (!o.addedToEmployeeHandle && System
             .currentTimeMillis() - order.createdTime < employeeTime) {
-          var qt = new QueueTask(order, TaskType.EmployeeDb, -1);
+          var qt = new QueueTask(order, TaskType.EMPLOYEE_DB, -1);
           updateQueue(qt);
           LOG.warn("Order " + order.id + ": Error in adding to employee db,"
               + " trying to queue task..");
@@ -520,21 +547,21 @@ public class Commander {
         LOG.trace("Order " + qt.order.id + ": This queue task of type " + qt.getType()
             + " does not need to be done anymore (timeout), dequeue..");
       } else {
-        if (qt.taskType.equals(TaskType.Payment)) {
-          if (!qt.order.paid.equals(PaymentStatus.Trying)) {
+        if (qt.taskType.equals(TaskType.PAYMENT)) {
+          if (!qt.order.paid.equals(PaymentStatus.TRYING)) {
             tryDequeue();
             LOG.trace("Order " + qt.order.id + ": This payment task already done, dequeueing..");
           } else {
             sendPaymentRequest(qt.order);
             LOG.debug("Order " + qt.order.id + ": Trying to connect to payment service..");
           }
-        } else if (qt.taskType.equals(TaskType.Messaging)) {
-          if (qt.order.messageSent.equals(MessageSent.PaymentFail)
-              || qt.order.messageSent.equals(MessageSent.PaymentSuccessful)) {
+        } else if (qt.taskType.equals(TaskType.MESSAGING)) {
+          if (qt.order.messageSent.equals(MessageSent.PAYMENT_FAIL)
+              || qt.order.messageSent.equals(MessageSent.PAYMENT_SUCCESSFUL)) {
             tryDequeue();
             LOG.trace("Order " + qt.order.id + ": This messaging task already done, dequeue..");
-          } else if (qt.messageType == 1 && (!qt.order.messageSent.equals(MessageSent.NoneSent)
-              || !qt.order.paid.equals(PaymentStatus.Trying))) {
+          } else if (qt.messageType == 1 && (!qt.order.messageSent.equals(MessageSent.NONE_SENT)
+              || !qt.order.paid.equals(PaymentStatus.TRYING))) {
             tryDequeue();
             LOG.trace("Order " + qt.order.id + ": This messaging task does not need to be done,"
                 + " dequeue..");
@@ -548,7 +575,7 @@ public class Commander {
             sendSuccessMessage(qt.order);
             LOG.debug("Order " + qt.order.id + ": Trying to connect to messaging service..");
           }
-        } else if (qt.taskType.equals(TaskType.EmployeeDb)) {
+        } else if (qt.taskType.equals(TaskType.EMPLOYEE_DB)) {
           if (qt.order.addedToEmployeeHandle) {
             tryDequeue();
             LOG.trace("Order " + qt.order.id + ": This employee handle task already done,"

commander/src/main/java/com/iluwatar/commander/Order.java

@@ -33,11 +33,11 @@ import java.util.Random;
 public class Order { //can store all transactions ids also
 
   enum PaymentStatus {
-    NotDone, Trying, Done
+    NOT_DONE, TRYING, DONE
   }
 
   enum MessageSent {
-    NoneSent, PaymentFail, PaymentTrying, PaymentSuccessful
+    NONE_SENT, PAYMENT_FAIL, PAYMENT_TRYING, PAYMENT_SUCCESSFUL
   }
 
   final User user;
@@ -65,8 +65,8 @@ public class Order { //can store all transactions ids also
     }
     this.id = id;
     USED_IDS.put(this.id, true);
-    this.paid = PaymentStatus.Trying;
-    this.messageSent = MessageSent.NoneSent;
+    this.paid = PaymentStatus.TRYING;
+    this.messageSent = MessageSent.NONE_SENT;
     this.addedToEmployeeHandle = false;
   }
 

commander/src/main/java/com/iluwatar/commander/messagingservice/MessagingService.java

@@ -38,7 +38,7 @@ public class MessagingService extends Service {
   private static final Logger LOGGER = LoggerFactory.getLogger(MessagingService.class);
 
   enum MessageToSend {
-    PaymentFail, PaymentTrying, PaymentSuccessful
+    PAYMENT_FAIL, PAYMENT_TRYING, PAYMENT_SUCCESSFUL
   }
 
   class MessageRequest {
@@ -63,11 +63,11 @@ public class MessagingService extends Service {
     var id = generateId();
     MessageToSend msg;
     if (messageToSend == 0) {
-      msg = MessageToSend.PaymentFail;
+      msg = MessageToSend.PAYMENT_FAIL;
     } else if (messageToSend == 1) {
-      msg = MessageToSend.PaymentTrying;
+      msg = MessageToSend.PAYMENT_TRYING;
     } else { //messageToSend == 2
-      msg = MessageToSend.PaymentSuccessful;
+      msg = MessageToSend.PAYMENT_SUCCESSFUL;
     }
     var req = new MessageRequest(id, msg);
     return updateDb(req);
@@ -84,10 +84,10 @@ public class MessagingService extends Service {
   }
 
   String sendMessage(MessageToSend m) {
-    if (m.equals(MessageToSend.PaymentSuccessful)) {
+    if (m.equals(MessageToSend.PAYMENT_SUCCESSFUL)) {
       return "Msg: Your order has been placed and paid for successfully!"
           + " Thank you for shopping with us!";
-    } else if (m.equals(MessageToSend.PaymentTrying)) {
+    } else if (m.equals(MessageToSend.PAYMENT_TRYING)) {
       return "Msg: There was an error in your payment process,"
           + " we are working on it and will return back to you shortly."
           + " Meanwhile, your order has been placed and will be shipped.";

commander/src/main/java/com/iluwatar/commander/queue/QueueTask.java

@@ -36,7 +36,7 @@ public class QueueTask {
    */
 
   public enum TaskType {
-    Messaging, Payment, EmployeeDb
+    MESSAGING, PAYMENT, EMPLOYEE_DB
   }
 
   public Order order;
@@ -68,7 +68,7 @@ public class QueueTask {
    * @return String representing type of task
    */
   public String getType() {
-    if (!this.taskType.equals(TaskType.Messaging)) {
+    if (!this.taskType.equals(TaskType.MESSAGING)) {
       return this.taskType.toString();
     } else {
       if (this.messageType == 0) {

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

@@ -23,15 +23,23 @@
 
 package com.iluwatar.composite;
 
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    Assertions.assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -25,14 +25,24 @@ package com.iluwatar.converter;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * App running test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void testMain() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 
 }

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

@@ -25,12 +25,22 @@ package com.iluwatar.dao;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that DAO example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
   @Test
-  public void test() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteDaoWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

data-locality/src/main/java/com/iluwatar/data/locality/game/component/AiComponent.java

@@ -43,6 +43,6 @@ public class AiComponent implements Component {
 
   @Override
   public void render() {
-
+    // Do Nothing.
   }
 }

data-locality/src/main/java/com/iluwatar/data/locality/game/component/manager/AiComponentManager.java

@@ -40,7 +40,7 @@ public class AiComponentManager {
 
   private final int numEntities;
 
