fix checkstyle errors

Co-authored-by: Subhrodip Mohanta <hello@subho.xyz>

.all-contributorsrc

@@ -1758,6 +1758,33 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "yuhangbin",
+      "name": "CharlieYu",
+      "avatar_url": "https://avatars.githubusercontent.com/u/17566866?v=4",
+      "profile": "https://github.com/yuhangbin",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "Leisterbecker",
+      "name": "Leisterbecker",
+      "avatar_url": "https://avatars.githubusercontent.com/u/20650323?v=4",
+      "profile": "https://github.com/Leisterbecker",
+      "contributions": [
+        "code"
+      ]
+    },
+    {
+      "login": "castleKing1997",
+      "name": "DragonDreamer",
+      "avatar_url": "https://avatars.githubusercontent.com/u/35420129?v=4",
+      "profile": "http://rosaecrucis.cn",
+      "contributions": [
+        "code"
+      ]
     }
   ],
   "contributorsPerLine": 7,

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-193-orange.svg?style=flat-square)](#contributors-)
+[![All Contributors](https://img.shields.io/badge/all_contributors-196-orange.svg?style=flat-square)](#contributors-)
 <!-- ALL-CONTRIBUTORS-BADGE:END -->
 
 <br/>
@@ -323,6 +323,9 @@ This project is licensed under the terms of the MIT license.
     <td align="center"><a href="http://no website"><img src="https://avatars.githubusercontent.com/u/47126749?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Kevin</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AKevinyl3" title="Reviewed Pull Requests">👀</a></td>
     <td align="center"><a href="https://github.com/Shrirang97"><img src="https://avatars.githubusercontent.com/u/28738668?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Shrirang</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/pulls?q=is%3Apr+reviewed-by%3AShrirang97" title="Reviewed Pull Requests">👀</a> <a href="https://github.com/iluwatar/java-design-patterns/commits?author=Shrirang97" title="Code">💻</a></td>
     <td align="center"><a href="https://github.com/interactwithankush"><img src="https://avatars.githubusercontent.com/u/18613127?v=4?s=100" width="100px;" alt=""/><br /><sub><b>interactwithankush</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=interactwithankush" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/yuhangbin"><img src="https://avatars.githubusercontent.com/u/17566866?v=4?s=100" width="100px;" alt=""/><br /><sub><b>CharlieYu</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=yuhangbin" title="Code">💻</a></td>
+    <td align="center"><a href="https://github.com/Leisterbecker"><img src="https://avatars.githubusercontent.com/u/20650323?v=4?s=100" width="100px;" alt=""/><br /><sub><b>Leisterbecker</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=Leisterbecker" title="Code">💻</a></td>
+    <td align="center"><a href="http://rosaecrucis.cn"><img src="https://avatars.githubusercontent.com/u/35420129?v=4?s=100" width="100px;" alt=""/><br /><sub><b>DragonDreamer</b></sub></a><br /><a href="https://github.com/iluwatar/java-design-patterns/commits?author=castleKing1997" title="Code">💻</a></td>
   </tr>
 </table>
 

event-aggregator/README.md

@@ -9,6 +9,10 @@ tags:
  - Reactive
 ---
 
+## Name
+
+Event Aggregator
+
 ## Intent
 A system with lots of objects can lead to complexities when a
 client wants to subscribe to events. The client has to find and register for
@@ -17,6 +21,136 @@ requires a separate subscription. An Event Aggregator acts as a single source
 of events for many objects. It registers for all the events of the many objects
 allowing clients to register with just the aggregator.
 
