docs: update .all-contributorsrc [skip ci]

Filterer pattern

.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 +1140,33 @@
       "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"
+      ]
     }
   ],
   "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-128-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
 # Introduction
@@ -252,10 +253,13 @@ 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>
 </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/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 +1,204 @@
+--- # this is so called 'Yaml Front Matter', read up on it here: http://jekyllrb.com/docs/frontmatter/
+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 system. We can have different types of threats and systems. We have a requirement that
+> system should be aware of threats that are present in it. In the design we have to take into consideration that new Threat types can be
+> added later. Also there is a requirement that a system can filter itself based on the threats that it possesses (system acts as container-like object for threats).
+>
+
+In plain words
+
+> We need to be able to filter different types of systems(container-like objects) based on properties of Threats that they contain.
+> Adding new properties for Threats should be easy (we still need the ability to filter by those new properties).
+
+**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);

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

@@ -25,13 +25,16 @@ package com.iluwatar.hexagonal;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Unit test for simple App.
  */
 class AppTest {
 
   @Test
-  void testApp() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+
   }
 }