-  private static final Component[] AI_COMPONENTS = new AiComponent[MAX_ENTITIES];
+  private final Component[] aiComponents = new AiComponent[MAX_ENTITIES];
 
   public AiComponentManager(int numEntities) {
     this.numEntities = numEntities;
@@ -51,7 +51,7 @@ public class AiComponentManager {
    */
   public void start() {
     LOGGER.info("Start AI Game Component");
-    IntStream.range(0, numEntities).forEach(i -> AI_COMPONENTS[i] = new AiComponent());
+    IntStream.range(0, numEntities).forEach(i -> aiComponents[i] = new AiComponent());
   }
 
   /**
@@ -60,7 +60,7 @@ public class AiComponentManager {
   public void update() {
     LOGGER.info("Update AI Game Component");
     IntStream.range(0, numEntities)
-        .filter(i -> AI_COMPONENTS.length > i && AI_COMPONENTS[i] != null)
-        .forEach(i -> AI_COMPONENTS[i].update());
+        .filter(i -> aiComponents.length > i && aiComponents[i] != null)
+        .forEach(i -> aiComponents[i].update());
   }
 }

data-locality/src/main/java/com/iluwatar/data/locality/game/component/manager/PhysicsComponentManager.java

@@ -40,7 +40,7 @@ public class PhysicsComponentManager {
 
   private final int numEntities;
 
-  private static final Component[] PHYSICS_COMPONENTS = new PhysicsComponent[MAX_ENTITIES];
+  private final Component[] physicsComponents = new PhysicsComponent[MAX_ENTITIES];
 
   public PhysicsComponentManager(int numEntities) {
     this.numEntities = numEntities;
@@ -51,7 +51,7 @@ public class PhysicsComponentManager {
    */
   public void start() {
     LOGGER.info("Start Physics Game Component ");
-    IntStream.range(0, numEntities).forEach(i -> PHYSICS_COMPONENTS[i] = new PhysicsComponent());
+    IntStream.range(0, numEntities).forEach(i -> physicsComponents[i] = new PhysicsComponent());
   }
 
 
@@ -62,7 +62,7 @@ public class PhysicsComponentManager {
     LOGGER.info("Update Physics Game Component ");
     // Process physics.
     IntStream.range(0, numEntities)
-        .filter(i -> PHYSICS_COMPONENTS.length > i && PHYSICS_COMPONENTS[i] != null)
-        .forEach(i -> PHYSICS_COMPONENTS[i].update());
+        .filter(i -> physicsComponents.length > i && physicsComponents[i] != null)
+        .forEach(i -> physicsComponents[i].update());
   }
 }

data-locality/src/main/java/com/iluwatar/data/locality/game/component/manager/RenderComponentManager.java

@@ -40,7 +40,7 @@ public class RenderComponentManager {
 
   private final int numEntities;
 
-  private static final Component[] RENDER_COMPONENTS = new RenderComponent[MAX_ENTITIES];
+  private final Component[] renderComponents = new RenderComponent[MAX_ENTITIES];
 
   public RenderComponentManager(int numEntities) {
     this.numEntities = numEntities;
@@ -51,7 +51,7 @@ public class RenderComponentManager {
    */
   public void start() {
     LOGGER.info("Start Render Game Component ");
-    IntStream.range(0, numEntities).forEach(i -> RENDER_COMPONENTS[i] = new RenderComponent());
+    IntStream.range(0, numEntities).forEach(i -> renderComponents[i] = new RenderComponent());
   }
 
 
@@ -62,7 +62,7 @@ public class RenderComponentManager {
     LOGGER.info("Update Render Game Component ");
     // Process Render.
     IntStream.range(0, numEntities)
-        .filter(i -> RENDER_COMPONENTS.length > i && RENDER_COMPONENTS[i] != null)
-        .forEach(i -> RENDER_COMPONENTS[i].render());
+        .filter(i -> renderComponents.length > i && renderComponents[i] != null)
+        .forEach(i -> renderComponents[i].render());
   }
 }

data-locality/src/test/java/com/iluwatar/data/locality/ApplicationTest.java

@@ -26,16 +26,22 @@ package com.iluwatar.data.locality;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Test Game Application
  */
 class ApplicationTest {
 
   /**
-   * Test run
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link Application#main(String[])}
+   * throws an exception.
    */
+
   @Test
-  void main() {
-    Application.main(new String[] {});
+  void shouldExecuteGameApplicationWithoutException() {
+    assertDoesNotThrow(() -> Application.main(new String[] {}));
   }
 }

data-mapper/src/test/java/com/iluwatar/datamapper/AppTest.java

@@ -24,14 +24,25 @@
 package com.iluwatar.datamapper;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.function.Executable;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 
 /**
  * Tests that Data-Mapper example runs without errors.
  */
-public final class AppTest {
+final class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main();
+  void shouldExecuteApplicationWithoutException() {
+
+    assertDoesNotThrow((Executable) App::main);
   }
 }

data-transfer-object/src/test/java/com/iluwatar/datatransfer/AppTest.java

@@ -25,9 +25,19 @@ package com.iluwatar.datatransfer;
 
 import org.junit.jupiter.api.Test;
 
-public class AppTest {
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+class AppTest {
+
+    /**
+     * Issue: Add at least one assertion to this test case.
+     *
+     * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+     * throws an exception.
+     */
+
     @Test
-    public void test() throws Exception {
-        App.main(new String[]{});
+    void shouldExecuteApplicationWithoutException() {
+        assertDoesNotThrow(() -> App.main(new String[]{}));
     }
 }

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

@@ -25,13 +25,22 @@ package com.iluwatar.decorator;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

delegation/src/test/java/com/iluwatar/delegation/simple/AppTest.java

@@ -25,14 +25,23 @@ package com.iluwatar.delegation.simple;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application Test Entry
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 
 }

dependency-injection/src/test/java/com/iluwatar/dependency/injection/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.dependency.injection;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

dirty-flag/pom.xml

@@ -31,7 +31,6 @@
     <artifactId>java-design-patterns</artifactId>
     <version>1.23.0-SNAPSHOT</version>
   </parent>
-  <groupId>com.iluwatar</groupId>
   <artifactId>dirty-flag</artifactId>
   <version>1.23.0-SNAPSHOT</version>
   <name>dirty-flag</name>
@@ -45,6 +44,12 @@
       <artifactId>junit-jupiter-engine</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-junit-jupiter</artifactId>
+      <version>3.5.0</version>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
   <build>
     <plugins>

dirty-flag/src/test/java/org/dirty/flag/AppTest.java

@@ -26,12 +26,22 @@ package org.dirty.flag;
 import com.iluwatar.dirtyflag.App;
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Dirty-Flag example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

dirty-flag/src/test/java/org/dirty/flag/DirtyFlagTest.java

@@ -23,29 +23,27 @@
 
 package org.dirty.flag;
 
-import static org.junit.jupiter.api.Assertions.assertFalse;
-import static org.junit.jupiter.api.Assertions.assertTrue;
-
 import com.iluwatar.dirtyflag.DataFetcher;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
 /**
  * Application test
  */
-public class DirtyFlagTest {
+class DirtyFlagTest {
 
   @Test
-  public void testIsDirty() {
+  void testIsDirty() {
     var df = new DataFetcher();
     var countries = df.fetch();
-    assertFalse(countries.isEmpty());
+    Assertions.assertFalse(countries.isEmpty());
   }
 