+## Explanation
+
+Real-world example
+
+> King Joffrey sits on the iron throne and rules the seven kingdoms of Westeros. He receives most
+> of his critical information from King's Hand, the second in command. King's hand has many
+> close advisors himself, feeding him with relevant information about events occurring in the
+> kingdom.
+
+In Plain Words
+
+> Event Aggregator is an event mediator that collects events from multiple sources and delivers
+> them to registered observers.
+
+**Programmatic Example**
+
+In our programmatic example, we demonstrate the implementation of an event aggregator pattern. Some of
+the objects are event listeners, some are event emitters, and the event aggregator does both.
+
+```java
+public interface EventObserver {
+  void onEvent(Event e);
+}
+
+public abstract class EventEmitter {
+
+  private final Map<Event, List<EventObserver>> observerLists;
+
+  public EventEmitter() {
+    observerLists = new HashMap<>();
+  }
+
+  public final void registerObserver(EventObserver obs, Event e) {
+    ...
+  }
+
+  protected void notifyObservers(Event e) {
+    ...
+  }
+}
+```
+
+`KingJoffrey` is listening to events from `KingsHand`.
+
+```java
+@Slf4j
+public class KingJoffrey implements EventObserver {
+  @Override
+  public void onEvent(Event e) {
+    LOGGER.info("Received event from the King's Hand: {}", e.toString());
+  }
+}
+```
+
+`KingsHand` is listening to events from his subordinates `LordBaelish`, `LordVarys`, and `Scout`.
+Whatever he hears from them, he delivers to `KingJoffrey`.
+
+```java
+public class KingsHand extends EventEmitter implements EventObserver {
+
+  public KingsHand() {
+  }
+
+  public KingsHand(EventObserver obs, Event e) {
+    super(obs, e);
+  }
+
+  @Override
+  public void onEvent(Event e) {
+    notifyObservers(e);
+  }
+}
+```
+
+For example, `LordVarys` finds a traitor every Sunday and notifies the `KingsHand`.
+
+```java
+@Slf4j
+public class LordVarys extends EventEmitter implements EventObserver {
+  @Override
+  public void timePasses(Weekday day) {
+    if (day == Weekday.SATURDAY) {
+      notifyObservers(Event.TRAITOR_DETECTED);
+    }
+  }
+}
+```
+
+The following snippet demonstrates how the objects are constructed and wired together.
+
+```java
+    var kingJoffrey = new KingJoffrey();
+
+    var kingsHand = new KingsHand();
+    kingsHand.registerObserver(kingJoffrey, Event.TRAITOR_DETECTED);
+    kingsHand.registerObserver(kingJoffrey, Event.STARK_SIGHTED);
+    kingsHand.registerObserver(kingJoffrey, Event.WARSHIPS_APPROACHING);
+    kingsHand.registerObserver(kingJoffrey, Event.WHITE_WALKERS_SIGHTED);
+
+    var varys = new LordVarys();
+    varys.registerObserver(kingsHand, Event.TRAITOR_DETECTED);
+    varys.registerObserver(kingsHand, Event.WHITE_WALKERS_SIGHTED);
+
+    var scout = new Scout();
+    scout.registerObserver(kingsHand, Event.WARSHIPS_APPROACHING);
+    scout.registerObserver(varys, Event.WHITE_WALKERS_SIGHTED);
+
+    var baelish = new LordBaelish(kingsHand, Event.STARK_SIGHTED);
+
+    var emitters = List.of(
+        kingsHand,
+        baelish,
+        varys,
+        scout
+    );
+
+    Arrays.stream(Weekday.values())
+        .<Consumer<? super EventEmitter>>map(day -> emitter -> emitter.timePasses(day))
+        .forEachOrdered(emitters::forEach);
+```
+
+The console output after running the example.
+
+```
+18:21:52.955 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Warships approaching
+18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: White walkers sighted
+18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Stark sighted
+18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Traitor detected
+```
+
 ## Class diagram
 ![alt text](./etc/classes.png "Event Aggregator")
 
@@ -26,9 +160,13 @@ Use the Event Aggregator pattern when
 * Event Aggregator is a good choice when you have lots of objects that are
   potential event sources. Rather than have the observer deal with registering
   with them all, you can centralize the registration logic to the Event
-  Aggregator. As well as simplifying registration, a Event Aggregator also
+  Aggregator. As well as simplifying registration, an Event Aggregator also
   simplifies the memory management issues in using observers.
 
+## Related patterns
+
+* [Observer](https://java-design-patterns.com/patterns/observer/)
+
 ## Credits
 
 * [Martin Fowler - Event Aggregator](http://martinfowler.com/eaaDev/EventAggregator.html)

event-aggregator/src/main/java/com/iluwatar/event/aggregator/App.java

@@ -49,13 +49,28 @@ public class App {
   public static void main(String[] args) {
 
     var kingJoffrey = new KingJoffrey();
-    var kingsHand = new KingsHand(kingJoffrey);
+
+    var kingsHand = new KingsHand();
+    kingsHand.registerObserver(kingJoffrey, Event.TRAITOR_DETECTED);
+    kingsHand.registerObserver(kingJoffrey, Event.STARK_SIGHTED);
+    kingsHand.registerObserver(kingJoffrey, Event.WARSHIPS_APPROACHING);
+    kingsHand.registerObserver(kingJoffrey, Event.WHITE_WALKERS_SIGHTED);
+
+    var varys = new LordVarys();
+    varys.registerObserver(kingsHand, Event.TRAITOR_DETECTED);
+    varys.registerObserver(kingsHand, Event.WHITE_WALKERS_SIGHTED);
+
+    var scout = new Scout();
+    scout.registerObserver(kingsHand, Event.WARSHIPS_APPROACHING);
+    scout.registerObserver(varys, Event.WHITE_WALKERS_SIGHTED);
+
+    var baelish = new LordBaelish(kingsHand, Event.STARK_SIGHTED);
 
     var emitters = List.of(
         kingsHand,
-        new LordBaelish(kingsHand),
-        new LordVarys(kingsHand),
-        new Scout(kingsHand)
+        baelish,
+        varys,
+        scout
     );
 
     Arrays.stream(Weekday.values())

event-aggregator/src/main/java/com/iluwatar/event/aggregator/Event.java

@@ -31,6 +31,7 @@ import lombok.RequiredArgsConstructor;
 @RequiredArgsConstructor
 public enum Event {
 
+  WHITE_WALKERS_SIGHTED("White walkers sighted"),
   STARK_SIGHTED("Stark sighted"),
   WARSHIPS_APPROACHING("Warships approaching"),
   TRAITOR_DETECTED("Traitor detected");

event-aggregator/src/main/java/com/iluwatar/event/aggregator/EventEmitter.java

@@ -23,31 +23,48 @@
 
 package com.iluwatar.event.aggregator;
 
