#590 add explanation for caching pattern

* docs: update README.md [skip ci]

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

Co-authored-by: allcontributors[bot] <46447321+allcontributors[bot]@users.noreply.github.com>
Co-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com>

.all-contributorsrc

@@ -1422,6 +1422,34 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "noamgrinch",
+      "name": "Noam Greenshtain",
+      "avatar_url": "https://avatars.githubusercontent.com/u/31648669?v=4",
+      "profile": "https://github.com/noamgrinch",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "qfxl",
+      "name": "yonghong Xu",
+      "avatar_url": "https://avatars.githubusercontent.com/u/14086462?v=4",
+      "profile": "https://xuyonghong.cn/",
+      "contributions": [
+        "doc"
+      ]
+    },
+    {
+      "login": "jinishavora",
+      "name": "jinishavora",
+      "avatar_url": "https://avatars.githubusercontent.com/u/40777762?v=4",
+      "profile": "https://www.linkedin.com/in/jinisha-vora",
+      "contributions": [
+        "review",
+        "code"
+      ]
     }
   ],
   "contributorsPerLine": 4,

README.md

@@ -10,7 +10,7 @@
 [![Coverage](https://sonarcloud.io/api/project_badges/measure?project=iluwatar_java-design-patterns&metric=coverage)](https://sonarcloud.io/dashboard?id=iluwatar_java-design-patterns)
 [![Join the chat at https://gitter.im/iluwatar/java-design-patterns](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/iluwatar/java-design-patterns?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 <!-- ALL-CONTRIBUTORS-BADGE:START - Do not remove or modify this section -->
-[![All Contributors](https://img.shields.io/badge/all_contributors-156-orange.svg?style=flat-square)](#contributors-)
+[![All Contributors](https://img.shields.io/badge/all_contributors-159-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
 <br/>
@@ -309,6 +309,11 @@ This project is licensed under the terms of the MIT license.
     <td align="center"><a href="https://github.com/richardmr36"><img src="https://avatars.githubusercontent.com/u/19147333?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Martel Richard</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=richardmr36" title="Code">💻</a></td>
     <td align="center"><a href="https://github.com/va1m"><img src="https://avatars.githubusercontent.com/u/17025445?v=4?s=100" width="100px;" alt=""/><br /><sub><b>va1m</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=va1m" title="Code">💻</a></td>
   </tr>
+  <tr>
+    <td align="center"><a href="https://github.com/noamgrinch"><img src="https://avatars.githubusercontent.com/u/31648669?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Noam Greenshtain</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=noamgrinch" title="Code">💻</a></td>
+    <td align="center"><a href="https://xuyonghong.cn/"><img src="https://avatars.githubusercontent.com/u/14086462?v=4?s=100" width="100px;" alt=""/><br /><sub><b>yonghong Xu</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=qfxl" title="Documentation">📖</a></td>
+    <td align="center"><a href="https://www.linkedin.com/in/jinisha-vora"><img src="https://avatars.githubusercontent.com/u/40777762?v=4?s=100" width="100px;" alt=""/><br /><sub><b>jinishavora</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3Ajinishavora" title="Reviewed Pull Requests">👀</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=jinishavora" title="Code">💻</a></td>
+  </tr>
 </table>
 
 <!-- markdownlint-restore -->

active-object/README.md

@@ -79,12 +79,12 @@ public abstract class ActiveCreature{
     return this.name;
   }
 }
-
 ```
 
 We can see that any class that will extend the ActiveCreature class will have its own thread of control to execute and invocate methods.
 
 For example, the Orc class:
+
 ```java
 public class Orc extends ActiveCreature {
 
@@ -96,6 +96,7 @@ public class Orc extends ActiveCreature {
 ```
 
 Now, we can create multiple creatures such as Orcs, tell them to eat and roam and they will execute it on their own thread of control:
+
 ```java
   public static void main(String[] args) {  
     var app = new App();
@@ -121,4 +122,4 @@ Now, we can create multiple creatures such as Orcs, tell them to eat and roam an
 
 ## Class diagram
 
-![alt text](./etc/active-object.urm.png "Active Object class diagram")
+![alt text](./etc/active-object.urm.PNG "Active Object class diagram")

active-object/src/main/java/com/iluwatar/activeobject/ActiveCreature.java

@@ -1,3 +1,26 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2021 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.activeobject;
 
 import java.util.concurrent.BlockingQueue;

active-object/src/main/java/com/iluwatar/activeobject/Orc.java

@@ -1,3 +1,26 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2021 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.activeobject;
 
 /**

active-object/src/test/java/com/iluwatar/activeobject/ActiveCreatureTest.java

@@ -1,3 +1,26 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2021 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.activeobject;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;

active-object/src/test/java/com/iluwatar/activeobject/AppTest.java

@@ -1,3 +1,26 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2021 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.activeobject;
 
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;

async-method-invocation/README.md

@@ -9,21 +9,160 @@ tags:
 ---
 
 ## Intent
-Asynchronous method invocation is pattern where the calling thread
+
+Asynchronous method invocation is a pattern where the calling thread
 is not blocked while waiting results of tasks. The pattern provides parallel
 processing of multiple independent tasks and retrieving the results via
 callbacks or waiting until everything is done. 
 
+## Explanation
+
+Real world example
+
+> Launching space rockets is an exciting business. The mission command gives an order to launch and 
+> after some undetermined time, the rocket either launches successfully or fails miserably.
+
+In plain words
+
+> Asynchronous method invocation starts task processing and returns immediately before the task is 
+> ready. The results of the task processing are returned to the caller later.
+
+Wikipedia says
+
+> In multithreaded computer programming, asynchronous method invocation (AMI), also known as 
+> asynchronous method calls or the asynchronous pattern is a design pattern in which the call site 
+> is not blocked while waiting for the called code to finish. Instead, the calling thread is 
+> notified when the reply arrives. Polling for a reply is an undesired option.
+
+**Programmatic Example**
+
+In this example, we are launching space rockets and deploying lunar rovers.
+
+The application demonstrates the async method invocation pattern. The key parts of the pattern are 
+`AsyncResult` which is an intermediate container for an asynchronously evaluated value, 
+`AsyncCallback` which can be provided to be executed on task completion and `AsyncExecutor` that 
+manages the execution of the async tasks.
+
+```java
+public interface AsyncResult<T> {
+  boolean isCompleted();
+  T getValue() throws ExecutionException;
+  void await() throws InterruptedException;
+}
+```
+
+```java
+public interface AsyncCallback<T> {
+  void onComplete(T value, Optional<Exception> ex);
+}
+```
+
+```java
+public interface AsyncExecutor {
+  <T> AsyncResult<T> startProcess(Callable<T> task);
+  <T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback);
+  <T> T endProcess(AsyncResult<T> asyncResult) throws ExecutionException, InterruptedException;
+}
+```
+
+`ThreadAsyncExecutor` is an implementation of `AsyncExecutor`. Some of its key parts are highlighted 
+next.
+
+```java
+public class ThreadAsyncExecutor implements AsyncExecutor {
+
+  @Override
+  public <T> AsyncResult<T> startProcess(Callable<T> task) {
+    return startProcess(task, null);
+  }
+
+  @Override
+  public <T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback) {
+    var result = new CompletableResult<>(callback);
+    new Thread(
+            () -> {
+              try {
+                result.setValue(task.call());
+              } catch (Exception ex) {
+                result.setException(ex);
+              }
+            },
+            "executor-" + idx.incrementAndGet())
+        .start();
+    return result;
+  }
+
+  @Override
+  public <T> T endProcess(AsyncResult<T> asyncResult)
+      throws ExecutionException, InterruptedException {
+    if (!asyncResult.isCompleted()) {
+      asyncResult.await();
+    }
+    return asyncResult.getValue();
+  }
+}
+```
+
+Then we are ready to launch some rockets to see how everything works together.
+
+```java
+public static void main(String[] args) throws Exception {
+  // construct a new executor that will run async tasks
+  var executor = new ThreadAsyncExecutor();
+
+  // start few async tasks with varying processing times, two last with callback handlers
+  final var asyncResult1 = executor.startProcess(lazyval(10, 500));
+  final var asyncResult2 = executor.startProcess(lazyval("test", 300));
+  final var asyncResult3 = executor.startProcess(lazyval(50L, 700));
+  final var asyncResult4 = executor.startProcess(lazyval(20, 400), callback("Deploying lunar rover"));
+  final var asyncResult5 =
+      executor.startProcess(lazyval("callback", 600), callback("Deploying lunar rover"));
+
+  // emulate processing in the current thread while async tasks are running in their own threads
+  Thread.sleep(350); // Oh boy, we are working hard here
+  log("Mission command is sipping coffee");
+
+  // wait for completion of the tasks
+  final var result1 = executor.endProcess(asyncResult1);
+  final var result2 = executor.endProcess(asyncResult2);
+  final var result3 = executor.endProcess(asyncResult3);
+  asyncResult4.await();
+  asyncResult5.await();
+
+  // log the results of the tasks, callbacks log immediately when complete
+  log("Space rocket <" + result1 + "> launch complete");
+  log("Space rocket <" + result2 + "> launch complete");
+  log("Space rocket <" + result3 + "> launch complete");
+}
+```
+
+Here's the program console output.
+
+```java
+21:47:08.227 [executor-2] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launched successfully
+21:47:08.269 [main] INFO com.iluwatar.async.method.invocation.App - Mission command is sipping coffee
+21:47:08.318 [executor-4] INFO com.iluwatar.async.method.invocation.App - Space rocket <20> launched successfully
+21:47:08.335 [executor-4] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <20>
+21:47:08.414 [executor-1] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launched successfully
+21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Space rocket <callback> launched successfully
+21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <callback>
+21:47:08.616 [executor-3] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launched successfully
+21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launch complete
+21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launch complete
+21:47:08.618 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launch complete
+```
+
 # Class diagram
+
 ![alt text](./etc/async-method-invocation.png "Async Method Invocation")
 
 ## Applicability
-Use async method invocation pattern when
+
+Use the async method invocation pattern when
 
 * You have multiple independent tasks that can run in parallel
 * You need to improve the performance of a group of sequential tasks
-* You have limited amount of processing capacity or long running tasks and the
-  caller should not wait the tasks to be ready
+* You have a limited amount of processing capacity or long-running tasks and the caller should not wait for the tasks to be ready
 
 ## Real world examples
 

async-method-invocation/src/main/java/com/iluwatar/async/method/invocation/App.java

@@ -27,10 +27,12 @@ import java.util.concurrent.Callable;
 import lombok.extern.slf4j.Slf4j;
 
 /**
- * This application demonstrates the async method invocation pattern. Key parts of the pattern are
- * <code>AsyncResult</code> which is an intermediate container for an asynchronously evaluated
- * value, <code>AsyncCallback</code> which can be provided to be executed on task completion and
- * <code>AsyncExecutor</code> that manages the execution of the async tasks.
+ * In this example, we are launching space rockets and deploying lunar rovers.
+ *
+ * <p>The application demonstrates the async method invocation pattern. The key parts of the
+ * pattern are <code>AsyncResult</code> which is an intermediate container for an asynchronously
+ * evaluated value, <code>AsyncCallback</code> which can be provided to be executed on task
+ * completion and <code>AsyncExecutor</code> that manages the execution of the async tasks.
  *
  * <p>The main method shows example flow of async invocations. The main thread starts multiple
  * tasks with variable durations and then continues its own work. When the main thread has done it's
@@ -68,13 +70,14 @@ public class App {
     final var asyncResult1 = executor.startProcess(lazyval(10, 500));
     final var asyncResult2 = executor.startProcess(lazyval("test", 300));
     final var asyncResult3 = executor.startProcess(lazyval(50L, 700));
-    final var asyncResult4 = executor.startProcess(lazyval(20, 400), callback("Callback result 4"));
+    final var asyncResult4 = executor.startProcess(lazyval(20, 400),
+        callback("Deploying lunar rover"));
     final var asyncResult5 =
-        executor.startProcess(lazyval("callback", 600), callback("Callback result 5"));
+        executor.startProcess(lazyval("callback", 600), callback("Deploying lunar rover"));
 