   @Test
-  public void testIsNotDirty() {
+  void testIsNotDirty() {
     var df = new DataFetcher();
     df.fetch();
     var countries = df.fetch();
-    assertTrue(countries.isEmpty());
+    Assertions.assertTrue(countries.isEmpty());
   }
 }

double-buffer/pom.xml

@@ -44,6 +44,11 @@
             <groupId>org.apache.commons</groupId>
             <artifactId>commons-lang3</artifactId>
         </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
     <build>
         <plugins>

double-buffer/src/test/java/com/iluwatar/doublebuffer/AppTest.java

@@ -25,14 +25,23 @@ package com.iluwatar.doublebuffer;
 
 import org.junit.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * App unit test.
  */
 public class AppTest {
 
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
   @Test
-  public void testMain() {
-    App.main(new String[]{});
+  public void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 
 }

double-checked-locking/src/test/java/com/iluwatar/doublechecked/locking/AppTest.java

@@ -25,13 +25,23 @@ package com.iluwatar.doublechecked.locking;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

double-dispatch/src/test/java/com/iluwatar/doubledispatch/AppTest.java

@@ -25,13 +25,23 @@ package com.iluwatar.doubledispatch;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

eip-aggregator/src/test/java/com/iluwatar/eip/aggregator/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.eip.aggregator;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Test for App class
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void testMain() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

eip-message-channel/src/test/java/com/iluwatar/eip/message/channel/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.eip.message.channel;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

eip-publish-subscribe/src/test/java/com/iluwatar/eip/publish/subscribe/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.eip.publish.subscribe;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

eip-splitter/src/test/java/com/iluwatar/eip/splitter/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.eip.splitter;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Test for App class
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void testMain() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

eip-wire-tap/src/test/java/com/iluwatar/eip/wiretap/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.eip.wiretap;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Test for App class
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void testMain() throws Exception {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

event-aggregator/src/test/java/com/iluwatar/event/aggregator/AppTest.java

@@ -25,13 +25,22 @@ package com.iluwatar.event.aggregator;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

event-asynchronous/src/test/java/com/iluwatar/event/asynchronous/AppTest.java

@@ -25,12 +25,22 @@ package com.iluwatar.event.asynchronous;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that EventAsynchronous example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

event-driven-architecture/src/test/java/com/iluwatar/eda/AppTest.java

@@ -25,12 +25,22 @@ package com.iluwatar.eda;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Event Driven Architecture example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

execute-around/src/test/java/com/iluwatar/execute/around/AppTest.java

@@ -29,19 +29,21 @@ import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests execute-around example.
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() throws IOException {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 
   @BeforeEach
   @AfterEach
-  public void cleanup() {
+  void cleanup() {
     var file = new File("testfile.txt");
     file.delete();
   }

extension-objects/src/main/java/concreteextensions/Commander.java

@@ -45,4 +45,8 @@ public class Commander implements CommanderExtension {
   public void commanderReady() {
     LOGGER.info("[Commander] " + unit.getName() + " is ready!");
   }
+
+  public CommanderUnit getUnit() {
+    return unit;
+  }
 }

extension-objects/src/main/java/concreteextensions/Sergeant.java

@@ -43,6 +43,10 @@ public class Sergeant implements SergeantExtension {
 
   @Override
   public void sergeantReady() {
-    LOGGER.info("[Sergeant] " + unit.getName() + " is ready! ");
+    LOGGER.info("[Sergeant] " + unit.getName() + " is ready!");
+  }
+
+  public SergeantUnit getUnit() {
+    return unit;
   }
 }

extension-objects/src/main/java/concreteextensions/Soldier.java

@@ -42,6 +42,10 @@ public class Soldier implements SoldierExtension {
 
   @Override
   public void soldierReady() {
-    LOGGER.info("[Solider] " + unit.getName() + "  is ready!");
+    LOGGER.info("[Soldier] " + unit.getName() + " is ready!");
+  }
+
+  public SoldierUnit getUnit() {
+    return unit;
   }
 }

extension-objects/src/test/java/AppTest.java

@@ -23,13 +23,16 @@
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Created by Srdjan on 03-May-17.
  */
-public class AppTest {
+class AppTest {
+
   @Test
-  public void main() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 
 }

extension-objects/src/test/java/concreteextensions/CommanderTest.java

@@ -23,17 +23,43 @@
 
 package concreteextensions;
 
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
 import org.junit.jupiter.api.Test;
+import org.slf4j.LoggerFactory;
 import units.CommanderUnit;
 
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 /**
  * Created by Srdjan on 03-May-17.
+ *
+ * Modified by ToxicDreamz on 15-Aug-20
  */
-public class CommanderTest {
+class CommanderTest {
+
   @Test
-  public void commanderReady() {
+  void shouldExecuteCommanderReady() {
+
+    Logger commanderLogger = (Logger) LoggerFactory.getLogger(Commander.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+
+    commanderLogger.addAppender(listAppender);
+
     final var commander = new Commander(new CommanderUnit("CommanderUnitTest"));
     commander.commanderReady();
+
+    List<ILoggingEvent> logsList = listAppender.list;
+    assertEquals("[Commander] " + commander.getUnit().getName() + " is ready!", logsList.get(0)
+            .getMessage());
+    assertEquals(Level.INFO, logsList.get(0)
+            .getLevel());
   }
 
 }

extension-objects/src/test/java/concreteextensions/SergeantTest.java

@@ -23,17 +23,41 @@
 
 package concreteextensions;
 
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
 import org.junit.jupiter.api.Test;
+import org.slf4j.LoggerFactory;
 import units.SergeantUnit;
 
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 /**
  * Created by Srdjan on 03-May-17.
  */
-public class SergeantTest {
+class SergeantTest {
+
   @Test
-  public void sergeantReady() {
+  void sergeantReady() {
+
+    Logger sergeantLogger = (Logger) LoggerFactory.getLogger(Sergeant.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+
+    sergeantLogger.addAppender(listAppender);
+
     final var sergeant = new Sergeant(new SergeantUnit("SergeantUnitTest"));
     sergeant.sergeantReady();
+
+    List<ILoggingEvent> logsList = listAppender.list;
+    assertEquals("[Sergeant] " + sergeant.getUnit().getName() + " is ready!", logsList.get(0)
+            .getMessage());
+    assertEquals(Level.INFO, logsList.get(0)
+            .getLevel());
   }
 
 }

extension-objects/src/test/java/concreteextensions/SoldierTest.java

@@ -23,17 +23,41 @@
 
 package concreteextensions;
 
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
 import org.junit.jupiter.api.Test;
+import org.slf4j.LoggerFactory;
 import units.SoldierUnit;
 
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 /**
  * Created by Srdjan on 03-May-17.
  */
-public class SoldierTest {
+class SoldierTest {
+
   @Test
-  public void soldierReady() {
+  void soldierReady() {
+
+    Logger soldierLogger = (Logger) LoggerFactory.getLogger(Soldier.class);
+
+    ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+    listAppender.start();
+
+    soldierLogger.addAppender(listAppender);
+
     final var soldier = new Soldier(new SoldierUnit("SoldierUnitTest"));
     soldier.soldierReady();
+
+    List<ILoggingEvent> logsList = listAppender.list;
+    assertEquals("[Soldier] " + soldier.getUnit().getName() + " is ready!", logsList.get(0)
+            .getMessage());
+    assertEquals(Level.INFO, logsList.get(0)
+            .getLevel());
   }
 