+import java.util.HashMap;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Map;
 
 /**
  * EventEmitter is the base class for event producers that can be observed.
  */
 public abstract class EventEmitter {
 
-  private final List<EventObserver> observers;
+  private final Map<Event, List<EventObserver>> observerLists;
 
   public EventEmitter() {
-    observers = new LinkedList<>();
+    observerLists = new HashMap<>();
   }
 
-  public EventEmitter(EventObserver obs) {
+  public EventEmitter(EventObserver obs, Event e) {
     this();
-    registerObserver(obs);
+    registerObserver(obs, e);
   }
 
-  public final void registerObserver(EventObserver obs) {
-    observers.add(obs);
+  /**
+  * Registers observer for specific event in the related list.
+  *
+  * @param obs the observer that observers this emitter
+  * @param e the specific event for that observation occurs
+  * */
+  public final void registerObserver(EventObserver obs, Event e) {
+    if (!observerLists.containsKey(e)) {
+      observerLists.put(e, new LinkedList<>());
+    }
+    if (!observerLists.get(e).contains(obs)) {
+      observerLists.get(e).add(obs);
+    }
   }
 
   protected void notifyObservers(Event e) {
-    observers.forEach(obs -> obs.onEvent(e));
+    if (observerLists.containsKey(e)) {
+      observerLists
+        .get(e)
+        .forEach(observer -> observer.onEvent(e));
+    }
   }
 
   public abstract void timePasses(Weekday day);

event-aggregator/src/main/java/com/iluwatar/event/aggregator/KingsHand.java

@@ -31,8 +31,8 @@ public class KingsHand extends EventEmitter implements EventObserver {
   public KingsHand() {
   }
 
-  public KingsHand(EventObserver obs) {
-    super(obs);
+  public KingsHand(EventObserver obs, Event e) {
+    super(obs, e);
   }
 
   @Override
@@ -42,6 +42,5 @@ public class KingsHand extends EventEmitter implements EventObserver {
 
   @Override
   public void timePasses(Weekday day) {
-    // NOP
   }
 }

event-aggregator/src/main/java/com/iluwatar/event/aggregator/LordBaelish.java

@@ -31,8 +31,8 @@ public class LordBaelish extends EventEmitter {
   public LordBaelish() {
   }
 
-  public LordBaelish(EventObserver obs) {
-    super(obs);
+  public LordBaelish(EventObserver obs, Event e) {
+    super(obs, e);
   }
 
   @Override

event-aggregator/src/main/java/com/iluwatar/event/aggregator/LordVarys.java

@@ -23,16 +23,19 @@
 
 package com.iluwatar.event.aggregator;
 
+import lombok.extern.slf4j.Slf4j;
+
 /**
  * LordVarys produces events.
  */
-public class LordVarys extends EventEmitter {
+@Slf4j
+public class LordVarys extends EventEmitter implements EventObserver {
 
   public LordVarys() {
   }
 
-  public LordVarys(EventObserver obs) {
-    super(obs);
+  public LordVarys(EventObserver obs, Event e) {
+    super(obs, e);
   }
 
   @Override
@@ -41,4 +44,10 @@ public class LordVarys extends EventEmitter {
       notifyObservers(Event.TRAITOR_DETECTED);
     }
   }
+
+
+  @Override
+  public void onEvent(Event e) {
+    notifyObservers(e);
+  }
 }

event-aggregator/src/main/java/com/iluwatar/event/aggregator/Scout.java

@@ -31,8 +31,8 @@ public class Scout extends EventEmitter {
   public Scout() {
   }
 
-  public Scout(EventObserver obs) {
-    super(obs);
+  public Scout(EventObserver obs, Event e) {
+    super(obs, e);
   }
 
   @Override
@@ -40,5 +40,8 @@ public class Scout extends EventEmitter {
     if (day == Weekday.TUESDAY) {
       notifyObservers(Event.WARSHIPS_APPROACHING);
     }
+    if (day == Weekday.WEDNESDAY) {
+      notifyObservers(Event.WHITE_WALKERS_SIGHTED);
+    }
   }
 }

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

@@ -31,6 +31,7 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
 import static org.mockito.Mockito.verifyZeroInteractions;
 
 import java.util.Objects;
+import java.util.function.BiFunction;
 import java.util.function.Function;
 import java.util.function.Supplier;
 import org.junit.jupiter.api.Test;
@@ -46,7 +47,7 @@ abstract class EventEmitterTest<E extends EventEmitter> {
   /**
    * Factory used to create a new instance of the test object with a default observer
    */
-  private final Function<EventObserver, E> factoryWithDefaultObserver;
+  private final BiFunction<EventObserver, Event, E> factoryWithDefaultObserver;
 