     // emulate processing in the current thread while async tasks are running in their own threads
-    Thread.sleep(350); // Oh boy I'm working hard here
-    log("Some hard work done");
+    Thread.sleep(350); // Oh boy, we are working hard here
+    log("Mission command is sipping coffee");
 
     // wait for completion of the tasks
     final var result1 = executor.endProcess(asyncResult1);
@@ -84,9 +87,9 @@ public class App {
     asyncResult5.await();
 
     // log the results of the tasks, callbacks log immediately when complete
-    log("Result 1: " + result1);
-    log("Result 2: " + result2);
-    log("Result 3: " + result3);
+    log("Space rocket <" + result1 + "> launch complete");
+    log("Space rocket <" + result2 + "> launch complete");
+    log("Space rocket <" + result3 + "> launch complete");
   }
 
   /**
@@ -99,7 +102,7 @@ public class App {
   private static <T> Callable<T> lazyval(T value, long delayMillis) {
     return () -> {
       Thread.sleep(delayMillis);
-      log("Task completed with: " + value);
+      log("Space rocket <" + value + "> launched successfully");
       return value;
     };
   }
@@ -115,7 +118,7 @@ public class App {
       if (ex.isPresent()) {
         log(name + " failed: " + ex.map(Exception::getMessage).orElse(""));
       } else {
-        log(name + ": " + value);
+        log(name + " <" + value + ">");
       }
     };
   }

balking/README.md

@@ -9,19 +9,131 @@ tags:
 ---
 
 ## Intent
-Balking Pattern is used to prevent an object from executing certain code if it is an
-incomplete or inappropriate state
+
+Balking Pattern is used to prevent an object from executing a certain code if it is in an incomplete 
+or inappropriate state.
+
+## Explanation
+
+Real world example
+
+> There's a start-button in a washing machine to initiate the laundry washing. When the washing 
+> machine is inactive the button works as expected, but if it's already washing the button does
+> nothing.
+
+In plain words
+
+> Using the balking pattern, a certain code executes only if the object is in particular state.
+
+Wikipedia says
+
+> The balking pattern is a software design pattern that only executes an action on an object when 
+> the object is in a particular state. For example, if an object reads ZIP files and a calling 
+> method invokes a get method on the object when the ZIP file is not open, the object would "balk" 
+> at the request.
+
+**Programmatic Example**
+
+In this example implementation, `WashingMachine` is an object that has two states in which it can 
+be: ENABLED and WASHING. If the machine is ENABLED, the state changes to WASHING using a thread-safe
+method. On the other hand, if it already has been washing and any other thread executes `wash()` 
+it won't do that and returns without doing anything.
+
+Here are the relevant parts of the `WashingMachine` class.
+
+```java
+@Slf4j
+public class WashingMachine {
+
+  private final DelayProvider delayProvider;
+  private WashingMachineState washingMachineState;
+
+  public WashingMachine(DelayProvider delayProvider) {
+    this.delayProvider = delayProvider;
+    this.washingMachineState = WashingMachineState.ENABLED;
+  }
+
+  public WashingMachineState getWashingMachineState() {
+    return washingMachineState;
+  }
+
+  public void wash() {
+    synchronized (this) {
+      var machineState = getWashingMachineState();
+      LOGGER.info("{}: Actual machine state: {}", Thread.currentThread().getName(), machineState);
+      if (this.washingMachineState == WashingMachineState.WASHING) {
+        LOGGER.error("Cannot wash if the machine has been already washing!");
+        return;
+      }
+      this.washingMachineState = WashingMachineState.WASHING;
+    }
+    LOGGER.info("{}: Doing the washing", Thread.currentThread().getName());
+    this.delayProvider.executeAfterDelay(50, TimeUnit.MILLISECONDS, this::endOfWashing);
+  }
+
+  public synchronized void endOfWashing() {
+    washingMachineState = WashingMachineState.ENABLED;
+    LOGGER.info("{}: Washing completed.", Thread.currentThread().getId());
+  }
+}
+```
+
+Here's the simple `DelayProvider` interface used by the `WashingMachine`.
+
+```java
+public interface DelayProvider {
+  void executeAfterDelay(long interval, TimeUnit timeUnit, Runnable task);
+}
+```
+
+Now we introduce the application using the `WashingMachine`.
+
+```java
+  public static void main(String... args) {
+    final var washingMachine = new WashingMachine();
+    var executorService = Executors.newFixedThreadPool(3);
+    for (int i = 0; i < 3; i++) {
+      executorService.execute(washingMachine::wash);
+    }
+    executorService.shutdown();
+    try {
+      executorService.awaitTermination(10, TimeUnit.SECONDS);
+    } catch (InterruptedException ie) {
+      LOGGER.error("ERROR: Waiting on executor service shutdown!");
+      Thread.currentThread().interrupt();
+    }
+  }
+```
+
+Here is the console output of the program.
+
+```
+14:02:52.268 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-2: Actual machine state: ENABLED
+14:02:52.272 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-2: Doing the washing
+14:02:52.272 [pool-1-thread-3] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-3: Actual machine state: WASHING
+14:02:52.273 [pool-1-thread-3] ERROR com.iluwatar.balking.WashingMachine - Cannot wash if the machine has been already washing!
+14:02:52.273 [pool-1-thread-1] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-1: Actual machine state: WASHING
+14:02:52.273 [pool-1-thread-1] ERROR com.iluwatar.balking.WashingMachine - Cannot wash if the machine has been already washing!
+14:02:52.324 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - 14: Washing completed.
+```
 
 ## Class diagram
+
 ![alt text](./etc/balking.png "Balking")
 
 ## Applicability
+
 Use the Balking pattern when
 
-* you want to invoke an action on an object only when it is in a particular state
-* objects are generally only in a state that is prone to balking temporarily
-but for an unknown amount of time
+* You want to invoke an action on an object only when it is in a particular state
+* Objects are generally only in a state that is prone to balking temporarily but for an unknown 
+  amount of time
 
 ## Related patterns
-* Guarded Suspension Pattern
-* Double Checked Locking Pattern
+
+* [Guarded Suspension Pattern](https://java-design-patterns.com/patterns/guarded-suspension/)
+* [Double Checked Locking Pattern](https://java-design-patterns.com/patterns/double-checked-locking/)
+
+## Credits
+
+* [Patterns in Java: A Catalog of Reusable Design Patterns Illustrated with UML, 2nd Edition, Volume 1](https://www.amazon.com/gp/product/0471227293/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0471227293&linkId=0e39a59ffaab93fb476036fecb637b99)

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

@@ -32,11 +32,11 @@ import lombok.extern.slf4j.Slf4j;
  * then the method will return without doing anything. Objects that use this pattern are generally
  * only in a state that is prone to balking temporarily but for an unknown amount of time
  *
- * <p>In this example implementation WashingMachine is an object that has two states in which it
- * can be: ENABLED and WASHING. If the machine is ENABLED the state is changed into WASHING that any
- * other thread can't invoke this action on this and then do the job. On the other hand if it have
- * been already washing and any other thread execute wash() it can't do that once again and returns
- * doing nothing.
+ * <p>In this example implementation, {@link WashingMachine} is an object that has two states in
+ * which it can be: ENABLED and WASHING. If the machine is ENABLED, the state changes to WASHING
+ * using a thread-safe method. On the other hand, if it already has been washing and any other
+ * thread executes {@link WashingMachine#wash()} it won't do that and returns without doing
+ * anything.
  */
 @Slf4j
 public class App {
@@ -54,11 +54,12 @@ public class App {
     }
     executorService.shutdown();
     try {
-      executorService.awaitTermination(10, TimeUnit.SECONDS);
+      if (!executorService.awaitTermination(10, TimeUnit.SECONDS)) {
+        executorService.shutdownNow();
+      }
     } catch (InterruptedException ie) {
       LOGGER.error("ERROR: Waiting on executor service shutdown!");
       Thread.currentThread().interrupt();
     }
   }
-
 }

business-delegate/README.md

@@ -9,21 +9,156 @@ tags:
 ---
 
 ## Intent
+
 The Business Delegate pattern adds an abstraction layer between
 presentation and business tiers. By using the pattern we gain loose coupling
 between the tiers and encapsulate knowledge about how to locate, connect to,
 and interact with the business objects that make up the application.
 
+## Explanation
+
+Real world example
+
+> A mobile phone application promises to stream any movie in existence to your phone. It captures
+> the user's search string and passes this on to the business delegate. The business delegate
+> selects the most suitable video streaming service and plays the video from there.
+
+In Plain Words
+
+> Business delegate adds an abstraction layer between the presentation and business tiers.
+
+Wikipedia says
+
+> Business delegate is a Java EE design pattern. This pattern is directing to reduce the coupling 
+> in between business services and the connected presentation tier, and to hide the implementation 
+> details of services (including lookup and accessibility of EJB architecture). Business delegates 
+> acts as an adaptor to invoke business objects from the presentation tier.
+
+**Programmatic Example**
+
+First, we have an abstraction for video streaming services and a couple of implementations.
+
+```java
+public interface VideoStreamingService {
+  void doProcessing();
+}
+
+@Slf4j
+public class NetflixService implements VideoStreamingService {
+  @Override
+  public void doProcessing() {
+    LOGGER.info("NetflixService is now processing");
+  }
+}
+
+@Slf4j
+public class YouTubeService implements VideoStreamingService {
+  @Override
+  public void doProcessing() {
+    LOGGER.info("YouTubeService is now processing");
+  }
+}
+```
+
+Then we have a lookup service that decides which video streaming service is used.
+
+```java
+@Setter
+public class BusinessLookup {
+
+  private NetflixService netflixService;
+  private YouTubeService youTubeService;
+
+  public VideoStreamingService getBusinessService(String movie) {
+    if (movie.toLowerCase(Locale.ROOT).contains("die hard")) {
+      return netflixService;
+    } else {
+      return youTubeService;
+    }
+  }
+}
+```
+
+The business delegate uses a business lookup to route movie playback requests to a suitable
+video streaming service.
+
+```java
+@Setter
+public class BusinessDelegate {
+
+  private BusinessLookup lookupService;
+
+  public void playbackMovie(String movie) {
+    VideoStreamingService videoStreamingService = lookupService.getBusinessService(movie);
+    videoStreamingService.doProcessing();
+  }
+}
+```
+
+The mobile client utilizes business delegate to call the business tier.
+
+```java
+public class MobileClient {
+
+  private final BusinessDelegate businessDelegate;
+
+  public MobileClient(BusinessDelegate businessDelegate) {
+    this.businessDelegate = businessDelegate;
+  }
+
+  public void playbackMovie(String movie) {
+    businessDelegate.playbackMovie(movie);
+  }
+}
+```
+
+Finally, we can show the full example in action.
+
+```java
+  public static void main(String[] args) {
+
+    // prepare the objects
+    var businessDelegate = new BusinessDelegate();
+    var businessLookup = new BusinessLookup();
+    businessLookup.setNetflixService(new NetflixService());
+    businessLookup.setYouTubeService(new YouTubeService());
+    businessDelegate.setLookupService(businessLookup);
+
+    // create the client and use the business delegate
+    var client = new MobileClient(businessDelegate);
+    client.playbackMovie("Die Hard 2");
+    client.playbackMovie("Maradona: The Greatest Ever");
+  }
+```
+
+Here is the console output.
+
+```
+21:15:33.790 [main] INFO com.iluwatar.business.delegate.NetflixService - NetflixService is now processing
+21:15:33.794 [main] INFO com.iluwatar.business.delegate.YouTubeService - YouTubeService is now processing
+```
+
 ## Class diagram
-![alt text](./etc/business-delegate.png "Business Delegate")
+
+![alt text](./etc/business-delegate.urm.png "Business Delegate")
+
+## Related patterns
+
+* [Service locator pattern](https://java-design-patterns.com/patterns/service-locator/)
 
 ## Applicability
+
 Use the Business Delegate pattern when
 
-* you want loose coupling between presentation and business tiers
-* you want to orchestrate calls to multiple business services
-* you want to encapsulate service lookups and service calls
+* You want loose coupling between presentation and business tiers
+* You want to orchestrate calls to multiple business services
+* You want to encapsulate service lookups and service calls
+
+## Tutorials
+
+* [Business Delegate Pattern at TutorialsPoint](https://www.tutorialspoint.com/design_pattern/business_delegate_pattern.htm)
 