 }

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

@@ -25,13 +25,15 @@ package com.iluwatar.facade;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

factory-kit/src/test/java/com/iluwatar/factorykit/app/AppTest.java

@@ -26,14 +26,16 @@ package com.iluwatar.factorykit.app;
 import com.iluwatar.factorykit.App;
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application Test Entrypoint
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }
 

factory-method/src/test/java/com/iluwatar/factory/method/AppTest.java

@@ -25,12 +25,15 @@ package com.iluwatar.factory.method;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Tests that Factory Method example runs without errors.
  */
-public class AppTest {
+class AppTest {
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

filterer/README.md

@@ -0,0 +1,239 @@
+---
+layout: pattern
+title: Filterer
+folder: filterer
+permalink: /patterns/filterer/
+description: Design pattern that helps container-like objects to return filtered version of themselves.# short meta description that shows in Google search results
+categories:
+ - Functional
+tags:
+ - Extensibility
+---
+
+## Name / classification
+
+Filterer
+
+## Intent
+
+The intent of this design pattern is to introduce a functional interface that will add a 
+functionality for container-like objects to easily return filtered versions of themselves.
+
+## Explanation
+
+Real world example
+
+> We are designing a threat (malware) detection software which can analyze target systems for 
+> threats that are present in it. In the design we have to take into consideration that new 
+> Threat types can be added later. Additionally, there is a requirement that the threat detection 
+> system can filter the detected threats based on different criteria (the target system acts as 
+> container-like object for threats).
+
+In plain words
+
+> Filterer pattern is a design pattern that helps container-like objects return filtered versions 
+> of themselves. 
+
+**Programmatic Example**
+
+To model the threat detection example presented above we introduce `Threat` and `ThreatAwareSystem` 
+interfaces.
+
+```java
+public interface Threat {
+  String name();
+  int id();
+  ThreatType type();
+}
+
+public interface ThreatAwareSystem {
+  String systemId();
+  List<? extends Threat> threats();
+  Filterer<? extends ThreatAwareSystem, ? extends Threat> filtered();
+
+}
+```
+
+Notice the `filtered` method that returns instance of `Filterer` interface which is defined as:
+
+```java
+@FunctionalInterface
+public interface Filterer<G, E> {
+  G by(Predicate<? super E> predicate);
+}
+```
+
+It is used to fulfill the requirement for system to be able to filter itself based on threat 
+properties. The container-like object (`ThreatAwareSystem` in our case) needs to have a method that 
+returns an instance of `Filterer`. This helper interface gives ability to covariantly specify a 
+lower bound of contravariant `Predicate` in the subinterfaces of interfaces representing the 
+container-like objects.
+
+In our example we will be able to pass a predicate that takes `? extends Threat` object and 
+return `?  extends ThreatAwareSystem` from `Filtered::by` method. A simple implementation 
+of `ThreatAwareSystem`:
+
+```java
+public class SimpleThreatAwareSystem implements ThreatAwareSystem {
+
+  private final String systemId;
+  private final ImmutableList<Threat> issues;
+
+  public SimpleThreatAwareSystem(final String systemId, final List<Threat> issues) {
+    this.systemId = systemId;
+    this.issues = ImmutableList.copyOf(issues);
+  }
+  
+  @Override
+  public String systemId() {
+    return systemId;
+  }
+  
+  @Override
+  public List<? extends Threat> threats() {
+    return new ArrayList<>(issues);
+  }
+
+  @Override
+  public Filterer<? extends ThreatAwareSystem, ? extends Threat> filtered() {
+    return this::filteredGroup;
+  }
+
+  private ThreatAwareSystem filteredGroup(Predicate<? super Threat> predicate) {
+    return new SimpleThreatAwareSystem(this.systemId, filteredItems(predicate));
+  }
+
+  private List<Threat> filteredItems(Predicate<? super Threat> predicate) {
+    return this.issues.stream()
+            .filter(predicate)
+            .collect(Collectors.toList());
+  }
+}
+```
+
+The `filtered` method is overridden to filter the threats list by given predicate.
+
+Now if we introduce a new subtype of `Threat` interface that adds probability with which given 
+threat can appear:
+
+```java
+public interface ProbableThreat extends Threat {
+  double probability();
+}
+```
+
+We can also introduce a new interface that represents a system that is aware of threats with their 
+probabilities:
+
+````java
+public interface ProbabilisticThreatAwareSystem extends ThreatAwareSystem {
+  @Override
+  List<? extends ProbableThreat> threats();
+
+  @Override
+  Filterer<? extends ProbabilisticThreatAwareSystem, ? extends ProbableThreat> filtered();
+}
+````
+
+Notice how we override the `filtered` method in `ProbabilisticThreatAwareSystem` and specify 
+different return covariant type by specifying different generic types. Our interfaces are clean and 
+not cluttered by default implementations. We we will be able to filter 
+`ProbabilisticThreatAwareSystem` by `ProbableThreat` properties:
+
+```java
+public class SimpleProbabilisticThreatAwareSystem implements ProbabilisticThreatAwareSystem {
+
+  private final String systemId;
+  private final ImmutableList<ProbableThreat> threats;
+
+  public SimpleProbabilisticThreatAwareSystem(final String systemId, final List<ProbableThreat> threats) {
+    this.systemId = systemId;
+    this.threats = ImmutableList.copyOf(threats);
+  }
+
+  @Override
+  public String systemId() {
+    return systemId;
+  }
+
+  @Override
+  public List<? extends ProbableThreat> threats() {
+    return threats;
+  }
+
+  @Override
+  public Filterer<? extends ProbabilisticThreatAwareSystem, ? extends ProbableThreat> filtered() {
+    return this::filteredGroup;
+  }
+
+  private ProbabilisticThreatAwareSystem filteredGroup(final Predicate<? super ProbableThreat> predicate) {
+    return new SimpleProbabilisticThreatAwareSystem(this.systemId, filteredItems(predicate));
+  }
+
+  private List<ProbableThreat> filteredItems(final Predicate<? super ProbableThreat> predicate) {
+    return this.threats.stream()
+            .filter(predicate)
+            .collect(Collectors.toList());
+  }
+}
+```
+
+Now if we want filter `ThreatAwareSystem` by threat type we can do:
+
+```java
+Threat rootkit = new SimpleThreat(ThreatType.ROOTKIT, 1, "Simple-Rootkit");
+Threat trojan = new SimpleThreat(ThreatType.TROJAN, 2, "Simple-Trojan");
+List<Threat> threats = List.of(rootkit, trojan);
+
+ThreatAwareSystem threatAwareSystem = new SimpleThreatAwareSystem("System-1", threats);
+
+ThreatAwareSystem rootkitThreatAwareSystem = threatAwareSystem.filtered()
+           .by(threat -> threat.type() == ThreatType.ROOTKIT);
+```
+
+Or if we want to filter `ProbabilisticThreatAwareSystem`:
+
+```java
+ProbableThreat malwareTroyan = new SimpleProbableThreat("Troyan-ArcBomb", 1, ThreatType.TROJAN, 0.99);
+ProbableThreat rootkit = new SimpleProbableThreat("Rootkit-System", 2, ThreatType.ROOTKIT, 0.8);
+List<ProbableThreat> probableThreats = List.of(malwareTroyan, rootkit);
+
+ProbabilisticThreatAwareSystem simpleProbabilisticThreatAwareSystem =new SimpleProbabilisticThreatAwareSystem("System-1", probableThreats);
+
+ProbabilisticThreatAwareSystem filtered = simpleProbabilisticThreatAwareSystem.filtered()
+           .by(probableThreat -> Double.compare(probableThreat.probability(), 0.99) == 0);
+```
+
+## Class diagram
+
+![Filterer](./etc/filterer.png "Filterer")
+
+## Applicability
+
+Pattern can be used when working with container-like objects that use subtyping, instead of 
+parametrizing (generics) for extensible class structure. It enables you to easily extend filtering 
+ability of container-like objects as business requirements change.
+
+## Tutorials
+
+* [Article about Filterer pattern posted on it's author's blog](https://blog.tlinkowski.pl/2018/filterer-pattern/)
+* [Application of Filterer pattern in domain of text analysis](https://www.javacodegeeks.com/2019/02/filterer-pattern-10-steps.html)
+
+## Known uses
+
+One of the uses is present on the blog presented in 
+[this](https://www.javacodegeeks.com/2019/02/filterer-pattern-10-steps.html) link. It presents how 
+to use `Filterer` pattern to create text issue analyzer with support for test cases used for unit 
+testing.
+
+## Consequences
+
+Pros:
+ * You can easily introduce new subtypes for container-like objects and subtypes for objects that are contained within them and still be able to filter easily be new properties of those new subtypes.
+
+Cons:
+ * Covariant return types mixed with generics can be sometimes tricky
+
+## Credits
+
+* Author of the pattern : [Tomasz Linkowski](https://tlinkowski.pl/)