   /**
    * Factory used to create a new instance of the test object without passing a default observer
@@ -67,7 +68,7 @@ abstract class EventEmitterTest<E extends EventEmitter> {
    * Create a new event emitter test, using the given test object factories, special day and event
    */
   EventEmitterTest(final Weekday specialDay, final Event event,
-                   final Function<EventObserver, E> factoryWithDefaultObserver,
+                   final BiFunction<EventObserver, Event, E> factoryWithDefaultObserver,
                    final Supplier<E> factoryWithoutDefaultObserver) {
 
     this.specialDay = specialDay;
@@ -129,8 +130,8 @@ abstract class EventEmitterTest<E extends EventEmitter> {
     final var observer2 = mock(EventObserver.class);
 
     final var emitter = this.factoryWithoutDefaultObserver.get();
-    emitter.registerObserver(observer1);
-    emitter.registerObserver(observer2);
+    emitter.registerObserver(observer1, event);
+    emitter.registerObserver(observer2, event);
 
     testAllDays(specialDay, event, emitter, observer1, observer2);
   }
@@ -146,9 +147,9 @@ abstract class EventEmitterTest<E extends EventEmitter> {
     final var observer1 = mock(EventObserver.class);
     final var observer2 = mock(EventObserver.class);
 
-    final var emitter = this.factoryWithDefaultObserver.apply(defaultObserver);
-    emitter.registerObserver(observer1);
-    emitter.registerObserver(observer2);
+    final var emitter = this.factoryWithDefaultObserver.apply(defaultObserver, event);
+    emitter.registerObserver(observer1, event);
+    emitter.registerObserver(observer2, event);
 
     testAllDays(specialDay, event, emitter, defaultObserver, observer1, observer2);
   }

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

@@ -55,7 +55,11 @@ class KingsHandTest extends EventEmitterTest<KingsHand> {
   @Test
   void testPassThrough() throws Exception {
     final var observer = mock(EventObserver.class);
-    final var kingsHand = new KingsHand(observer);
+    final var kingsHand = new KingsHand();
+    kingsHand.registerObserver(observer, Event.STARK_SIGHTED);
+    kingsHand.registerObserver(observer, Event.WARSHIPS_APPROACHING);
+    kingsHand.registerObserver(observer, Event.TRAITOR_DETECTED);
+    kingsHand.registerObserver(observer, Event.WHITE_WALKERS_SIGHTED);
 
     // The kings hand should not pass any events before he received one
     verifyZeroInteractions(observer);

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

@@ -34,7 +34,9 @@ class ScoutTest extends EventEmitterTest<Scout> {
    * Create a new test instance, using the correct object factory
    */
   public ScoutTest() {
-    super(Weekday.TUESDAY, Event.WARSHIPS_APPROACHING, Scout::new, Scout::new);
+
+      super(Weekday.TUESDAY, Event.WARSHIPS_APPROACHING, Scout::new, Scout::new);
+
   }
 
 }

execute-around/README.md

@@ -17,10 +17,10 @@ the user to specify only what to do with the resource.
 
 ## Explanation
 
-Real world example
+Real-world example
 
-> We need to provide a class that can be used to write text strings to files. To make it easy for 
-> the user we let our service class open and close the file automatically, the user only has to 
+> A class needs to be provided for writing text strings to files. To make it easy for 
+> the user, the service class opens and closes the file automatically. The user only has to 
 > specify what is written into which file.       
 
 In plain words
@@ -35,35 +35,50 @@ In plain words
 
 **Programmatic Example**
 
-Let's introduce our file writer class.
+`SimpleFileWriter` class implements the Execute Around idiom. It takes `FileWriterAction` as a
+constructor argument allowing the user to specify what gets written into the file.
 
 ```java
 @FunctionalInterface
 public interface FileWriterAction {
-
   void writeFile(FileWriter writer) throws IOException;
-
 }
 
+@Slf4j
 public class SimpleFileWriter {
-
-  public SimpleFileWriter(String filename, FileWriterAction action) throws IOException {
-    try (var writer = new FileWriter(filename)) {
-      action.writeFile(writer);
+    public SimpleFileWriter(String filename, FileWriterAction action) throws IOException {
+        LOGGER.info("Opening the file");
+        try (var writer = new FileWriter(filename)) {
+            LOGGER.info("Executing the action");
+            action.writeFile(writer);
+            LOGGER.info("Closing the file");
+        }
     }
-  }
 }
 ```
 
-To utilize the file writer the following code is needed.
+The following code demonstrates how `SimpleFileWriter` is used. `Scanner` is used to print the file
+contents after the writing finishes.
 
 ```java
-    FileWriterAction writeHello = writer -> {
-      writer.write("Hello");
-      writer.append(" ");
-      writer.append("there!");
-    };
-    new SimpleFileWriter("testfile.txt", writeHello);
+FileWriterAction writeHello = writer -> {
+    writer.write("Gandalf was here");
+};
+new SimpleFileWriter("testfile.txt", writeHello);
+
+var scanner = new Scanner(new File("testfile.txt"));
+while (scanner.hasNextLine()) {
+LOGGER.info(scanner.nextLine());
+}
+```
+
+Here's the console output.
+
+```
+21:18:07.185 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Opening the file
+21:18:07.188 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Executing the action
+21:18:07.189 [main] INFO com.iluwatar.execute.around.SimpleFileWriter - Closing the file
+21:18:07.199 [main] INFO com.iluwatar.execute.around.App - Gandalf was here
 ```
 
 ## Class diagram
@@ -74,8 +89,7 @@ To utilize the file writer the following code is needed.
 
 Use the Execute Around idiom when
 
-* You use an API that requires methods to be called in pairs such as open/close or 
-allocate/deallocate.
+* An API requires methods to be called in pairs such as open/close or allocate/deallocate.
 