 ## Credits
 
 * [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)
+* [Core J2EE Patterns: Best Practices and Design Strategies](https://www.amazon.com/gp/product/0130648841/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0130648841&linkId=a0100de2b28c71ede8db1757fb2b5947)

business-delegate/etc/business-delegate.ucls

@@ -1,136 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true" 
-  realizations="true" associations="true" dependencies="false" nesting-relationships="true">  
-  <class id="1" language="java" name="com.iluwatar.business.delegate.BusinessDelegate" project="business-delegate" 
-    file="/business-delegate/src/main/java/com/iluwatar/business/delegate/BusinessDelegate.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="272" y="219"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="2" language="java" name="com.iluwatar.business.delegate.Client" project="business-delegate" 
-    file="/business-delegate/src/main/java/com/iluwatar/business/delegate/Client.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="272" y="64"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <enumeration id="3" language="java" name="com.iluwatar.business.delegate.ServiceType" project="business-delegate" 
-    file="/business-delegate/src/main/java/com/iluwatar/business/delegate/ServiceType.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="89" y="383"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </enumeration>  
-  <interface id="4" language="java" name="com.iluwatar.business.delegate.BusinessService" project="business-delegate" 
-    file="/business-delegate/src/main/java/com/iluwatar/business/delegate/BusinessService.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="630" y="365"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </interface>  
-  <class id="5" language="java" name="com.iluwatar.business.delegate.JmsService" project="business-delegate" 
-    file="/business-delegate/src/main/java/com/iluwatar/business/delegate/JmsService.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="489" y="210"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="6" language="java" name="com.iluwatar.business.delegate.BusinessLookup" project="business-delegate" 
-    file="/business-delegate/src/main/java/com/iluwatar/business/delegate/BusinessLookup.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="360" y="399"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="7" language="java" name="com.iluwatar.business.delegate.EjbService" project="business-delegate" 
-    file="/business-delegate/src/main/java/com/iluwatar/business/delegate/EjbService.java" binary="false" 
-    corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="668" y="210"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <realization id="8">    
-    <end type="SOURCE" refId="7"/>    
-    <end type="TARGET" refId="4"/>  
-  </realization>  
-  <association id="9">    
-    <end type="SOURCE" refId="1" navigable="false">      
-      <attribute id="10" name="lookupService">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </attribute>      
-      <multiplicity id="11" minimum="0" maximum="1">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </multiplicity>    
-    </end>    
-    <end type="TARGET" refId="6" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <association id="12">    
-    <end type="SOURCE" refId="1" navigable="false">      
-      <attribute id="13" name="serviceType">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </attribute>      
-      <multiplicity id="14" minimum="0" maximum="1">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </multiplicity>    
-    </end>    
-    <end type="TARGET" refId="3" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <realization id="15">    
-    <end type="SOURCE" refId="5"/>    
-    <end type="TARGET" refId="4"/>  
-  </realization>  
-  <association id="16">    
-    <end type="SOURCE" refId="2" navigable="false">      
-      <attribute id="17" name="businessDelegate">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </attribute>      
-      <multiplicity id="18" minimum="0" maximum="1">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </multiplicity>    
-    </end>    
-    <end type="TARGET" refId="1" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <association id="19">    
-    <end type="SOURCE" refId="1" navigable="false">      
-      <attribute id="20" name="businessService">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </attribute>      
-      <multiplicity id="21" minimum="0" maximum="1">        
-        <position height="0" width="0" x="0" y="0"/>      
-      </multiplicity>    
-    </end>    
-    <end type="TARGET" refId="4" navigable="true"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-    sort-features="false" accessors="true" visibility="true">    
-    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
-    <operations public="true" package="true" protected="true" private="true" static="true"/>  
-  </classifier-display>  
-  <association-display labels="true" multiplicity="true"/>
-</class-diagram>

business-delegate/etc/business-delegate.urm.puml

@@ -5,53 +5,42 @@ package com.iluwatar.business.delegate {
     + main(args : String[]) {static}
   }
   class BusinessDelegate {
-    - businessService : BusinessService
     - lookupService : BusinessLookup
-    - serviceType : ServiceType
     + BusinessDelegate()
-    + doTask()
-    + setLookupService(businessLookup : BusinessLookup)
-    + setServiceType(serviceType : ServiceType)
+    + playbackMovie(movie : String)
+    + setLookupService(lookupService : BusinessLookup)
   }
   class BusinessLookup {
-    - ejbService : EjbService
-    - jmsService : JmsService
+    - netflixService : NetflixService
+    - youTubeService : YouTubeService
     + BusinessLookup()
-    + getBusinessService(serviceType : ServiceType) : BusinessService
-    + setEjbService(ejbService : EjbService)
-    + setJmsService(jmsService : JmsService)
+    + getBusinessService(movie : String) : VideoStreamingService
+    + setNetflixService(netflixService : NetflixService)
+    + setYouTubeService(youTubeService : YouTubeService)
   }
-  interface BusinessService {
+  class MobileClient {
+    - businessDelegate : BusinessDelegate
+    + MobileClient(businessDelegate : BusinessDelegate)
+    + playbackMovie(movie : String)
+  }
+  class NetflixService {
+    - LOGGER : Logger {static}
+    + NetflixService()
+    + doProcessing()
+  }
+  interface VideoStreamingService {
     + doProcessing() {abstract}
   }
-  class Client {
-    - businessDelegate : BusinessDelegate
-    + Client(businessDelegate : BusinessDelegate)
-    + doTask()
-  }
-  class EjbService {
+  class YouTubeService {
     - LOGGER : Logger {static}
-    + EjbService()
+    + YouTubeService()
     + doProcessing()
   }
-  class JmsService {
-    - LOGGER : Logger {static}
-    + JmsService()
-    + doProcessing()
-  }
-  enum ServiceType {
-    + EJB {static}
-    + JMS {static}
-    + valueOf(name : String) : ServiceType {static}
-    + values() : ServiceType[] {static}
-  }
 }
-BusinessLookup -->  "-ejbService" EjbService
-BusinessDelegate -->  "-serviceType" ServiceType
-Client -->  "-businessDelegate" BusinessDelegate
-BusinessDelegate -->  "-businessService" BusinessService
+BusinessLookup -->  "-netflixService" NetflixService
+BusinessLookup -->  "-youTubeService" YouTubeService
+MobileClient -->  "-businessDelegate" BusinessDelegate
 BusinessDelegate -->  "-lookupService" BusinessLookup
-BusinessLookup -->  "-jmsService" JmsService
-EjbService ..|> BusinessService 
-JmsService ..|> BusinessService 
+NetflixService ..|> VideoStreamingService 
+YouTubeService ..|> VideoStreamingService 
 @enduml

business-delegate/src/main/java/com/iluwatar/business/delegate/App.java

@@ -33,9 +33,9 @@ package com.iluwatar.business.delegate;
  * retrieved through service lookups. The Business Delegate itself may contain business logic too
  * potentially tying together multiple service calls, exception handling, retrying etc.
  *
- * <p>In this example the client ({@link Client}) utilizes a business delegate (
- * {@link BusinessDelegate}) to execute a task. The Business Delegate then selects the appropriate
- * service and makes the service call.
+ * <p>In this example the client ({@link MobileClient}) utilizes a business delegate (
+ * {@link BusinessDelegate}) to search for movies in video streaming services. The Business Delegate
+ * then selects the appropriate service and makes the service call.
  */
 public class App {
 
@@ -46,18 +46,16 @@ public class App {
    */
   public static void main(String[] args) {
 
+    // prepare the objects
     var businessDelegate = new BusinessDelegate();
     var businessLookup = new BusinessLookup();
-    businessLookup.setEjbService(new EjbService());
-    businessLookup.setJmsService(new JmsService());
-
+    businessLookup.setNetflixService(new NetflixService());
+    businessLookup.setYouTubeService(new YouTubeService());
     businessDelegate.setLookupService(businessLookup);
-    businessDelegate.setServiceType(ServiceType.EJB);
 
-    var client = new Client(businessDelegate);
-    client.doTask();
-
-    businessDelegate.setServiceType(ServiceType.JMS);
-    client.doTask();
+    // create the client and use the business delegate
+    var client = new MobileClient(businessDelegate);
+    client.playbackMovie("Die Hard 2");
+    client.playbackMovie("Maradona: The Greatest Ever");
   }
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/BusinessDelegate.java

@@ -23,24 +23,18 @@
 
 package com.iluwatar.business.delegate;
 
+import lombok.Setter;
+
 /**
  * BusinessDelegate separates the presentation and business tiers.
  */
+@Setter
 public class BusinessDelegate {
 
   private BusinessLookup lookupService;
-  private ServiceType serviceType;
 
-  public void setLookupService(BusinessLookup businessLookup) {
-    this.lookupService = businessLookup;
-  }
-
-  public void setServiceType(ServiceType serviceType) {
-    this.serviceType = serviceType;
-  }
-
-  public void doTask() {
-    BusinessService businessService = lookupService.getBusinessService(serviceType);
-    businessService.doProcessing();
+  public void playbackMovie(String movie) {
+    VideoStreamingService videoStreamingService = lookupService.getBusinessService(movie);
+    videoStreamingService.doProcessing();
   }
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/BusinessLookup.java

@@ -23,6 +23,7 @@
 
 package com.iluwatar.business.delegate;
 
+import java.util.Locale;
 import lombok.Setter;
 
 /**
@@ -31,21 +32,21 @@ import lombok.Setter;
 @Setter
 public class BusinessLookup {
 
-  private EjbService ejbService;
+  private NetflixService netflixService;
 
-  private JmsService jmsService;
+  private YouTubeService youTubeService;
 
   /**
-   * Gets service instance based on service type.
+   * Gets service instance based on given movie search string.
    *
-   * @param serviceType Type of service instance to be returned.
+   * @param movie Search string for the movie.
    * @return Service instance.
    */
-  public BusinessService getBusinessService(ServiceType serviceType) {
-    if (serviceType.equals(ServiceType.EJB)) {
-      return ejbService;
+  public VideoStreamingService getBusinessService(String movie) {
+    if (movie.toLowerCase(Locale.ROOT).contains("die hard")) {
+      return netflixService;
     } else {
-      return jmsService;
+      return youTubeService;
     }
   }
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/MobileClient.java

@@ -24,17 +24,17 @@
 package com.iluwatar.business.delegate;
 
 /**
- * Client utilizes BusinessDelegate to call the business tier.
+ * MobileClient utilizes BusinessDelegate to call the business tier.
  */
-public class Client {
+public class MobileClient {
 
   private final BusinessDelegate businessDelegate;
 
-  public Client(BusinessDelegate businessDelegate) {
+  public MobileClient(BusinessDelegate businessDelegate) {
     this.businessDelegate = businessDelegate;
   }
 
-  public void doTask() {
-    businessDelegate.doTask();
+  public void playbackMovie(String movie) {
+    businessDelegate.playbackMovie(movie);
   }
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/NetflixService.java

@@ -26,13 +26,13 @@ package com.iluwatar.business.delegate;
 import lombok.extern.slf4j.Slf4j;
 
 /**
- * Service EJB implementation.
+ * NetflixService implementation.
  */
 @Slf4j
-public class EjbService implements BusinessService {
+public class NetflixService implements VideoStreamingService {
 
   @Override
   public void doProcessing() {
-    LOGGER.info("EjbService is now processing");
+    LOGGER.info("NetflixService is now processing");
   }
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/VideoStreamingService.java

@@ -24,9 +24,9 @@
 package com.iluwatar.business.delegate;
 
 /**
- * Interface for service implementations.
+ * Interface for video streaming service implementations.
  */
-public interface BusinessService {
+public interface VideoStreamingService {
 
   void doProcessing();
 }

business-delegate/src/main/java/com/iluwatar/business/delegate/YouTubeService.java

@@ -26,13 +26,13 @@ package com.iluwatar.business.delegate;
 import lombok.extern.slf4j.Slf4j;
 
 /**
- * Service JMS implementation.
+ * YouTubeService implementation.
  */
 @Slf4j
-public class JmsService implements BusinessService {
+public class YouTubeService implements VideoStreamingService {
 