filterer/etc/filterer.urm.puml

@@ -0,0 +1,96 @@
+@startuml
+package com.iluwatar.filterer.domain {
+  interface Filterer<G, E> {
+    + by(Predicate<? super E>) : G {abstract}
+  }
+}
+package com.iluwatar.filterer {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    - filteringSimpleProbableThreats() {static}
+    - filteringSimpleThreats() {static}
+    + main(args : String[]) {static}
+  }
+}
+package com.iluwatar.filterer.threat {
+  interface ProbabilisticThreatAwareSystem {
+    + filtered() : Filterer<? extends ProbabilisticThreatAwareSystem, ? extends ProbableThreat> {abstract}
+    + threats() : List<? extends ProbableThreat> {abstract}
+  }
+  interface ProbableThreat {
+    + probability() : double {abstract}
+  }
+  class SimpleProbabilisticThreatAwareSystem {
+    - systemId : String
+    - threats : ImmutableList<ProbableThreat>
+    + SimpleProbabilisticThreatAwareSystem(systemId : String, threats : List<ProbableThreat>)
+    + equals(o : Object) : boolean
+    + filtered() : Filterer<? extends ProbabilisticThreatAwareSystem, ? extends ProbableThreat>
+    - filteredGroup(predicate : Predicate<? super ProbableThreat>) : ProbabilisticThreatAwareSystem
+    - filteredItems(predicate : Predicate<? super ProbableThreat>) : List<ProbableThreat>
+    + hashCode() : int
+    + systemId() : String
+    + threats() : List<? extends ProbableThreat>
+    + toString() : String
+  }
+  class SimpleProbableThreat {
+    - probability : double
+    + SimpleProbableThreat(name : String, id : int, threatType : ThreatType, probability : double)
+    + equals(o : Object) : boolean
+    + hashCode() : int
+    + probability() : double
+    + toString() : String
+  }
+  class SimpleThreat {
+    - id : int
+    - name : String
+    - threatType : ThreatType
+    + SimpleThreat(threatType : ThreatType, id : int, name : String)
+    + id() : int
+    + name() : String
+    + toString() : String
+    + type() : ThreatType
+  }
+  class SimpleThreatAwareSystem {
+    - issues : ImmutableList<Threat>
+    - systemId : String
+    + SimpleThreatAwareSystem(systemId : String, issues : List<Threat>)
+    + equals(o : Object) : boolean
+    + filtered() : Filterer<? extends ThreatAwareSystem, ? extends Threat>
+    - filteredGroup(predicate : Predicate<? super Threat>) : ThreatAwareSystem
+    - filteredItems(predicate : Predicate<? super Threat>) : List<Threat>
+    + hashCode() : int
+    + systemId() : String
+    + threats() : List<? extends Threat>
+    + toString() : String
+  }
+  interface Threat {
+    + id() : int {abstract}
+    + name() : String {abstract}
+    + type() : ThreatType {abstract}
+  }
+  interface ThreatAwareSystem {
+    + filtered() : Filterer<? extends ThreatAwareSystem, ? extends Threat> {abstract}
+    + systemId() : String {abstract}
+    + threats() : List<? extends Threat> {abstract}
+  }
+  enum ThreatType {
+    + ROOTKIT {static}
+    + TROJAN {static}
+    + WORM {static}
+    + valueOf(name : String) : ThreatType {static}
+    + values() : ThreatType[] {static}
+  }
+}
+SimpleThreatAwareSystem -->  "-issues" Threat
+SimpleThreat -->  "-threatType" ThreatType
+SimpleProbabilisticThreatAwareSystem -->  "-threats" ProbableThreat
+ProbabilisticThreatAwareSystem --|> ThreatAwareSystem 
+ProbableThreat --|> Threat 
+SimpleProbabilisticThreatAwareSystem ..|> ProbabilisticThreatAwareSystem 
+SimpleProbableThreat ..|> ProbableThreat 
+SimpleProbableThreat --|> SimpleThreat 
+SimpleThreat ..|> Threat 
+SimpleThreatAwareSystem ..|> ThreatAwareSystem 
+@enduml

filterer/pom.xml

@@ -0,0 +1,73 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2019 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>java-design-patterns</artifactId>
+        <groupId>com.iluwatar</groupId>
+        <version>1.23.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>filterer</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.google.guava</groupId>
+            <artifactId>guava</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <configuration>
+                            <archive>
+                                <manifest>
+                                    <mainClass>com.iluwatar.filterer.App</mainClass>
+                                </manifest>
+                            </archive>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