 ## Credits
 

execute-around/src/main/java/com/iluwatar/execute/around/App.java

@@ -23,10 +23,14 @@
 
 package com.iluwatar.execute.around;
 
+import java.io.File;
 import java.io.IOException;
+import java.util.Scanner;
+
+import lombok.extern.slf4j.Slf4j;
 
 /**
- * The Execute Around idiom specifies some code to be executed before and after a method. Typically
+ * The Execute Around idiom specifies executable code before and after a method. Typically
  * the idiom is used when the API has methods to be executed in pairs, such as resource
  * allocation/deallocation or lock acquisition/release.
  *
@@ -34,6 +38,7 @@ import java.io.IOException;
  * the user. The user specifies only what to do with the file by providing the {@link
  * FileWriterAction} implementation.
  */
+@Slf4j
 public class App {
 
   /**
@@ -41,11 +46,16 @@ public class App {
    */
   public static void main(String[] args) throws IOException {
 
+    // create the file writer and execute the custom action
     FileWriterAction writeHello = writer -> {
-      writer.write("Hello");
-      writer.append(" ");
-      writer.append("there!");
+      writer.write("Gandalf was here");
     };
     new SimpleFileWriter("testfile.txt", writeHello);
+
+    // print the file contents
+    var scanner = new Scanner(new File("testfile.txt"));
+    while (scanner.hasNextLine()) {
+      LOGGER.info(scanner.nextLine());
+    }
   }
 }

execute-around/src/main/java/com/iluwatar/execute/around/SimpleFileWriter.java

@@ -26,18 +26,24 @@ package com.iluwatar.execute.around;
 import java.io.FileWriter;
 import java.io.IOException;
 
+import lombok.extern.slf4j.Slf4j;
+
 /**
  * SimpleFileWriter handles opening and closing file for the user. The user only has to specify what
  * to do with the file resource through {@link FileWriterAction} parameter.
  */
+@Slf4j
 public class SimpleFileWriter {
 
   /**
    * Constructor.
    */
   public SimpleFileWriter(String filename, FileWriterAction action) throws IOException {
+    LOGGER.info("Opening the file");
     try (var writer = new FileWriter(filename)) {
+      LOGGER.info("Executing the action");
       action.writeFile(writer);
+      LOGGER.info("Closing the file");
     }
   }
 }

factory-kit/README.md

@@ -10,19 +10,115 @@ tags:
 ---
 
 ## Intent
+
 Define a factory of immutable content with separated builder and factory interfaces.
 
+## Explanation
+
+Real-world example
+
+> Imagine a magical weapon factory that can create any type of weapon wished for. When the factory
+> is unboxed, the master recites the weapon types needed to prepare it. After that, any of those
+> weapon types can be summoned in an instant. 
+
+In plain words
+
+> Factory kit is a configurable object builder.
+
+**Programmatic Example**
+
+Let's first define the simple `Weapon` hierarchy.
+
+```java
+public interface Weapon {
+}
+
+public enum WeaponType {
+    SWORD,
+    AXE,
+    BOW,
+    SPEAR
+}
+
+public class Sword implements Weapon {
+    @Override
+    public String toString() {
+        return "Sword";
+    }
+}
+
+// Axe, Bow, and Spear are defined similarly
+```
+
+Next, we define a functional interface that allows adding a builder with a name to the factory.
+
+```java
+public interface Builder {
+  void add(WeaponType name, Supplier<Weapon> supplier);
+}
+```
+
+The meat of the example is the `WeaponFactory` interface that effectively implements the factory
+kit pattern. The method `#factory` is used to configure the factory with the classes it needs to
+be able to construct. The method `#create` is then used to create object instances.
+
+```java
+public interface WeaponFactory {
+
+  static WeaponFactory factory(Consumer<Builder> consumer) {
+      var map = new HashMap<WeaponType, Supplier<Weapon>>();
+      consumer.accept(map::put);
+      return name -> map.get(name).get();
+  }
+    
+  Weapon create(WeaponType name);
+}
+```
+
+Now, we can show how `WeaponFactory` can be used.
+
+```java
+var factory = WeaponFactory.factory(builder -> {
+  builder.add(WeaponType.SWORD, Sword::new);
+  builder.add(WeaponType.AXE, Axe::new);
+  builder.add(WeaponType.SPEAR, Spear::new);
+  builder.add(WeaponType.BOW, Bow::new);
+});
+var list = new ArrayList<Weapon>();
+list.add(factory.create(WeaponType.AXE));
+list.add(factory.create(WeaponType.SPEAR));
+list.add(factory.create(WeaponType.SWORD));
+list.add(factory.create(WeaponType.BOW));
+list.stream().forEach(weapon -> LOGGER.info("{}", weapon.toString()));
+```
+
+Here is the console output when the example is run.
+
+```
+21:15:49.709 [main] INFO com.iluwatar.factorykit.App - Axe
+21:15:49.713 [main] INFO com.iluwatar.factorykit.App - Spear
+21:15:49.713 [main] INFO com.iluwatar.factorykit.App - Sword
+21:15:49.713 [main] INFO com.iluwatar.factorykit.App - Bow
+```
+
 ## Class diagram
+
 ![alt text](./etc/factory-kit.png "Factory Kit")
 