   @Override
   public void doProcessing() {
-    LOGGER.info("JmsService is now processing");
+    LOGGER.info("YouTubeService is now processing");
   }
 }

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

@@ -26,25 +26,20 @@ package com.iluwatar.business.delegate;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+
+import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
 /**
- * The Business Delegate pattern adds an abstraction layer between the presentation and business
- * tiers. By using the pattern we gain loose coupling between the tiers. The Business Delegate
- * encapsulates knowledge about how to locate, connect to, and interact with the business objects
- * that make up the application.
- *
- * <p>Some of the services the Business Delegate uses are instantiated directly, and some can be
- * retrieved through service lookups. The Business Delegate itself may contain business logic too
- * potentially tying together multiple service calls, exception handling, retrying etc.
+ * Tests for the {@link BusinessDelegate}
  */
 class BusinessDelegateTest {
 
-  private EjbService ejbService;
+  private NetflixService netflixService;
 
-  private JmsService jmsService;
+  private YouTubeService youTubeService;
 
   private BusinessDelegate businessDelegate;
 
@@ -54,19 +49,19 @@ class BusinessDelegateTest {
    */
   @BeforeEach
   public void setup() {
-    ejbService = spy(new EjbService());
-    jmsService = spy(new JmsService());
+    netflixService = spy(new NetflixService());
+    youTubeService = spy(new YouTubeService());
 
     BusinessLookup businessLookup = spy(new BusinessLookup());
-    businessLookup.setEjbService(ejbService);
-    businessLookup.setJmsService(jmsService);
+    businessLookup.setNetflixService(netflixService);
+    businessLookup.setYouTubeService(youTubeService);
 
     businessDelegate = spy(new BusinessDelegate());
     businessDelegate.setLookupService(businessLookup);
   }
 
   /**
-   * In this example the client ({@link Client}) utilizes a business delegate (
+   * In this example the client ({@link MobileClient}) utilizes a business delegate (
    * {@link BusinessDelegate}) to execute a task. The Business Delegate then selects the appropriate
    * service and makes the service call.
    */
@@ -74,26 +69,20 @@ class BusinessDelegateTest {
   void testBusinessDelegate() {
 
     // setup a client object
-    var client = new Client(businessDelegate);
-
-    // set the service type
-    businessDelegate.setServiceType(ServiceType.EJB);
+    var client = new MobileClient(businessDelegate);
 
     // action
-    client.doTask();
+    client.playbackMovie("Die hard");
 
-    // verifying that the businessDelegate was used by client during doTask() method.
-    verify(businessDelegate).doTask();
-    verify(ejbService).doProcessing();
-
-    // set the service type
-    businessDelegate.setServiceType(ServiceType.JMS);
+    // verifying that the businessDelegate was used by client during playbackMovie() method.
+    verify(businessDelegate).playbackMovie(anyString());
+    verify(netflixService).doProcessing();
 
     // action
-    client.doTask();
+    client.playbackMovie("Maradona");
 
     // verifying that the businessDelegate was used by client during doTask() method.
-    verify(businessDelegate, times(2)).doTask();
-    verify(jmsService).doProcessing();
+    verify(businessDelegate, times(2)).playbackMovie(anyString());
+    verify(youTubeService).doProcessing();
   }
 }

bytecode/README.md

@@ -9,18 +9,234 @@ tags:
 ---
 
 ## Intent
-Allows to encode behaviour as instructions for virtual machine.
+
+Allows encoding behavior as instructions for a virtual machine.
+
+## Explanation
+
+Real world example
+
+> A team is working on a new game where wizards battle against each other. The wizard behavior 
+> needs to be carefully adjusted and iterated hundreds of times through playtesting. It's not 
+> optimal to ask the programmer to make changes each time the game designer wants to vary the 
+> behavior, so the wizard behavior is implemented as a data-driven virtual machine.
+
+In plain words
+
+> Bytecode pattern enables behavior driven by data instead of code.
+
+[Gameprogrammingpatterns.com](https://gameprogrammingpatterns.com/bytecode.html) documentation 
+states:
+
+> An instruction set defines the low-level operations that can be performed. A series of 
+> instructions is encoded as a sequence of bytes. A virtual machine executes these instructions one 
+> at a time, using a stack for intermediate values. By combining instructions, complex high-level 
+> behavior can be defined.
+
+**Programmatic Example**
+
+One of the most important game objects is the `Wizard` class.
+
+```java
+@AllArgsConstructor
+@Setter
+@Getter
+@Slf4j
+public class Wizard {
+
+  private int health;
+  private int agility;
+  private int wisdom;
+  private int numberOfPlayedSounds;
+  private int numberOfSpawnedParticles;
+
+  public void playSound() {
+    LOGGER.info("Playing sound");
+    numberOfPlayedSounds++;
+  }
+
+  public void spawnParticles() {
+    LOGGER.info("Spawning particles");
+    numberOfSpawnedParticles++;
+  }
+}
+```
+
+Next, we show the available instructions for our virtual machine. Each of the instructions has its 
+own semantics on how it operates with the stack data. For example, the ADD instruction takes the top
+two items from the stack, adds them together and pushes the result to the stack.
+
+```java
+@AllArgsConstructor
+@Getter
+public enum Instruction {
+
+  LITERAL(1),         // e.g. "LITERAL 0", push 0 to stack
+  SET_HEALTH(2),      // e.g. "SET_HEALTH", pop health and wizard number, call set health
+  SET_WISDOM(3),      // e.g. "SET_WISDOM", pop wisdom and wizard number, call set wisdom
+  SET_AGILITY(4),     // e.g. "SET_AGILITY", pop agility and wizard number, call set agility
+  PLAY_SOUND(5),      // e.g. "PLAY_SOUND", pop value as wizard number, call play sound
+  SPAWN_PARTICLES(6), // e.g. "SPAWN_PARTICLES", pop value as wizard number, call spawn particles
+  GET_HEALTH(7),      // e.g. "GET_HEALTH", pop value as wizard number, push wizard's health
+  GET_AGILITY(8),     // e.g. "GET_AGILITY", pop value as wizard number, push wizard's agility
+  GET_WISDOM(9),      // e.g. "GET_WISDOM", pop value as wizard number, push wizard's wisdom
+  ADD(10),            // e.g. "ADD", pop 2 values, push their sum
+  DIVIDE(11);         // e.g. "DIVIDE", pop 2 values, push their division
+  // ...
+}
+```
+
+At the heart of our example is the `VirtualMachine` class. It takes instructions as input and
+executes them to provide the game object behavior.
+
+```java
+@Getter
+@Slf4j
+public class VirtualMachine {
+
+  private final Stack<Integer> stack = new Stack<>();
+
+  private final Wizard[] wizards = new Wizard[2];
+
+  public VirtualMachine() {
+    wizards[0] = new Wizard(randomInt(3, 32), randomInt(3, 32), randomInt(3, 32),
+        0, 0);
+    wizards[1] = new Wizard(randomInt(3, 32), randomInt(3, 32), randomInt(3, 32),
+        0, 0);
+  }
+
+  public VirtualMachine(Wizard wizard1, Wizard wizard2) {
+    wizards[0] = wizard1;
+    wizards[1] = wizard2;
+  }
+
+  public void execute(int[] bytecode) {
+    for (var i = 0; i < bytecode.length; i++) {
+      Instruction instruction = Instruction.getInstruction(bytecode[i]);
+      switch (instruction) {
+        case LITERAL:
+          // Read the next byte from the bytecode.
+          int value = bytecode[++i];
+          // Push the next value to stack
+          stack.push(value);
+          break;
+        case SET_AGILITY:
+          var amount = stack.pop();
+          var wizard = stack.pop();
+          setAgility(wizard, amount);
+          break;
+        case SET_WISDOM:
+          amount = stack.pop();
+          wizard = stack.pop();
+          setWisdom(wizard, amount);
+          break;
+        case SET_HEALTH:
+          amount = stack.pop();
+          wizard = stack.pop();
+          setHealth(wizard, amount);
+          break;
+        case GET_HEALTH:
+          wizard = stack.pop();
+          stack.push(getHealth(wizard));
+          break;
+        case GET_AGILITY:
+          wizard = stack.pop();
+          stack.push(getAgility(wizard));
+          break;
+        case GET_WISDOM:
+          wizard = stack.pop();
+          stack.push(getWisdom(wizard));
+          break;
+        case ADD:
+          var a = stack.pop();
+          var b = stack.pop();
+          stack.push(a + b);
+          break;
+        case DIVIDE:
+          a = stack.pop();
+          b = stack.pop();
+          stack.push(b / a);
+          break;
+        case PLAY_SOUND:
+          wizard = stack.pop();
+          getWizards()[wizard].playSound();
+          break;
+        case SPAWN_PARTICLES:
+          wizard = stack.pop();
+          getWizards()[wizard].spawnParticles();
+          break;
+        default:
+          throw new IllegalArgumentException("Invalid instruction value");
+      }
+      LOGGER.info("Executed " + instruction.name() + ", Stack contains " + getStack());
+    }
+  }
+
+  public void setHealth(int wizard, int amount) {
+    wizards[wizard].setHealth(amount);
+  }
+  // other setters ->
+  // ...
+}
+```
+
+Now we can show the full example utilizing the virtual machine.
+
+```java
+  public static void main(String[] args) {
+
+    var vm = new VirtualMachine(
+        new Wizard(45, 7, 11, 0, 0),
+        new Wizard(36, 18, 8, 0, 0));
+
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("GET_HEALTH"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("GET_AGILITY"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("GET_WISDOM"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("ADD"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 2"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("DIVIDE"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("ADD"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("SET_HEALTH"));
+  }
+```
+
+Here is the console output.
+
+```
+16:20:10.193 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed LITERAL, Stack contains [0]
+16:20:10.196 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed LITERAL, Stack contains [0, 0]
+16:20:10.197 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed GET_HEALTH, Stack contains [0, 45]
+16:20:10.197 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed LITERAL, Stack contains [0, 45, 0]
+16:20:10.197 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed GET_AGILITY, Stack contains [0, 45, 7]
+16:20:10.197 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed LITERAL, Stack contains [0, 45, 7, 0]
+16:20:10.197 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed GET_WISDOM, Stack contains [0, 45, 7, 11]
+16:20:10.197 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed ADD, Stack contains [0, 45, 18]
+16:20:10.197 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed LITERAL, Stack contains [0, 45, 18, 2]
+16:20:10.198 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed DIVIDE, Stack contains [0, 45, 9]
+16:20:10.198 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed ADD, Stack contains [0, 54]
+16:20:10.198 [main] INFO com.iluwatar.bytecode.VirtualMachine - Executed SET_HEALTH, Stack contains []
+```
 
 ## Class diagram
+
 ![alt text](./etc/bytecode.urm.png "Bytecode class diagram")
 
 ## Applicability
+
 Use the Bytecode pattern when you have a lot of behavior you need to define and your
 game’s implementation language isn’t a good fit because:
 
-* it’s too low-level, making it tedious or error-prone to program in.
-* iterating on it takes too long due to slow compile times or other tooling issues.
-* it has too much trust. If you want to ensure the behavior being defined can’t break the game, you need to sandbox it from the rest of the codebase.
+* It’s too low-level, making it tedious or error-prone to program in.
+* Iterating on it takes too long due to slow compile times or other tooling issues.
+* It has too much trust. If you want to ensure the behavior being defined can’t break the game, you need to sandbox it from the rest of the codebase.
+
+## Related patterns
+
+* [Interpreter](https://java-design-patterns.com/patterns/interpreter/)
 