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

@@ -0,0 +1,108 @@
+/*
+ * 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.filterer;
+
+import com.iluwatar.filterer.threat.ProbableThreat;
+import com.iluwatar.filterer.threat.SimpleProbabilisticThreatAwareSystem;
+import com.iluwatar.filterer.threat.SimpleProbableThreat;
+import com.iluwatar.filterer.threat.SimpleThreat;
+import com.iluwatar.filterer.threat.SimpleThreatAwareSystem;
+import com.iluwatar.filterer.threat.Threat;
+import com.iluwatar.filterer.threat.ThreatAwareSystem;
+import com.iluwatar.filterer.threat.ThreatType;
+
+import java.util.List;
+import java.util.function.Predicate;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This demo class represent how {@link com.iluwatar.filterer.domain.Filterer} pattern is used to
+ * filter container-like objects to return filtered versions of themselves. The container like
+ * objects are systems that are aware of threats that they can be vulnerable to. We would like
+ * to have a way to create copy of different system objects but with filtered threats.
+ * The thing is to keep it simple if we add new subtype of {@link Threat}
+ * (for example {@link ProbableThreat}) - we still need to be able to filter by it's properties.
+ */
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  public static void main(String[] args) {
+    filteringSimpleThreats();
+    filteringSimpleProbableThreats();
+  }
+
+  /**
+   * Demonstrates how to filter {@link com.iluwatar.filterer.threat.ProbabilisticThreatAwareSystem}
+   * based on probability property. The @{@link com.iluwatar.filterer.domain.Filterer#by(Predicate)}
+   * method is able to use {@link com.iluwatar.filterer.threat.ProbableThreat}
+   * as predicate argument.
+   */
+  private static void filteringSimpleProbableThreats() {
+    LOGGER.info(" ### Filtering ProbabilisticThreatAwareSystem by probability ###");
+
+    var trojanArcBomb = new SimpleProbableThreat("Trojan-ArcBomb", 1, ThreatType.TROJAN, 0.99);
+    var rootkit = new SimpleProbableThreat("Rootkit-Kernel", 2, ThreatType.ROOTKIT, 0.8);
+
+    List<ProbableThreat> probableThreats = List.of(trojanArcBomb, rootkit);
+
+    var probabilisticThreatAwareSystem =
+            new SimpleProbabilisticThreatAwareSystem("Sys-1", probableThreats);
+
+    LOGGER.info("Filtering ProbabilisticThreatAwareSystem. Initial : "
+            + probabilisticThreatAwareSystem);
+
+    //Filtering using filterer
+    var filteredThreatAwareSystem = probabilisticThreatAwareSystem.filtered()
+            .by(probableThreat -> Double.compare(probableThreat.probability(), 0.99) == 0);
+
+    LOGGER.info("Filtered by probability = 0.99 : " + filteredThreatAwareSystem);
+  }
+
+  /**
+   * Demonstrates how to filter {@link ThreatAwareSystem} based on startingOffset property
+   * of {@link SimpleThreat}. The @{@link com.iluwatar.filterer.domain.Filterer#by(Predicate)}
+   * method is able to use {@link Threat} as predicate argument.
+   */
+  private static void filteringSimpleThreats() {
+    LOGGER.info("### Filtering ThreatAwareSystem by ThreatType ###");
+
+    var rootkit = new SimpleThreat(ThreatType.ROOTKIT, 1, "Simple-Rootkit");
+    var trojan = new SimpleThreat(ThreatType.TROJAN, 2, "Simple-Trojan");
+    List<Threat> threats = List.of(rootkit, trojan);
+
+    var threatAwareSystem = new SimpleThreatAwareSystem("Sys-1", threats);
+
+    LOGGER.info("Filtering ThreatAwareSystem. Initial : " + threatAwareSystem);
+
+    //Filtering using Filterer
+    var rootkitThreatAwareSystem = threatAwareSystem.filtered()
+            .by(threat -> threat.type() == ThreatType.ROOTKIT);
+
+    LOGGER.info("Filtered by threatType = ROOTKIT : " + rootkitThreatAwareSystem);
+  }
+
+}

filterer/src/main/java/com/iluwatar/filterer/domain/Filterer.java

@@ -0,0 +1,36 @@
+/*
+ * 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.filterer.domain;
+
+import java.util.function.Predicate;
+
+/**
+ * Filterer helper interface.
+ * @param <G> type of the container-like object.
+ * @param <E> type of the elements contained within this container-like object.
+ */
+@FunctionalInterface
+public interface Filterer<G, E> {
+  G by(Predicate<? super E> predicate);
+}

filterer/src/main/java/com/iluwatar/filterer/threat/ProbabilisticThreatAwareSystem.java

@@ -0,0 +1,49 @@
+/*
+ * 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.filterer.threat;
+
+import com.iluwatar.filterer.domain.Filterer;
+
+import java.util.List;
+
+/**
+ * Represents system that is aware of it's threats with given probability of their occurrence.
+ */
+public interface ProbabilisticThreatAwareSystem extends ThreatAwareSystem {
+
+  /**
+   * {@inheritDoc}
+   * @return
+   */
+  @Override
+  List<? extends ProbableThreat> threats();
+
+  /**
+   * {@inheritDoc}
+   * @return
+   */
+  @Override
+  Filterer<? extends ProbabilisticThreatAwareSystem, ? extends ProbableThreat> filtered();
+}
+

filterer/src/main/java/com/iluwatar/filterer/threat/ProbableThreat.java

@@ -0,0 +1,35 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.filterer.threat;
+
+/**
+ * Represents threat that might be a threat with given probability.
+ */
+public interface ProbableThreat extends Threat {
+  /**
+   * Returns probability of occurrence of given threat.
+   * @return probability of occurrence of given threat.
+   */
+  double probability();
+}

filterer/src/main/java/com/iluwatar/filterer/threat/SimpleProbabilisticThreatAwareSystem.java

@@ -0,0 +1,113 @@
+/*
+ * 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.filterer.threat;
+
+import com.google.common.collect.ImmutableList;
+import com.iluwatar.filterer.domain.Filterer;
+
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+/**
+ * {@inheritDoc}
+ */
+public class SimpleProbabilisticThreatAwareSystem implements ProbabilisticThreatAwareSystem {
+
+  private final String systemId;
+  private final ImmutableList<ProbableThreat> threats;
+
+  public SimpleProbabilisticThreatAwareSystem(
+          final String systemId,
+          final List<ProbableThreat> threats
+  ) {
+    this.systemId = systemId;
+    this.threats = ImmutableList.copyOf(threats);
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public String systemId() {
+    return systemId;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public List<? extends ProbableThreat> threats() {
+    return threats;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public Filterer<? extends ProbabilisticThreatAwareSystem, ? extends ProbableThreat> filtered() {
+    return this::filteredGroup;
+  }
+
+  private ProbabilisticThreatAwareSystem filteredGroup(
+          final Predicate<? super ProbableThreat> predicate
+  ) {
+    return new SimpleProbabilisticThreatAwareSystem(this.systemId, filteredItems(predicate));
+  }
+
+  private List<ProbableThreat> filteredItems(
+          final Predicate<? super ProbableThreat> predicate
+  ) {
+    return this.threats.stream()
+            .filter(predicate)
+            .collect(Collectors.toList());
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    var that = (SimpleProbabilisticThreatAwareSystem) o;
+    return systemId.equals(that.systemId)
+            && threats.equals(that.threats);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(systemId, threats);
+  }
+
+  @Override
+  public String toString() {
+    return "SimpleProbabilisticThreatAwareSystem{"
+            + "systemId='" + systemId + '\''
+            + ", threats=" + threats
+            + '}';
+  }
+}

filterer/src/main/java/com/iluwatar/filterer/threat/SimpleProbableThreat.java

@@ -0,0 +1,79 @@
+/*
+ * 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.filterer.threat;
+
+import java.util.Objects;
+
+/**
+ * {@inheritDoc}
+ */
+public class SimpleProbableThreat extends SimpleThreat implements ProbableThreat {
+
+  private final double probability;
+
+  public SimpleProbableThreat(final String name,
+                              final int id,
+                              final ThreatType threatType,
+                              final double probability
+  ) {
+    super(threatType, id, name);
+    this.probability = probability;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public double probability() {
+    return probability;
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    if (!super.equals(o)) {
+      return false;
+    }
+    var that = (SimpleProbableThreat) o;
+    return Double.compare(that.probability, probability) == 0;
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(super.hashCode(), probability);
+  }
+
+  @Override
+  public String toString() {
+    return "SimpleProbableThreat{"
+            + "probability=" + probability
+            + "} "
+            + super.toString();
+  }
+}