 ## Applicability
+
 Use the Factory Kit pattern when
 
-* a class can't anticipate the class of objects it must create
-* you just want a new instance of a custom builder instead of the global one
-* you explicitly want to define types of objects, that factory can build
-* you want a separated builder and creator interface
+* The factory class can't anticipate the types of objects it must create
+* A new instance of a custom builder is needed instead of a global one
+* The types of objects that the factory can build need to be defined outside the class
+* The builder and creator interfaces need to be separated
+
+## Related patterns
+
+* [Builder](https://java-design-patterns.com/patterns/builder/)
+* [Factory](https://java-design-patterns.com/patterns/factory/)
 
 ## Credits
 
-* [Design Pattern Reloaded by Remi Forax: ](https://www.youtube.com/watch?v=-k2X7guaArU)
+* [Design Pattern Reloaded by Remi Forax](https://www.youtube.com/watch?v=-k2X7guaArU)

factory-kit/src/main/java/com/iluwatar/factorykit/App.java

@@ -23,14 +23,16 @@
 
 package com.iluwatar.factorykit;
 
+import java.util.ArrayList;
+
 import lombok.extern.slf4j.Slf4j;
 
 /**
- * Factory-kit is a creational pattern which defines a factory of immutable content with separated
+ * Factory kit is a creational pattern that defines a factory of immutable content with separated
  * builder and factory interfaces to deal with the problem of creating one of the objects specified
- * directly in the factory-kit instance.
+ * directly in the factory kit instance.
  *
- * <p>In the given example {@link WeaponFactory} represents the factory-kit, that contains four
+ * <p>In the given example {@link WeaponFactory} represents the factory kit, that contains four
  * {@link Builder}s for creating new objects of the classes implementing {@link Weapon} interface.
  *
  * <p>Each of them can be called with {@link WeaponFactory#create(WeaponType)} method, with
@@ -52,7 +54,11 @@ public class App {
       builder.add(WeaponType.SPEAR, Spear::new);
       builder.add(WeaponType.BOW, Bow::new);
     });
-    var axe = factory.create(WeaponType.AXE);
-    LOGGER.info(axe.toString());
+    var list = new ArrayList<Weapon>();
+    list.add(factory.create(WeaponType.AXE));
+    list.add(factory.create(WeaponType.SPEAR));
+    list.add(factory.create(WeaponType.SWORD));
+    list.add(factory.create(WeaponType.BOW));
+    list.stream().forEach(weapon -> LOGGER.info("{}", weapon.toString()));
   }
 }