 ## Credits
 

bytecode/etc/bytecode.ucls

@@ -1,49 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<class-diagram version="1.2.3" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
-  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
-  <class id="1" language="java" name="com.iluwatar.bytecode.VirtualMachine" project="bytecode" 
-    file="/bytecode/src/main/java/com/iluwatar/bytecode/VirtualMachine.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="455" y="173"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="2" language="java" name="com.iluwatar.bytecode.App" project="bytecode" 
-    file="/bytecode/src/main/java/com/iluwatar/bytecode/App.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="148" y="110"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <class id="3" language="java" name="com.iluwatar.bytecode.Wizard" project="bytecode" 
-    file="/bytecode/src/main/java/com/iluwatar/bytecode/Wizard.java" binary="false" corner="BOTTOM_RIGHT">    
-    <position height="-1" width="-1" x="148" y="416"/>    
-    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-      sort-features="false" accessors="true" visibility="true">      
-      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
-      <operations public="true" package="true" protected="true" private="true" static="true"/>    
-    </display>  
-  </class>  
-  <association id="4">    
-    <end type="SOURCE" refId="1" navigable="false" variant="ASSOCIATION">      
-      <attribute id="5" name="wizards">        
-        <position height="18" width="48" x="296" y="291"/>      
-      </attribute>      
-      <multiplicity id="6" minimum="0" maximum="2147483647">        
-        <position height="0" width="0" x="-327" y="-27"/>      
-      </multiplicity>    
-    </end>    
-    <end type="TARGET" refId="3" navigable="true" variant="ASSOCIATION"/>    
-    <display labels="true" multiplicity="true"/>  
-  </association>  
-  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
-    sort-features="false" accessors="true" visibility="true">    
-    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
-    <operations public="true" package="true" protected="true" private="true" static="true"/>  
-  </classifier-display>  
-  <association-display labels="true" multiplicity="true"/>
-</class-diagram>

bytecode/etc/bytecode.urm.puml

@@ -3,7 +3,6 @@ package com.iluwatar.bytecode {
   class App {
     - LOGGER : Logger {static}
     + App()
-    - interpretInstruction(instruction : String, vm : VirtualMachine) {static}
     + main(args : String[]) {static}
   }
   enum Instruction {
@@ -18,22 +17,25 @@ package com.iluwatar.bytecode {
     + SET_HEALTH {static}
     + SET_WISDOM {static}
     + SPAWN_PARTICLES {static}
-    - value : int
+    - intValue : int
     + getInstruction(value : int) : Instruction {static}
     + getIntValue() : int
     + valueOf(name : String) : Instruction {static}
     + values() : Instruction[] {static}
   }
   class VirtualMachine {
+    - LOGGER : Logger {static}
     - stack : Stack<Integer>
     - wizards : Wizard[]
     + VirtualMachine()
+    + VirtualMachine(wizard1 : Wizard, wizard2 : Wizard)
     + execute(bytecode : int[])
     + getAgility(wizard : int) : int
     + getHealth(wizard : int) : int
     + getStack() : Stack<Integer>
     + getWisdom(wizard : int) : int
     + getWizards() : Wizard[]
+    - randomInt(min : int, max : int) : int
     + setAgility(wizard : int, amount : int)
     + setHealth(wizard : int, amount : int)
     + setWisdom(wizard : int, amount : int)
@@ -45,7 +47,7 @@ package com.iluwatar.bytecode {
     - numberOfPlayedSounds : int
     - numberOfSpawnedParticles : int
     - wisdom : int
-    + Wizard()
+    + Wizard(health : int, agility : int, wisdom : int, numberOfPlayedSounds : int, numberOfSpawnedParticles : int)
     + getAgility() : int
     + getHealth() : int
     + getNumberOfPlayedSounds() : int
@@ -54,6 +56,8 @@ package com.iluwatar.bytecode {
     + playSound()
     + setAgility(agility : int)
     + setHealth(health : int)
+    + setNumberOfPlayedSounds(numberOfPlayedSounds : int)
+    + setNumberOfSpawnedParticles(numberOfSpawnedParticles : int)
     + setWisdom(wisdom : int)
     + spawnParticles()
   }

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

@@ -49,33 +49,21 @@ public class App {
    */
   public static void main(String[] args) {
 
-    var wizard = new Wizard();
-    wizard.setHealth(45);
-    wizard.setAgility(7);
-    wizard.setWisdom(11);
+    var vm = new VirtualMachine(
+        new Wizard(45, 7, 11, 0, 0),
+        new Wizard(36, 18, 8, 0, 0));
 
-    var vm = new VirtualMachine();
-    vm.getWizards()[0] = wizard;
-
-    String literal = "LITERAL 0";
-
-    interpretInstruction(literal, vm);
-    interpretInstruction(literal, vm);
-    interpretInstruction("GET_HEALTH", vm);
-    interpretInstruction(literal, vm);
-    interpretInstruction("GET_AGILITY", vm);
-    interpretInstruction(literal, vm);
-    interpretInstruction("GET_WISDOM ", vm);
-    interpretInstruction("ADD", vm);
-    interpretInstruction("LITERAL 2", vm);
-    interpretInstruction("DIVIDE", vm);
-    interpretInstruction("ADD", vm);
-    interpretInstruction("SET_HEALTH", vm);
-  }
-
-  private static void interpretInstruction(String instruction, VirtualMachine vm) {
-    vm.execute(InstructionConverterUtil.convertToByteCode(instruction));
-    var stack = vm.getStack();
-    LOGGER.info(instruction + String.format("%" + (12 - instruction.length()) + "s", "") + stack);
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("GET_HEALTH"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("GET_AGILITY"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 0"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("GET_WISDOM"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("ADD"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("LITERAL 2"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("DIVIDE"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("ADD"));
+    vm.execute(InstructionConverterUtil.convertToByteCode("SET_HEALTH"));
   }
 }

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

@@ -33,17 +33,17 @@ import lombok.Getter;
 @Getter
 public enum Instruction {
 
-  LITERAL(1),
-  SET_HEALTH(2),
-  SET_WISDOM(3),
-  SET_AGILITY(4),
-  PLAY_SOUND(5),
-  SPAWN_PARTICLES(6),
-  GET_HEALTH(7),
-  GET_AGILITY(8),
-  GET_WISDOM(9),
-  ADD(10),
-  DIVIDE(11);
+  LITERAL(1),         // e.g. "LITERAL 0", push 0 to stack
+  SET_HEALTH(2),      // e.g. "SET_HEALTH", pop health and wizard number, call set health
+  SET_WISDOM(3),      // e.g. "SET_WISDOM", pop wisdom and wizard number, call set wisdom
+  SET_AGILITY(4),     // e.g. "SET_AGILITY", pop agility and wizard number, call set agility
+  PLAY_SOUND(5),      // e.g. "PLAY_SOUND", pop value as wizard number, call play sound
+  SPAWN_PARTICLES(6), // e.g. "SPAWN_PARTICLES", pop value as wizard number, call spawn particles
+  GET_HEALTH(7),      // e.g. "GET_HEALTH", pop value as wizard number, push wizard's health
+  GET_AGILITY(8),     // e.g. "GET_AGILITY", pop value as wizard number, push wizard's agility
+  GET_WISDOM(9),      // e.g. "GET_WISDOM", pop value as wizard number, push wizard's wisdom
+  ADD(10),            // e.g. "ADD", pop 2 values, push their sum
+  DIVIDE(11);         // e.g. "DIVIDE", pop 2 values, push their division
 
   private final int intValue;
 

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

@@ -24,12 +24,15 @@
 package com.iluwatar.bytecode;
 
 import java.util.Stack;
+import java.util.concurrent.ThreadLocalRandom;
 import lombok.Getter;
+import lombok.extern.slf4j.Slf4j;
 
 /**
  * Implementation of virtual machine.
  */
 @Getter
+@Slf4j
 public class VirtualMachine {
 
   private final Stack<Integer> stack = new Stack<>();
@@ -37,12 +40,21 @@ public class VirtualMachine {
   private final Wizard[] wizards = new Wizard[2];
 
   /**
-   * Constructor.
+   * No-args constructor.
    */
   public VirtualMachine() {
-    for (var i = 0; i < wizards.length; i++) {
-      wizards[i] = new Wizard();
-    }
+    wizards[0] = new Wizard(randomInt(3, 32), randomInt(3, 32), randomInt(3, 32),
+        0, 0);
+    wizards[1] = new Wizard(randomInt(3, 32), randomInt(3, 32), randomInt(3, 32),
+        0, 0);
+  }
+
+  /**
+   * Constructor taking the wizards as arguments.
+   */
+  public VirtualMachine(Wizard wizard1, Wizard wizard2) {
+    wizards[0] = wizard1;
+    wizards[1] = wizard2;
   }
 
   /**
@@ -57,6 +69,7 @@ public class VirtualMachine {
         case LITERAL:
           // Read the next byte from the bytecode.
           int value = bytecode[++i];
+          // Push the next value to stack
           stack.push(value);
           break;
         case SET_AGILITY:
@@ -107,6 +120,7 @@ public class VirtualMachine {
         default:
           throw new IllegalArgumentException("Invalid instruction value");
       }
+      LOGGER.info("Executed " + instruction.name() + ", Stack contains " + getStack());
     }
   }
 
@@ -133,4 +147,8 @@ public class VirtualMachine {
   public int getAgility(int wizard) {
     return wizards[wizard].getAgility();
   }
+
+  private int randomInt(int min, int max) {
+    return ThreadLocalRandom.current().nextInt(min, max + 1);
+  }
 }

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

@@ -23,6 +23,7 @@
 
 package com.iluwatar.bytecode;
 
+import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.Setter;
 import lombok.extern.slf4j.Slf4j;
@@ -31,16 +32,15 @@ import lombok.extern.slf4j.Slf4j;
  * This class represent game objects which properties can be changed by instructions interpreted by
  * virtual machine.
  */
+@AllArgsConstructor
 @Setter
 @Getter
 @Slf4j
 public class Wizard {
 
   private int health;
-
   private int agility;
   private int wisdom;
-
   private int numberOfPlayedSounds;
   private int numberOfSpawnedParticles;
 
@@ -53,5 +53,4 @@ public class Wizard {
     LOGGER.info("Spawning particles");
     numberOfSpawnedParticles++;
   }
-
 }

bytecode/src/main/java/com/iluwatar/bytecode/util/InstructionConverterUtil.java

@@ -73,6 +73,4 @@ public class InstructionConverterUtil {
       return false;
     }
   }
-
-
 }