filterer/src/main/java/com/iluwatar/filterer/threat/SimpleThreat.java

@@ -0,0 +1,101 @@
+/*
+ * 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.filterer.threat;
+
+import java.util.Objects;
+
+/**
+ * Represents a simple threat.
+ */
+public class SimpleThreat implements Threat {
+
+  private final ThreatType threatType;
+  private final int id;
+  private final String name;
+
+  /**
+   * Constructor.
+   *
+   * @param threatType {@link ThreatType}.
+   * @param id         threat id.
+   * @param name       threat name.
+   */
+  public SimpleThreat(final ThreatType threatType, final int id, String name) {
+    this.threatType = threatType;
+    this.id = id;
+    this.name = name;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public String name() {
+    return name;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public int id() {
+    return id;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public ThreatType type() {
+    return threatType;
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    var that = (SimpleThreat) o;
+    return id == that.id
+            && threatType == that.threatType
+            && Objects.equals(name, that.name);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(threatType, id, name);
+  }
+
+  @Override
+  public String toString() {
+    return "SimpleThreat{"
+            + "threatType=" + threatType
+            + ", id=" + id
+            + ", name='" + name + '\''
+            + '}';
+  }
+}

filterer/src/main/java/com/iluwatar/filterer/threat/SimpleThreatAwareSystem.java

@@ -0,0 +1,107 @@
+/*
+ * 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.filterer.threat;
+
+import com.google.common.collect.ImmutableList;
+import com.iluwatar.filterer.domain.Filterer;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+/**
+ * {@inheritDoc}
+ */
+public class SimpleThreatAwareSystem implements ThreatAwareSystem {
+
+  private final String systemId;
+  private final ImmutableList<Threat> issues;
+
+  public SimpleThreatAwareSystem(final String systemId, final List<Threat> issues) {
+    this.systemId = systemId;
+    this.issues = ImmutableList.copyOf(issues);
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public String systemId() {
+    return systemId;
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public List<? extends Threat> threats() {
+    return new ArrayList<>(issues);
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  @Override
+  public Filterer<? extends ThreatAwareSystem, ? extends Threat> filtered() {
+    return this::filteredGroup;
+  }
+
+  private ThreatAwareSystem filteredGroup(Predicate<? super Threat> predicate) {
+    return new SimpleThreatAwareSystem(this.systemId, filteredItems(predicate));
+  }
+
+  private List<Threat> filteredItems(Predicate<? super Threat> predicate) {
+    return this.issues.stream()
+            .filter(predicate)
+            .collect(Collectors.toList());
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    var that = (SimpleThreatAwareSystem) o;
+    return systemId.equals(that.systemId)
+            && issues.equals(that.issues);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(systemId, issues);
+  }
+
+  @Override
+  public String toString() {
+    return "SimpleThreatAwareSystem{"
+            + "systemId='" + systemId
+            + '\'' + ", issues=" + issues
+            + '}';
+  }
+}

filterer/src/main/java/com/iluwatar/filterer/threat/Threat.java

@@ -0,0 +1,49 @@
+/*
+ * 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.filterer.threat;
+
+/**
+ * Represents a threat that can be detected in given system.
+ */
+public interface Threat {
+  /**
+   * Returns name of the threat.
+   *
+   * @return value representing name of the threat.
+   */
+  String name();
+
+  /**
+   * Returns unique id of the threat.
+   *
+   * @return value representing threat id.
+   */
+  int id();
+
+  /**
+   * Returns threat type.
+   * @return {@link ThreatType}
+   */
+  ThreatType type();
+}

filterer/src/main/java/com/iluwatar/filterer/threat/ThreatAwareSystem.java

@@ -0,0 +1,55 @@
+/*
+ * 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.filterer.threat;
+
+import com.iluwatar.filterer.domain.Filterer;
+
+import java.util.List;
+
+/**
+ * Represents system that is aware of threats that are present in it.
+ */
+public interface ThreatAwareSystem {
+
+  /**
+   * Returns the system id.
+   *
+   * @return system id.
+   */
+  String systemId();
+
+  /**
+   * Returns list of threats for this system.
+   * @return list of threats for this system.
+   */
+  List<? extends Threat> threats();
+
+  /**
+   * Returns the instance of {@link Filterer} helper interface that allows to covariantly
+   * specify lower bound for predicate that we want to filter by.
+   * @return an instance of {@link Filterer} helper interface.
+   */
+  Filterer<? extends ThreatAwareSystem, ? extends Threat> filtered();
+
+}

filterer/src/main/java/com/iluwatar/filterer/threat/ThreatType.java

@@ -0,0 +1,26 @@
+/*
+ * 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.filterer.threat;
+
+public enum ThreatType { TROJAN, WORM, ROOTKIT }

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

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

filterer/src/test/java/com/iluwatar/filterer/threat/SimpleProbabilisticThreatAwareSystemTest.java

@@ -0,0 +1,51 @@
+/*
+ * 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.filterer.threat;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class SimpleProbabilisticThreatAwareSystemTest {
+  @Test
+  void shouldFilterByProbability() {
+    //given
+    var trojan = new SimpleProbableThreat("Troyan-ArcBomb", 1, ThreatType.TROJAN, 0.99);
+    var rootkit = new SimpleProbableThreat("Rootkit-System", 2, ThreatType.ROOTKIT, 0.8);
+    List<ProbableThreat> probableThreats = List.of(trojan, rootkit);
+
+    var simpleProbabilisticThreatAwareSystem =
+            new SimpleProbabilisticThreatAwareSystem("System-1", probableThreats);
+
+    //when
+    var filtered = simpleProbabilisticThreatAwareSystem.filtered()
+            .by(probableThreat -> Double.compare(probableThreat.probability(), 0.99) == 0);
+
+    //then
+    assertEquals(filtered.threats().size(), 1);
+    assertEquals(filtered.threats().get(0), trojan);
+  }
+}

filterer/src/test/java/com/iluwatar/filterer/threat/SimpleThreatAwareSystemTest.java

@@ -0,0 +1,50 @@
+/*
+ * 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.filterer.threat;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SimpleThreatAwareSystemTest {
+  @Test
+  void shouldFilterByThreatType() {
+    //given
+    var rootkit = new SimpleThreat(ThreatType.ROOTKIT, 1, "Simple-Rootkit");
+    var trojan = new SimpleThreat(ThreatType.TROJAN, 2, "Simple-Trojan");
+    List<Threat> threats = List.of(rootkit, trojan);
+
+    var threatAwareSystem = new SimpleThreatAwareSystem("System-1", threats);
+
+    //when
+    var rootkitThreatAwareSystem = threatAwareSystem.filtered()
+            .by(threat -> threat.type() == ThreatType.ROOTKIT);
+
+    //then
+    assertEquals(rootkitThreatAwareSystem.threats().size(), 1);
+    assertEquals(rootkitThreatAwareSystem.threats().get(0), rootkit);
+  }
+}

fluentinterface/src/test/java/com/iluwatar/fluentinterface/app/AppTest.java

@@ -25,13 +25,15 @@ package com.iluwatar.fluentinterface.app;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application Test Entry
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

flux/src/main/java/com/iluwatar/flux/dispatcher/Dispatcher.java

@@ -57,15 +57,10 @@ public final class Dispatcher {
    */
   public void menuItemSelected(MenuItem menuItem) {
     dispatchAction(new MenuAction(menuItem));
-    switch (menuItem) {
-      case HOME:
-      case PRODUCTS:
-      default:
-        dispatchAction(new ContentAction(Content.PRODUCTS));
-        break;
-      case COMPANY:
-        dispatchAction(new ContentAction(Content.COMPANY));
-        break;
+    if (menuItem == MenuItem.COMPANY) {
+      dispatchAction(new ContentAction(Content.COMPANY));
+    } else {
+      dispatchAction(new ContentAction(Content.PRODUCTS));
     }
   }
 

flux/src/test/java/com/iluwatar/flux/app/AppTest.java

@@ -25,13 +25,15 @@ package com.iluwatar.flux.app;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -25,13 +25,15 @@ package com.iluwatar.flyweight;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

front-controller/src/test/java/com/iluwatar/front/controller/AppTest.java

@@ -25,13 +25,15 @@ package com.iluwatar.front.controller;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

game-loop/pom.xml

@@ -39,6 +39,12 @@
             <groupId>junit</groupId>
             <artifactId>junit</artifactId>
         </dependency>
+
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
     <build>
         <plugins>