metadata-mapping/README.md

@@ -0,0 +1,182 @@
+---
+layout: pattern
+title: Metadata Mapping
+folder: metadata-mapping
+permalink: /patterns/metadata-mapping/
+categories: Architectural
+language: en
+tags:
+ - Data access
+---
+
+## Intent
+
+Holds details of object-relational mapping in the metadata.
+
+## Explanation
+
+Real world example
+
+> Hibernate ORM Tool uses Metadata Mapping Pattern to specify the mapping between classes and tables either using XML or annotations in code. 
+
+In plain words
+
+> Metadata Mapping specifies the mapping between classes and tables so that we could treat a table of any database like a Java class.
+
+Wikipedia says
+
+> Create a "virtual [object database](https://en.wikipedia.org/wiki/Object_database)" that can be used from within the programming language.
+
+**Programmatic Example**
+
+We give an example about visiting the information of `USER` table in `h2` database. Firstly, we create `USER` table with `h2`:
+
+```java
+@Slf4j
+public class DatabaseUtil {
+  private static final String DB_URL = "jdbc:h2:mem:metamapping";
+  private static final String CREATE_SCHEMA_SQL = "DROP TABLE IF EXISTS `user`;"
+      + "CREATE TABLE `user` (\n"
+      + "  `id` int(11) NOT NULL AUTO_INCREMENT,\n"
+      + "  `username` varchar(255) NOT NULL,\n"
+      + "  `password` varchar(255) NOT NULL,\n"
+      + "  PRIMARY KEY (`id`)\n"
+      + ");";
+    
+  /**
+   * Create database.
+   */
+  static {
+    LOGGER.info("create h2 database");
+    var source = new JdbcDataSource();
+    source.setURL(DB_URL);
+    try (var statement = source.getConnection().createStatement()) {
+      statement.execute(CREATE_SCHEMA_SQL);
+    } catch (SQLException e) {
+      LOGGER.error("unable to create h2 data source", e);
+    }
+  }
+}
+```
+
+Correspondingly, here's the basic `User` entity.
+
+```java
+@Setter
+@Getter
+@ToString
+public class User {
+  private Integer id;
+  private String username;
+  private String password;
+    
+  /**
+   * Get a user.
+   * @param username user name
+   * @param password user password
+   */
+  public User(String username, String password) {
+    this.username = username;
+    this.password = password;
+  }
+}
+```
+
+Then we write a `xml` file to show the mapping between the table and the object:
+
+```xml
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE hibernate-mapping PUBLIC
+    "-//Hibernate/Hibernate Mapping DTD//EN"
+    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
+
+<hibernate-mapping>
+  <class name="com.iluwatar.metamapping.model.User" table="user">
+    <id name="id" type="java.lang.Integer" column="id">
+      <generator class="native"/>
+    </id>
+    <property name="username" column="username" type="java.lang.String"/>
+    <property name="password" column="password" type="java.lang.String"/>
+  </class>
+</hibernate-mapping>
+```
+
+We use `Hibernate` to resolve the mapping and connect to our database, here's its configuration:
+
+```xml
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE hibernate-configuration PUBLIC
+    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
+    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
+<hibernate-configuration>
+  <session-factory>
+    <!-- JDBC Database connection settings -->
+    <property name="connection.url">jdbc:h2:mem:metamapping</property>
+    <property name="connection.driver_class">org.h2.Driver</property>
+    <!-- JDBC connection pool settings ... using built-in test pool -->
+    <property name="connection.pool_size">1</property>
+    <!-- Select our SQL dialect -->
+    <property name="dialect">org.hibernate.dialect.H2Dialect</property>
+    <!-- Echo the SQL to stdout -->
+    <property name="show_sql">false</property>
+    <!-- Drop and re-create the database schema on startup -->
+    <property name="hbm2ddl.auto">create-drop</property>
+    <mapping resource="com/iluwatar/metamapping/model/User.hbm.xml" />
+  </session-factory>
+</hibernate-configuration>
+```
+
+Then we can get access to the table just like an object with `Hibernate`, here's some CRUDs:
+
+```java
+@Slf4j
+public class UserService {
+  private static final SessionFactory factory = HibernateUtil.getSessionFactory();
+
+  /**
+   * List all users.
+   * @return list of users
+   */
+  public List<User> listUser() {
+    LOGGER.info("list all users.");
+    List<User> users = new ArrayList<>();
+    try (var session = factory.openSession()) {
+      var tx = session.beginTransaction();
+      List<User> userIter = session.createQuery("FROM User").list();
+      for (var iterator = userIter.iterator(); iterator.hasNext();) {
+        users.add(iterator.next());
+      }
+      tx.commit();
+    } catch (HibernateException e) {
+      LOGGER.debug("fail to get users", e);
+    }
+    return users;
+  }
+  
+  // other CRUDs ->
+  ...
+    
+  public void close() {
+    HibernateUtil.shutdown();
+  }
+}
+```
+
+## Class diagram
+
+![metamapping](etc/metamapping.png)
+
+## Applicability
+
+Use the Metadata Mapping when:
+
+- you want reduce the amount of work needed to handle database mapping.
+
+## Known uses
+
+[Hibernate](https://hibernate.org/), [EclipseLink](https://www.eclipse.org/eclipselink/), [MyBatis](https://blog.mybatis.org/)......
+
+## 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)
+

metadata-mapping/etc/metamapping.puml

@@ -0,0 +1,32 @@
+@startuml
+interface com.iluwatar.metamapping.service.UserService {
++ List<User> listUser()
++ int createUser(User)
++ void updateUser(Integer,User)
++ void deleteUser(Integer)
++ User getUser(Integer)
++ void close()
+}
+class com.iluwatar.metamapping.utils.DatabaseUtil {
++ {static} void createDataSource()
+}
+class com.iluwatar.metamapping.model.User {
+- Integer id
+- String username
+- String password
++ User(String username, String password)
+}
+class com.iluwatar.metamapping.utils.HibernateUtil {
++ {static} SessionFactory getSessionFactory()
++ {static} void shutdown()
+}
+class com.iluwatar.metamapping.App {
++ {static} void main(String[])
++ {static} List<User> generateSampleUsers()
+}
+
+com.iluwatar.metamapping.service.UserService <.. com.iluwatar.metamapping.App
+com.iluwatar.metamapping.model.User <.. com.iluwatar.metamapping.service.UserService
+com.iluwatar.metamapping.utils.HibernateUtil <.. com.iluwatar.metamapping.service.UserService
+com.iluwatar.metamapping.utils.DatabaseUtil <-- com.iluwatar.metamapping.utils.HibernateUtil
+@enduml

metadata-mapping/pom.xml

@@ -0,0 +1,87 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+  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.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <artifactId>java-design-patterns</artifactId>
+    <groupId>com.iluwatar</groupId>
+    <version>1.26.0-SNAPSHOT</version>
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+
+  <artifactId>metadata-mapping</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter-engine</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>com.h2database</groupId>
+      <artifactId>h2</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.hibernate</groupId>
+      <artifactId>hibernate-core</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>com.h2database</groupId>
+      <artifactId>h2</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>javax.xml.bind</groupId>
+      <artifactId>jaxb-api</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>com.sun.xml.bind</groupId>
+      <artifactId>jaxb-impl</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>com.sun.istack</groupId>
+      <artifactId>istack-commons-runtime</artifactId>
+    </dependency>
+  </dependencies>
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-assembly-plugin</artifactId>
+        <executions>
+          <execution>
+            <configuration>
+              <archive>
+                <manifest>
+                  <mainClass>com.iluwatar.metamapping.App</mainClass>
+                </manifest>
+              </archive>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>