caching/README.md

@@ -10,20 +10,326 @@ tags:
 ---
 
 ## Intent
-To avoid expensive re-acquisition of resources by not releasing
-the resources immediately after their use. The resources retain their identity, are kept in some
-fast-access storage, and are re-used to avoid having to acquire them again.
+
+The caching pattern avoids expensive re-acquisition of resources by not releasing them immediately 
+after use. The resources retain their identity, are kept in some fast-access storage, and are 
+re-used to avoid having to acquire them again.
+
+## Explanation
+
+Real world example
+
+> A team is working on a website that provides new homes for abandoned cats. People can post their
+> cats on the website after registering, but all the new posts require approval from one of the
+> site moderators. The user accounts of the site moderators contain a specific flag and the data
+> is stored in a MongoDB database. Checking for the moderator flag each time a post is viewed 
+> becomes expensive and it's a good idea to utilize caching here.
+
+In plain words
+
+> Caching pattern keeps frequently needed data in fast-access storage to improve performance.
+
+Wikipedia says:
+
+> In computing, a cache is a hardware or software component that stores data so that future 
+> requests for that data can be served faster; the data stored in a cache might be the result of 
+> an earlier computation or a copy of data stored elsewhere. A cache hit occurs when the requested 
+> data can be found in a cache, while a cache miss occurs when it cannot. Cache hits are served by 
+> reading data from the cache, which is faster than recomputing a result or reading from a slower 
+> data store; thus, the more requests that can be served from the cache, the faster the system 
+> performs.
+
+**Programmatic Example**
+
+Let's first look at the data layer of our application. The interesting classes are `UserAccount`
+which is a simple Java object containing the user account details, and `DbManager` which handles
+reading and writing of these objects to/from MongoDB database.
+
+```java
+@Setter
+@Getter
+@AllArgsConstructor
+@ToString
+public class UserAccount {
+  private String userId;
+  private String userName;
+  private String additionalInfo;
+}
+
+@Slf4j
+public final class DbManager {
+
+  private static MongoClient mongoClient;
+  private static MongoDatabase db;
+
+  private DbManager() { /*...*/ }
+
+  public static void createVirtualDb() { /*...*/ }
+
+  public static void connect() throws ParseException { /*...*/ }
+
+  public static UserAccount readFromDb(String userId) { /*...*/ }
+
+  public static void writeToDb(UserAccount userAccount) { /*...*/ }
+
+  public static void updateDb(UserAccount userAccount) { /*...*/ }
+
+  public static void upsertDb(UserAccount userAccount) { /*...*/ }
+}
+```
+
+In the example, we are demonstrating various different caching policies
+
+* Write-through writes data to the cache and DB in a single transaction
+* Write-around writes data immediately into the DB instead of the cache
+* Write-behind writes data into the cache initially whilst the data is only written into the DB 
+  when the cache is full
+* Cache-aside pushes the responsibility of keeping the data synchronized in both data sources to 
+  the application itself
+* Read-through strategy is also included in the aforementioned strategies and it returns data from 
+  the cache to the caller if it exists, otherwise queries from DB and stores it into the cache for 
+  future use.
+  
+The cache implementation in `LruCache` is a hash table accompanied by a doubly 
+linked-list. The linked-list helps in capturing and maintaining the LRU data in the cache. When 
+data is queried (from the cache), added (to the cache), or updated, the data is moved to the front 
+of the list to depict itself as the most-recently-used data. The LRU data is always at the end of 
+the list.
+
+```java
+@Slf4j
+public class LruCache {
+
+  static class Node {
+    String userId;
+    UserAccount userAccount;
+    Node previous;
+    Node next;
+
+    public Node(String userId, UserAccount userAccount) {
+      this.userId = userId;
+      this.userAccount = userAccount;
+    }
+  }
+  
+  /* ... omitted details ... */
+
+  public LruCache(int capacity) {
+    this.capacity = capacity;
+  }
+
+  public UserAccount get(String userId) {
+    if (cache.containsKey(userId)) {
+      var node = cache.get(userId);
+      remove(node);
+      setHead(node);
+      return node.userAccount;
+    }
+    return null;
+  }
+
+  public void set(String userId, UserAccount userAccount) {
+    if (cache.containsKey(userId)) {
+      var old = cache.get(userId);
+      old.userAccount = userAccount;
+      remove(old);
+      setHead(old);
+    } else {
+      var newNode = new Node(userId, userAccount);
+      if (cache.size() >= capacity) {
+        LOGGER.info("# Cache is FULL! Removing {} from cache...", end.userId);
+        cache.remove(end.userId); // remove LRU data from cache.
+        remove(end);
+        setHead(newNode);
+      } else {
+        setHead(newNode);
+      }
+      cache.put(userId, newNode);
+    }
+  }
+
+  public boolean contains(String userId) {
+    return cache.containsKey(userId);
+  }
+  
+  public void remove(Node node) { /* ... */ }
+  public void setHead(Node node) { /* ... */ }
+  public void invalidate(String userId) { /* ... */ }
+  public boolean isFull() { /* ... */ }
+  public UserAccount getLruData() { /* ... */ }
+  public void clear() { /* ... */ }
+  public List<UserAccount> getCacheDataInListForm() { /* ... */ }
+  public void setCapacity(int newCapacity) { /* ... */ }
+}
+```
+
+The next layer we are going to look at is `CacheStore` which implements the different caching
+strategies.
+
+```java
+@Slf4j
+public class CacheStore {
+
+  private static LruCache cache;
+
+  /* ... details omitted ... */
+
+  public static UserAccount readThrough(String userId) {
+    if (cache.contains(userId)) {
+      LOGGER.info("# Cache Hit!");
+      return cache.get(userId);
+    }
+    LOGGER.info("# Cache Miss!");
+    UserAccount userAccount = DbManager.readFromDb(userId);
+    cache.set(userId, userAccount);
+    return userAccount;
+  }
+
+  public static void writeThrough(UserAccount userAccount) {
+    if (cache.contains(userAccount.getUserId())) {
+      DbManager.updateDb(userAccount);
+    } else {
+      DbManager.writeToDb(userAccount);
+    }
+    cache.set(userAccount.getUserId(), userAccount);
+  }
+
+  public static void clearCache() {
+    if (cache != null) {
+      cache.clear();
+    }
+  }
+
+  public static void flushCache() {
+    LOGGER.info("# flushCache...");
+    Optional.ofNullable(cache)
+        .map(LruCache::getCacheDataInListForm)
+        .orElse(List.of())
+        .forEach(DbManager::updateDb);
+  }
+
+  /* ... omitted the implementation of other caching strategies ... */
+
+}
+```
+
+`AppManager` helps to bridge the gap in communication between the main class and the application's
+back-end. DB connection is initialized through this class. The chosen caching strategy/policy is
+also initialized here. Before the cache can be used, the size of the cache has to be set. Depending 
+on the chosen caching policy, `AppManager` will call the appropriate function in the `CacheStore` 
+class.
+
+```java
+@Slf4j
+public final class AppManager {
+
+  private static CachingPolicy cachingPolicy;
+
+  private AppManager() {
+  }
+
+  public static void initDb(boolean useMongoDb) { /* ... */ }
+
+  public static void initCachingPolicy(CachingPolicy policy) { /* ... */ }
+
+  public static void initCacheCapacity(int capacity) { /* ... */ }
+
+  public static UserAccount find(String userId) {
+    if (cachingPolicy == CachingPolicy.THROUGH || cachingPolicy == CachingPolicy.AROUND) {
+      return CacheStore.readThrough(userId);
+    } else if (cachingPolicy == CachingPolicy.BEHIND) {
+      return CacheStore.readThroughWithWriteBackPolicy(userId);
+    } else if (cachingPolicy == CachingPolicy.ASIDE) {
+      return findAside(userId);
+    }
+    return null;
+  }
+
+  public static void save(UserAccount userAccount) {
+    if (cachingPolicy == CachingPolicy.THROUGH) {
+      CacheStore.writeThrough(userAccount);
+    } else if (cachingPolicy == CachingPolicy.AROUND) {
+      CacheStore.writeAround(userAccount);
+    } else if (cachingPolicy == CachingPolicy.BEHIND) {
+      CacheStore.writeBehind(userAccount);
+    } else if (cachingPolicy == CachingPolicy.ASIDE) {
+      saveAside(userAccount);
+    }
+  }
+
+  public static String printCacheContent() {
+    return CacheStore.print();
+  }
+
+  /* ... details omitted ... */
+}
+```
+
+Here is what we do in the main class of the application.
+
+```java
+@Slf4j
+public class App {
+
+  public static void main(String[] args) {
+    AppManager.initDb(false);
+    AppManager.initCacheCapacity(3);
+    var app = new App();
+    app.useReadAndWriteThroughStrategy();
+    app.useReadThroughAndWriteAroundStrategy();
+    app.useReadThroughAndWriteBehindStrategy();
+    app.useCacheAsideStategy();
+  }
+
+  public void useReadAndWriteThroughStrategy() {
+    LOGGER.info("# CachingPolicy.THROUGH");
+    AppManager.initCachingPolicy(CachingPolicy.THROUGH);
+    var userAccount1 = new UserAccount("001", "John", "He is a boy.");
+    AppManager.save(userAccount1);
+    LOGGER.info(AppManager.printCacheContent());
+    AppManager.find("001");
+    AppManager.find("001");
+  }
+
+  public void useReadThroughAndWriteAroundStrategy() { /* ... */ }
+
+  public void useReadThroughAndWriteBehindStrategy() { /* ... */ }
+
+  public void useCacheAsideStategy() { /* ... */ }
+}
+```
+
+Finally, here is some of the console output from the program.
+
+```
+12:32:53.845 [main] INFO com.iluwatar.caching.App - # CachingPolicy.THROUGH
+12:32:53.900 [main] INFO com.iluwatar.caching.App - 
+--CACHE CONTENT--
+UserAccount(userId=001, userName=John, additionalInfo=He is a boy.)
+----
+```
 
 ## Class diagram
+
 ![alt text](./etc/caching.png "Caching")
 
 ## Applicability
+
 Use the Caching pattern(s) when
 
-* Repetitious acquisition, initialization, and release of the same resource causes unnecessary performance overhead.
+* Repetitious acquisition, initialization, and release of the same resource cause unnecessary 
+  performance overhead.
+
+## Related patterns
+
+* [Proxy](https://java-design-patterns.com/patterns/proxy/)
 
 ## Credits
 
 * [Write-through, write-around, write-back: Cache explained](http://www.computerweekly.com/feature/Write-through-write-around-write-back-Cache-explained)
 * [Read-Through, Write-Through, Write-Behind, and Refresh-Ahead Caching](https://docs.oracle.com/cd/E15357_01/coh.360/e15723/cache_rtwtwbra.htm#COHDG5177)
 * [Cache-Aside pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/cache-aside)
+* [Java EE 8 High Performance: Master techniques such as memory optimization, caching, concurrency, and multithreading to achieve maximum performance from your enterprise applications](https://www.amazon.com/gp/product/178847306X/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=178847306X&linkId=e948720055599f248cdac47da9125ff4)
+* [Java Performance: In-Depth Advice for Tuning and Programming Java 8, 11, and Beyond](https://www.amazon.com/gp/product/1492056111/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1492056111&linkId=7e553581559b9ec04221259e52004b08)
+* [Effective Java](https://www.amazon.com/gp/product/B078H61SCH/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=B078H61SCH&linkId=f06607a0b48c76541ef19c5b8b9e7882)
+* [Java Performance: The Definitive Guide: Getting the Most Out of Your Code](https://www.amazon.com/gp/product/1449358454/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1449358454&linkId=475c18363e350630cc0b39ab681b2687)

caching/src/main/java/com/iluwatar/caching/AppManager.java

@@ -25,6 +25,7 @@ package com.iluwatar.caching;
 
 import java.text.ParseException;
 import java.util.Optional;
+import lombok.extern.slf4j.Slf4j;
 
 /**
  * AppManager helps to bridge the gap in communication between the main class and the application's
@@ -33,6 +34,7 @@ import java.util.Optional;
  * Depending on the chosen caching policy, AppManager will call the appropriate function in the
  * CacheStore class.
  */
+@Slf4j
 public final class AppManager {
 
   private static CachingPolicy cachingPolicy;
@@ -50,7 +52,7 @@ public final class AppManager {
       try {
         DbManager.connect();
       } catch (ParseException e) {
-        e.printStackTrace();
+        LOGGER.error("Error connecting to MongoDB", e);
       }
     } else {
       DbManager.createVirtualDb();

caching/src/main/java/com/iluwatar/caching/DbManager.java

@@ -30,6 +30,7 @@ import com.mongodb.client.model.UpdateOptions;
 import java.text.ParseException;
 import java.util.HashMap;
 import java.util.Map;
+import lombok.extern.slf4j.Slf4j;
 import org.bson.Document;
 
 /**
@@ -41,6 +42,7 @@ import org.bson.Document;
  * underlying data storage (connect()) or a simple Java data structure to (temporarily) store the
  * data/objects during runtime (createVirtualDB()).</p>
  */
+@Slf4j
 public final class DbManager {
 
   private static MongoClient mongoClient;
@@ -83,7 +85,7 @@ public final class DbManager {
       try {
         connect();
       } catch (ParseException e) {
-        e.printStackTrace();
+        LOGGER.error("Error connecting to MongoDB", e);
       }
     }
     var iterable = db
@@ -110,7 +112,7 @@ public final class DbManager {
       try {
         connect();
       } catch (ParseException e) {
-        e.printStackTrace();
+        LOGGER.error("Error connecting to MongoDB", e);
       }
     }
     db.getCollection(CachingConstants.USER_ACCOUNT).insertOne(
@@ -132,7 +134,7 @@ public final class DbManager {
       try {
         connect();
       } catch (ParseException e) {
-        e.printStackTrace();
+        LOGGER.error("Error connecting to MongoDB", e);
       }
     }
     db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(
@@ -153,7 +155,7 @@ public final class DbManager {
       try {
         connect();
       } catch (ParseException e) {
-        e.printStackTrace();
+        LOGGER.error("Error connecting to MongoDB", e);
       }
     }
     db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(

factory/README.md

@@ -105,7 +105,7 @@ Program output:
 
 ```java
 This is Ford.
-This Ferrari.
+This is Ferrari.
 ```
 