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

@@ -25,14 +25,16 @@ package com.iluwatar.gameloop;
 
 import org.junit.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * App unit test class.
  */
 public class AppTest {
 
   @Test
-  public void testMain() {
-    new App().main(new String[]{});
+  public void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 
 }

half-sync-half-async/src/test/java/com/iluwatar/halfsynchalfasync/AppTest.java

@@ -25,13 +25,15 @@ package com.iluwatar.halfsynchalfasync;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
 
   @Test
-  public void test() {
-    App.main(null);
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(null));
   }
 }

hexagonal/src/main/java/com/iluwatar/hexagonal/database/MongoTicketRepository.java

@@ -46,6 +46,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
   private static final String DEFAULT_DB = "lotteryDB";
   private static final String DEFAULT_TICKETS_COLLECTION = "lotteryTickets";
   private static final String DEFAULT_COUNTERS_COLLECTION = "counters";
+  private static final String TICKET_ID = "ticketId";
 
   private MongoClient mongoClient;
   private MongoDatabase database;
@@ -93,7 +94,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
   }
 
   private void initCounters() {
-    var doc = new Document("_id", "ticketId").append("seq", 1);
+    var doc = new Document("_id", TICKET_ID).append("seq", 1);
     countersCollection.insertOne(doc);
   }
 
@@ -103,7 +104,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
    * @return next ticket id
    */
   public int getNextId() {
-    var find = new Document("_id", "ticketId");
+    var find = new Document("_id", TICKET_ID);
     var increase = new Document("seq", 1);
     var update = new Document("$inc", increase);
     var result = countersCollection.findOneAndUpdate(find, update);
@@ -131,7 +132,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
   @Override
   public Optional<LotteryTicket> findById(LotteryTicketId id) {
     return ticketsCollection
-        .find(new Document("ticketId", id.getId()))
+        .find(new Document(TICKET_ID, id.getId()))
         .limit(1)
         .into(new ArrayList<>())
         .stream()
@@ -142,7 +143,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
   @Override
   public Optional<LotteryTicketId> save(LotteryTicket ticket) {
     var ticketId = getNextId();
-    var doc = new Document("ticketId", ticketId);
+    var doc = new Document(TICKET_ID, ticketId);
     doc.put("email", ticket.getPlayerDetails().getEmail());
     doc.put("bank", ticket.getPlayerDetails().getBankAccount());
     doc.put("phone", ticket.getPlayerDetails().getPhoneNumber());
@@ -173,7 +174,7 @@ public class MongoTicketRepository implements LotteryTicketRepository {
         .map(Integer::parseInt)
         .collect(Collectors.toSet());
     var lotteryNumbers = LotteryNumbers.create(numbers);
-    var ticketId = new LotteryTicketId(doc.getInteger("ticketId"));
+    var ticketId = new LotteryTicketId(doc.getInteger(TICKET_ID));
     return new LotteryTicket(ticketId, playerDetails, lotteryNumbers);
   }
 }

hexagonal/src/main/java/com/iluwatar/hexagonal/eventlog/MongoEventLog.java

@@ -36,6 +36,9 @@ public class MongoEventLog implements LotteryEventLog {
 
   private static final String DEFAULT_DB = "lotteryDB";
   private static final String DEFAULT_EVENTS_COLLECTION = "events";
+  private static final String EMAIL = "email";
+  private static final String PHONE = "phone";
+  public static final String MESSAGE = "message";
 
   private MongoClient mongoClient;
   private MongoDatabase database;
@@ -107,41 +110,41 @@ public class MongoEventLog implements LotteryEventLog {
 
   @Override
   public void ticketSubmitted(PlayerDetails details) {
-    var document = new Document("email", details.getEmail());
-    document.put("phone", details.getPhoneNumber());
+    var document = new Document(EMAIL, details.getEmail());
+    document.put(PHONE, details.getPhoneNumber());
     document.put("bank", details.getBankAccount());
     document
-        .put("message", "Lottery ticket was submitted and bank account was charged for 3 credits.");
+        .put(MESSAGE, "Lottery ticket was submitted and bank account was charged for 3 credits.");
     eventsCollection.insertOne(document);
     stdOutEventLog.ticketSubmitted(details);
   }
 
   @Override
   public void ticketSubmitError(PlayerDetails details) {
-    var document = new Document("email", details.getEmail());
-    document.put("phone", details.getPhoneNumber());
+    var document = new Document(EMAIL, details.getEmail());
+    document.put(PHONE, details.getPhoneNumber());
     document.put("bank", details.getBankAccount());
-    document.put("message", "Lottery ticket could not be submitted because lack of funds.");
+    document.put(MESSAGE, "Lottery ticket could not be submitted because lack of funds.");
     eventsCollection.insertOne(document);
     stdOutEventLog.ticketSubmitError(details);
   }
 
   @Override
   public void ticketDidNotWin(PlayerDetails details) {
-    var document = new Document("email", details.getEmail());
-    document.put("phone", details.getPhoneNumber());
+    var document = new Document(EMAIL, details.getEmail());
+    document.put(PHONE, details.getPhoneNumber());
     document.put("bank", details.getBankAccount());
-    document.put("message", "Lottery ticket was checked and unfortunately did not win this time.");
+    document.put(MESSAGE, "Lottery ticket was checked and unfortunately did not win this time.");
     eventsCollection.insertOne(document);
     stdOutEventLog.ticketDidNotWin(details);
   }
 
   @Override
   public void ticketWon(PlayerDetails details, int prizeAmount) {
-    var document = new Document("email", details.getEmail());
-    document.put("phone", details.getPhoneNumber());
+    var document = new Document(EMAIL, details.getEmail());
+    document.put(PHONE, details.getPhoneNumber());
     document.put("bank", details.getBankAccount());
-    document.put("message", String
+    document.put(MESSAGE, String
         .format("Lottery ticket won! The bank account was deposited with %d credits.",
             prizeAmount));
     eventsCollection.insertOne(document);
@@ -150,10 +153,10 @@ public class MongoEventLog implements LotteryEventLog {
 
   @Override
   public void prizeError(PlayerDetails details, int prizeAmount) {
-    var document = new Document("email", details.getEmail());
-    document.put("phone", details.getPhoneNumber());
+    var document = new Document(EMAIL, details.getEmail());
+    document.put(PHONE, details.getPhoneNumber());
     document.put("bank", details.getBankAccount());
-    document.put("message", String
+    document.put(MESSAGE, String
         .format("Lottery ticket won! Unfortunately the bank credit transfer of %d failed.",
             prizeAmount));
     eventsCollection.insertOne(document);