 ## Class Diagram

model-view-viewmodel/README.md

@@ -0,0 +1,145 @@
+---
+layout: pattern
+title: Model-View-ViewModel
+folder: model-view-viewmodel
+permalink: /patterns/model-view-viewmodel/
+categories: Architectural
+tags:
+ - Decoupling
+---
+
+## Also known as
+
+Model–View–Binder
+
+## Intent
+
+To apply "[Separation of Concerns](https://java-design-patterns.com/principles/#separation-of-concerns)" to separate the logic from the UI components and allow developers to work on UI without affecting the logic and vice versa.
+
+## Explanation
+
+Wikipedia says
+
+> Model–view–viewmodel (MVVM) is a software architectural pattern that facilitates the separation of the development of the graphical user interface (the view) – be it via a markup language or GUI code – from the development of the business logic or back-end logic (the model) so that the view is not dependent on any specific model platform. 
+
+**Programmatic Example**
+
+Zkoss implementation:
+
+> ViewModel will hold the business logic and expose the data from model to View
+
+```java
+public class BookViewModel {
+  @WireVariable
+  private List<Book> bookList;
+  private Book selectedBook;
+  private BookService bookService = new BookServiceImpl();
+  
+  public Book getSelectedBook() {
+    return selectedBook;
+  }
+
+  @NotifyChange("selectedBook")
+  public void setSelectedBook(Book selectedBook) {
+    this.selectedBook = selectedBook;
+  }
+
+  public List<Book> getBookList() {
+    return bookService.load();
+  }
+  
+  /** Deleting a book.
+   */
+  @Command
+  @NotifyChange({"selectedBook","bookList"})
+  public void deleteBook() {
+    if (selectedBook != null) {
+      getBookList().remove(selectedBook);
+      selectedBook = null;
+    }
+}
+```
+
+> View will have no logic, only UI elements
+
+```xml
+<zk>
+<window title="List of Books" border="normal" width="600px" apply="org.zkoss.bind.BindComposer" viewModel="@id('vm') @init('com.iluwatar.model.view.viewmodel.BookViewModel')">
+    <vbox hflex="true">
+        <listbox model="@bind(vm.bookList)" selectedItem="@bind(vm.selectedBook)" height="400px" mold="paging">
+            <listhead>
+                <listheader label="Book Name"/>
+                <listheader label="Author"/>               
+            </listhead>
+            <template name="model" var="book">
+                <listitem >
+                    <listcell label="@bind(book.name)"/>
+                    <listcell label="@bind(book.author)"/>
+                </listitem>
+            </template>
+        </listbox>
+    </vbox>
+    <toolbar>
+        <button label="Delete" onClick="@command('deleteBook')" disabled="@load(empty vm.selectedBook)" />
+    </toolbar>
+    <hbox style="margin-top:20px" visible="@bind(not empty vm.selectedBook)">
+		<vbox>
+			<hlayout>
+				Book Name : <label value="@bind(vm.selectedBook.name)" style="font-weight:bold"/>
+			</hlayout>
+			<hlayout>
+				Book Author : <label value="@bind(vm.selectedBook.author)" style="font-weight:bold"/>
+			</hlayout>
+			<hlayout>
+				Book Description : <label value="@bind(vm.selectedBook.description)" style="font-weight:bold"/>
+			</hlayout>
+		</vbox>
+	</hbox>
+</window>
+</zk>
+```
+
+To deploy the example, go to model-view-viewmodel folder and run:
+
+* `mvn clean install`
+* `mvn jetty:run -Djetty.http.port=9911`
+* Open browser to address: http://localhost:9911/model-view-viewmodel/
+
+## Class diagram
+
+![alt text](./etc/model-view-viewmodel.png "MVVM pattern class diagram")
+
+## Applicability
+
+* When looking for clean architecture, with better reusability, testability and maintainability.
+
+## Tutorials
+
+* [Zkoss Demo](https://www.zkoss.org/zkdemo/getting_started/mvvm)
+* [Learn MVVM](https://www.learnmvvm.com/)
+* [Android Developer CodeLabs](https://codelabs.developers.google.com/codelabs/android-databinding)
+
+## Typical Use Case
+
+* Android apps
+* .NET framework applications
+* JavaScript applications
+
+## Real world examples
+
+* ZK Framework [zkoss.org](https://www.zkoss.org/)
+* KnockoutJS [knockoutjs.com](https://knockoutjs.com/)
+
+## Consequences
+
+* John Gossman has criticized the MVVM pattern and its application in specific uses, stating that MVVM can be "overkill" when creating simple user interfaces. For larger applications, he believes that generalizing the viewmodel upfront can be difficult, and that large-scale data binding can lead to lower performance - Ref: [MVVM-Wiki](https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93viewmodel)
+
+* Can be hard to design ViewModel for larger applications.
+* For complex databinding, debugging can be difficult.
+
+## Credits
+
+* [ZK MVVM](https://www.zkoss.org/wiki/ZK%20Developer's%20Reference/MVVM)
+* [GeeksforGeeks  MVVM Intro](https://www.geeksforgeeks.org/introduction-to-model-view-view-model-mvvm/)
+* [ZK MVVM Book](http://books.zkoss.org/zk-mvvm-book/9.5/)
+* [Microsoft MVVM](https://docs.microsoft.com/en-us/archive/msdn-magazine/2009/february/patterns-wpf-apps-with-the-model-view-viewmodel-design-pattern)

model-view-viewmodel/etc/model-view-viewmodel.urm.puml

@@ -0,0 +1,43 @@
+@startuml
+package com.iluwatar.model.view.viewmodel {
+  class Book {
+    - author : String
+    - description : String
+    - name : String
+    + Book(name : String, author : String, description : String)
+    # canEqual(other : Object) : boolean
+    + equals(o : Object) : boolean
+    + getAuthor() : String
+    + getDescription() : String
+    + getName() : String
+    + hashCode() : int
+    + setAuthor(author : String)
+    + setDescription(description : String)
+    + setName(name : String)
+    + toString() : String
+  }
+  interface BookService {
+    + load() : List<Book> {abstract}
+  }
+  class BookServiceImpl {
+    - designPatternBooks : List<Book>
+    + BookServiceImpl()
+    + load() : List<Book>
+  }
+  class BookViewModel {
+    - bookList : List<Book>
+    - bookService : BookService
+    - selectedBook : Book
+    + BookViewModel()
+    + deleteBook()
+    + getBookList() : List<Book>
+    + getSelectedBook() : Book
+    + setSelectedBook(selectedBook : Book)
+  }
+}
+BookViewModel -->  "-bookService" BookService
+BookServiceImpl -->  "-designPatternBooks" Book
+BookViewModel -->  "-bookList" Book
+BookViewModel -->  "-selectedBook" Book
+BookServiceImpl ..|> BookService 
+@enduml

model-view-viewmodel/lombok.config

@@ -0,0 +1,2 @@
+config.stopBubbling = true
+lombok.addLombokGeneratedAnnotation = true

model-view-viewmodel/pom.xml

@@ -0,0 +1,115 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<parent>
+		<artifactId>java-design-patterns</artifactId>
+		<groupId>com.iluwatar</groupId>
+		<version>1.24.0-SNAPSHOT</version>
+	</parent>
+	<groupId>com.iluwatar</groupId>
+	<artifactId>model-view-viewmodel</artifactId>
+	<version>1.24.0-SNAPSHOT</version>
+	<properties>
+		<zk.version>9.0.0</zk.version>
+		<guava.version>19.0</guava.version>
+		<jetty-maven-plugin.version>9.4.28.v20200408</jetty-maven-plugin.version>
+		<maven-war-plugin.version>2.1.1</maven-war-plugin.version>
+		<maven-assembly-plugin.version>2.2</maven-assembly-plugin.version>
+		<maven.build.timestamp.format>yyyy-MM-dd</maven.build.timestamp.format>
+		<packname>-${project.version}-FL-${maven.build.timestamp}</packname>
+	</properties>
+	<packaging>war</packaging>
+	<name>model-view-viewmodel</name>
+	<description>model-view-viewmodel</description>
+	<licenses>
+		<license>
+			<name>GNU LESSER GENERAL PUBLIC LICENSE, Version 3</name>
+			<url>https://www.gnu.org/licenses/lgpl.html</url>
+			<distribution>repo</distribution>
+		</license>
+	</licenses>
+	<repositories>
+		<repository>
+			<id>ZK CE</id>
+			<name>ZK CE Repository</name>
+			<url>https://mavensync.zkoss.org/maven2</url>
+		</repository>
+		<repository>
+			<id>ZK EVAL</id>
+			<name>ZK Evaluation Repository</name>
+			<url>https://mavensync.zkoss.org/eval</url>
+		</repository>
+	</repositories>
+	<pluginRepositories>
+		<pluginRepository>
+			<id>zkmaven</id>
+			<name>ZK Maven Plugin Repository</name>
+			<url>https://mavensync.zkoss.org/maven2/</url>
+		</pluginRepository>
+	</pluginRepositories>
+	<dependencies>
+		<dependency>
+			<groupId>org.zkoss.zk</groupId>
+			<artifactId>zkbind</artifactId>
+			<version>${zk.version}</version>
+		</dependency>
+		<dependency>
+			<groupId>org.junit.jupiter</groupId>
+			<artifactId>junit-jupiter-engine</artifactId>
+			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>com.google.guava</groupId>
+			<artifactId>guava-testlib</artifactId>
+			<version>${guava.version}</version>
+			<scope>test</scope>
+		</dependency>
+	</dependencies>
+	<build>
+		<finalName>${project.artifactId}</finalName>
+		<plugins>
+			<!-- Run with Jetty -->
+			<plugin>
+				<groupId>org.eclipse.jetty</groupId>
+				<artifactId>jetty-maven-plugin</artifactId>
+				<version>${jetty-maven-plugin.version}</version>
+				<configuration>
+					<webApp>
+						<contextPath>/${project.artifactId}</contextPath>
+						<allowDuplicateFragmentNames>true</allowDuplicateFragmentNames>
+					</webApp>
+					<scanIntervalSeconds>5</scanIntervalSeconds>
+				</configuration>
+			</plugin>
+			<!-- Build war -->
+			<plugin>
+				<artifactId>maven-war-plugin</artifactId>
+				<groupId>org.apache.maven.plugins</groupId>
+				<version>${maven-war-plugin.version}</version>
+			</plugin>
+			<!-- Pack zips -->
+			<plugin>
+				<artifactId>maven-assembly-plugin</artifactId>
+				<version>${maven-assembly-plugin.version}</version>
+				<executions>
+					<execution>
+						<id>webapp</id>
+						<phase>package</phase>
+						<goals>
+							<goal>single</goal>
+						</goals>
+						<configuration>
+							<finalName>model-view-viewmodel${packname}</finalName>
+							<appendAssemblyId>false</appendAssemblyId>
+							<descriptors>
+								<descriptor>src/main/assembly/webapp.xml</descriptor>
+							</descriptors>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+</project>

model-view-viewmodel/src/main/assembly/webapp.xml

@@ -0,0 +1,25 @@
+<assembly
+	xmlns="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.0"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/plugins/maven-assembly-plugin/assembly/1.1.0 http://maven.apache.org/xsd/assembly-1.1.0.xsd">
+	<id>webapp</id>
+	<formats>
+		<format>zip</format>
+	</formats>
+	<fileSets>
+		<fileSet>
+			<directory>${project.basedir}/src/main/java</directory>
+			<outputDirectory>/${project.artifactId}/src</outputDirectory>
+		</fileSet>
+		<fileSet>
+			<directory>${project.basedir}/src/main/webapp</directory>
+			<outputDirectory>/${project.artifactId}/WebContent</outputDirectory>
+		</fileSet>
+	</fileSets>
+	<files>
+		<file>
+			<source>${project.build.directory}/${project.artifactId}.war</source>
+			<outputDirectory>/</outputDirectory>
+		</file>
+	</files>
+</assembly>

model-view-viewmodel/src/main/java/com/iluwatar/model/view/viewmodel/Book.java

@@ -21,13 +21,17 @@
  * THE SOFTWARE.
  */
 
-package com.iluwatar.business.delegate;
+package com.iluwatar.model.view.viewmodel;
 
-/**
- * Enumeration for service types.
- */
-public enum ServiceType {
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@AllArgsConstructor
+@Data
+public class Book {
+
+  private String name;
+  private String author;
+  private String description;
 
-  EJB,
-  JMS
 }

model-view-viewmodel/src/main/java/com/iluwatar/model/view/viewmodel/BookService.java

@@ -0,0 +1,10 @@
+package com.iluwatar.model.view.viewmodel;
+
+import java.util.List;
+
+public interface BookService {
+  /* List all books
+   * @return all books
+   */
+  public List<Book> load();
+}

model-view-viewmodel/src/main/java/com/iluwatar/model/view/viewmodel/BookServiceImpl.java

@@ -0,0 +1,38 @@
+package com.iluwatar.model.view.viewmodel;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class BookServiceImpl implements BookService {
+  private List<Book> designPatternBooks = new ArrayList<>();
+
+  /** Initializes Book Data.
+  * To be used and passed along in load method
+  * In this case, list design pattern books are initialized to be loaded.
+  */
+  public BookServiceImpl() {
+    designPatternBooks.add(new Book(
+        "Head First Design Patterns: A Brain-Friendly Guide",
+        "Eric Freeman, Bert Bates, Kathy Sierra, Elisabeth Robson",
+        "Head First Design Patterns Description"));
+    designPatternBooks.add(new Book(
+        "Design Patterns: Elements of Reusable Object-Oriented Software",
+        "Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides",
+        "Design Patterns Description"));
+    designPatternBooks.add(new Book(
+        "Patterns of Enterprise Application Architecture", "Martin Fowler",
+        "Patterns of Enterprise Application Architecture Description"));
+    designPatternBooks.add(new Book(
+        "Design Patterns Explained", "Alan Shalloway, James Trott",
+        "Design Patterns Explained Description"));
+    designPatternBooks.add(new Book(
+        "Applying UML and Patterns: An Introduction to "
+         + "Object-Oriented Analysis and Design and Iterative Development",
+        "Craig Larman", "Applying UML and Patterns Description"));
+  }
+
+  public List<Book> load() {
+    return designPatternBooks;
+  }
+
+}

model-view-viewmodel/src/main/java/com/iluwatar/model/view/viewmodel/BookViewModel.java

@@ -0,0 +1,43 @@
+package com.iluwatar.model.view.viewmodel;
+
+import java.util.List;
+
+import org.zkoss.bind.annotation.Command;
+import org.zkoss.bind.annotation.NotifyChange;
+import org.zkoss.zk.ui.select.annotation.WireVariable;
+
+public class BookViewModel {
+  
+  @WireVariable
+  private List<Book> bookList;
+  private Book selectedBook;
+  private BookService bookService = new BookServiceImpl();
+  
+  public Book getSelectedBook() {
+    return selectedBook;
+  }
+
+  @NotifyChange("selectedBook")
+  public void setSelectedBook(Book selectedBook) {
+    this.selectedBook = selectedBook;
+  }
+
+  public List<Book> getBookList() {
+    return bookService.load();
+  }
+  
+  /** Deleting a book.
+   * When event is triggered on click of Delete button, 
+   * this method will be notified with the selected entry that will be referenced
+   * and used to delete the selected book from the list of books. 
+   */
+  @Command
+  @NotifyChange({"selectedBook","bookList"})
+  public void deleteBook() {
+    if (selectedBook != null) {
+      getBookList().remove(selectedBook);
+      selectedBook = null;
+    }
+  }
+
+}

model-view-viewmodel/src/main/webapp/META-INF/MANIFEST.MF

@@ -0,0 +1,8 @@
+Manifest-Version: 1.0
+Specification-Title: ZK Application
+Specification-Version: 1.0
+Specification-Vendor: 
+Implementation-Title: 
+Implementation-URL: http://your-website/
+Implementation-Version: 1.0
+Implementation-Vendor: 

model-view-viewmodel/src/main/webapp/WEB-INF/web.xml

@@ -0,0 +1,114 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
+ xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+ xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> 
+
+	<description><![CDATA[My ZK Application]]></description>
+	<display-name>model-view-viewmodel</display-name>
+
+	<!-- ZK -->
+	<listener>
+		<description>ZK listener for session cleanup</description>
+		<listener-class>org.zkoss.zk.ui.http.HttpSessionListener</listener-class>
+	</listener>
+	<servlet>
+		<description>ZK loader for ZUML pages</description>
+		<servlet-name>zkLoader</servlet-name>
+		<servlet-class>org.zkoss.zk.ui.http.DHtmlLayoutServlet</servlet-class>
+
+		<!-- Must. Specifies URI of the update engine (DHtmlUpdateServlet).
+		It must be the same as <url-pattern> for the update engine.
+		-->
+		<init-param>
+			<param-name>update-uri</param-name>
+			<param-value>/zkau</param-value>
+		</init-param>
+		<load-on-startup>1</load-on-startup><!-- Must -->
+	</servlet>
+	<servlet-mapping>
+		<servlet-name>zkLoader</servlet-name>
+		<url-pattern>*.zul</url-pattern>
+	</servlet-mapping>
+	<servlet-mapping>
+		<servlet-name>zkLoader</servlet-name>
+		<url-pattern>*.zhtml</url-pattern>
+	</servlet-mapping>
+	<servlet>
+		<description>The asynchronous update engine for ZK</description>
+		<servlet-name>auEngine</servlet-name>
+		<servlet-class>org.zkoss.zk.au.http.DHtmlUpdateServlet</servlet-class>
+	</servlet>
+	<servlet-mapping>
+		<servlet-name>auEngine</servlet-name>
+		<url-pattern>/zkau/*</url-pattern>
+	</servlet-mapping>
+	<session-config>
+		<session-timeout>60</session-timeout>
+	</session-config>
+
+	<!-- [Optional] MIME mapping -->
+	<mime-mapping>
+		<extension>doc</extension>
+		<mime-type>application/vnd.ms-word</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>gif</extension>
+		<mime-type>image/gif</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>htm</extension>
+		<mime-type>text/html</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>html</extension>
+		<mime-type>text/html</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>jpeg</extension>
+		<mime-type>image/jpeg</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>jpg</extension>
+		<mime-type>image/jpeg</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>js</extension>
+		<mime-type>text/javascript</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>pdf</extension>
+		<mime-type>application/pdf</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>png</extension>
+		<mime-type>image/png</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>txt</extension>
+		<mime-type>text/plain</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>xls</extension>
+		<mime-type>application/vnd.ms-excel</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>xml</extension>
+		<mime-type>text/xml</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>zhtml</extension>
+		<mime-type>text/html</mime-type>
+	</mime-mapping>
+	<mime-mapping>
+		<extension>zul</extension>
+		<mime-type>text/html</mime-type>
+	</mime-mapping>
+
+	<welcome-file-list>
+		<welcome-file>index.zul</welcome-file>
+		<welcome-file>index.zhtml</welcome-file>
+		<welcome-file>index.html</welcome-file>
+		<welcome-file>index.htm</welcome-file>
+	</welcome-file-list>
+</web-app>

model-view-viewmodel/src/main/webapp/index.zul

@@ -0,0 +1,34 @@
+<zk>
+<window title="List of Books" border="normal" width="600px" apply="org.zkoss.bind.BindComposer" viewModel="@id('vm') @init('com.iluwatar.model.view.viewmodel.BookViewModel')">
+    <vbox hflex="true">
+        <listbox model="@bind(vm.bookList)" selectedItem="@bind(vm.selectedBook)" height="400px" mold="paging">
+            <listhead>
+                <listheader label="Book Name"/>
+                <listheader label="Author"/>               
+            </listhead>
+            <template name="model" var="book">
+                <listitem >
+                    <listcell label="@bind(book.name)"/>
+                    <listcell label="@bind(book.author)"/>
+                </listitem>
+            </template>
+        </listbox>
+    </vbox>
+    <toolbar>
+        <button label="Delete" onClick="@command('deleteBook')" disabled="@load(empty vm.selectedBook)" />
+    </toolbar>
+    <hbox style="margin-top:20px" visible="@bind(not empty vm.selectedBook)">
+		<vbox>
+			<hlayout>
+				Book Name : <label value="@bind(vm.selectedBook.name)" style="font-weight:bold"/>
+			</hlayout>
+			<hlayout>
+				Book Author : <label value="@bind(vm.selectedBook.author)" style="font-weight:bold"/>
+			</hlayout>
+			<hlayout>
+				Book Description : <label value="@bind(vm.selectedBook.description)" style="font-weight:bold"/>
+			</hlayout>
+		</vbox>
+	</hbox>
+</window>
+</zk>

model-view-viewmodel/src/test/java/com/iluwatar/model/view/viewmodel/BookTest.java

@@ -0,0 +1,108 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2021 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.model.view.viewmodel;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.CoreMatchers.not;
+import static org.junit.Assert.assertThat;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import java.util.List;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import com.google.common.testing.EqualsTester;
+
+class BookTest {
+
+  BookViewModel bvm;
+  Book testBook;
+  List<Book> testBookList;
+  Book testBookTwo;
+  Book testBookThree;
+  
+  @BeforeEach
+  void setUp() {
+    bvm = new BookViewModel();
+    testBook = new Book("Head First Design Patterns: A Brain-Friendly Guide",
+    		"Eric Freeman, Bert Bates, Kathy Sierra, Elisabeth Robson",
+    		"Head First Design Patterns Description");
+    testBookList = bvm.getBookList();
+    testBookTwo = new Book("Head First Design Patterns: A Brain-Friendly Guide",
+	  		"Eric Freeman, Bert Bates, Kathy Sierra, Elisabeth Robson",
+	  		"Head First Design Patterns Description");
+    testBookThree = new Book("Design Patterns: Elements of Reusable Object-Oriented Software",
+            "Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides",
+            "Design Patterns Description");
+  }
+
+  @Test
+  void testBookModel() {
+	assertNotNull(testBook);
+  }
+  
+  @Test
+  void testEquals() {
+    new EqualsTester().addEqualityGroup(testBook, testBookTwo).testEquals();
+  }
+
+  @Test
+  void testToString() {
+    assertThat(testBook.toString(), is(testBookTwo.toString()));
+    assertThat(testBook.toString(), is(not(testBookThree.toString())));
+  }
+  
+  @Test
+  void testHashCode() {
+    assertTrue(testBook.equals(testBookTwo) && testBookTwo.equals(testBook));
+    assertEquals(testBook.hashCode(), testBookTwo.hashCode());
+  }
+  
+  @Test
+  void testLoadData() {
+    assertNotNull(testBookList);
+    assertTrue(testBookList.get(0).toString().contains("Head First Design Patterns"));
+  }
+
+  @Test
+  void testSelectedData() {
+	bvm.setSelectedBook(testBook);
+    assertNotNull(bvm.getSelectedBook());
+    assertEquals(testBook.toString(), bvm.getSelectedBook().toString());
+    assertTrue(true, bvm.getSelectedBook().toString());
+  }
+
+  @Test
+  void testDeleteData() {
+    bvm.setSelectedBook(testBook);
+    assertNotNull(bvm.getSelectedBook());
+    assertTrue(testBookList.get(0).toString().contains("Head First Design Patterns"));
+    bvm.deleteBook();
+    assertNull(bvm.getSelectedBook());
+    assertFalse(testBookList.get(0).toString().contains("Head First Design Patterns"));
+  }
+
+}

pom.xml

@@ -75,6 +75,7 @@
         <directory-maven-plugin.version>0.3.1</directory-maven-plugin.version>
         <license-maven-plugin.version>3.0</license-maven-plugin.version>
         <urm-maven-plugin.version>1.4.8</urm-maven-plugin.version>
+	<commons-io.version>2.6</commons-io.version>
 
         <!-- SonarCloud -->
         <sonar.host.url>https://sonarcloud.io</sonar.host.url>
@@ -222,6 +223,7 @@
         <module>special-case</module>
         <module>parameter-object</module>
         <module>active-object</module>
+        <module>model-view-viewmodel</module>
     </modules>
 
     <repositories>
@@ -369,9 +371,13 @@
                 <version>${system-lambda.version}</version>
                 <scope>test</scope>
             </dependency>
-        </dependencies>
+		    <dependency>
+			    <groupId>commons-io</groupId>
+			    <artifactId>commons-io</artifactId>
+			    <version>${commons-io.version}</version>
+		        </dependency>
+            </dependencies>
     </dependencyManagement>
-
     <dependencies>
         <dependency>
             <groupId>org.slf4j</groupId>