metadata-mapping/src/main/java/com/iluwatar/metamapping/App.java

@@ -0,0 +1,72 @@
+package com.iluwatar.metamapping;
+
+import com.iluwatar.metamapping.model.User;
+import com.iluwatar.metamapping.service.UserService;
+import com.iluwatar.metamapping.utils.DatabaseUtil;
+import java.util.List;
+import lombok.extern.slf4j.Slf4j;
+import org.hibernate.service.ServiceRegistry;
+
+/**
+ * Metadata Mapping specifies the mapping
+ * between classes and tables so that
+ * we could treat a table of any database like a Java class.
+ *
+ * <p>With hibernate, we achieve list/create/update/delete/get operations:
+ * 1)Create the H2 Database in {@link DatabaseUtil}.
+ * 2)Hibernate resolve hibernate.cfg.xml and generate service like save/list/get/delete.
+ *    For learning metadata mapping pattern, we go deeper into Hibernate here:
+ * a)read properties from hibernate.cfg.xml and mapping from *.hbm.xml
+ * b)create session factory to generate session interacting with database
+ * c)generate session with factory pattern
+ * d)create query object or use basic api with session,
+ *      hibernate will convert all query to database query according to metadata
+ * 3)We encapsulate hibernate service in {@link UserService} for our use.
+ * @see org.hibernate.cfg.Configuration#configure(String)
+ * @see org.hibernate.cfg.Configuration#buildSessionFactory(ServiceRegistry)
+ * @see org.hibernate.internal.SessionFactoryImpl#openSession()
+ */
+@Slf4j
+public class App {
+  /**
+   * Program entry point.
+   *
+   * @param args command line args.
+   * @throws Exception if any error occurs.
+   */
+  public static void main(String[] args)  throws Exception {
+    // get service
+    var userService = new UserService();
+    // use create service to add users
+    for (var user: generateSampleUsers()) {
+      var id = userService.createUser(user);
+      LOGGER.info("Add user" + user + "at" + id + ".");
+    }
+    // use list service to get users
+    var users = userService.listUser();
+    LOGGER.info(String.valueOf(users));
+    // use get service to get a user
+    var user = userService.getUser(1);
+    LOGGER.info(String.valueOf(user));
+    // change password of user 1
+    user.setPassword("new123");
+    // use update service to update user 1
+    userService.updateUser(1, user);
+    // use delete service to delete user 2
+    userService.deleteUser(2);
+    // close service
+    userService.close();
+  }
+
+  /**
+   * Generate users.
+   *
+   * @return list of users.
+   */
+  public static List<User> generateSampleUsers() {
+    final var user1 = new User("ZhangSan", "zhs123");
+    final var user2 = new User("LiSi", "ls123");
+    final var user3 = new User("WangWu", "ww123");
+    return List.of(user1, user2, user3);
+  }
+}

metadata-mapping/src/main/java/com/iluwatar/metamapping/model/User.java

@@ -0,0 +1,29 @@
+package com.iluwatar.metamapping.model;
+
+import lombok.Getter;
+import lombok.Setter;
+import lombok.ToString;
+
+/**
+ * User Entity.
+ */
+@Setter
+@Getter
+@ToString
+public class User {
+  private Integer id;
+  private String username;
+  private String password;
+
+  public User() {}
+
+  /**
+   * Get a user.
+   * @param username user name
+   * @param password user password
+   */
+  public User(String username, String password) {
+    this.username = username;
+    this.password = password;
+  }
+}

metadata-mapping/src/main/java/com/iluwatar/metamapping/service/UserService.java

@@ -0,0 +1,114 @@
+package com.iluwatar.metamapping.service;
+
+import com.iluwatar.metamapping.model.User;
+import com.iluwatar.metamapping.utils.HibernateUtil;
+import java.util.ArrayList;
+import java.util.List;
+import lombok.extern.slf4j.Slf4j;
+import org.hibernate.HibernateException;
+import org.hibernate.SessionFactory;
+
+/**
+ * Service layer for user.
+ */
+@Slf4j
+public class UserService {
+  private static final SessionFactory factory = HibernateUtil.getSessionFactory();
+
+  /**
+   * List all users.
+   * @return list of users
+   */
+  public List<User> listUser() {
+    LOGGER.info("list all users.");
+    List<User> users = new ArrayList<>();
+    try (var session = factory.openSession()) {
+      var tx = session.beginTransaction();
+      List<User> userIter = session.createQuery("FROM User").list();
+      for (var iterator = userIter.iterator(); iterator.hasNext();) {
+        users.add(iterator.next());
+      }
+      tx.commit();
+    } catch (HibernateException e) {
+      LOGGER.debug("fail to get users", e);
+    }
+    return users;
+  }
+
+  /**
+   * Add a user.
+   * @param user user entity
+   * @return user id
+   */
+  public int createUser(User user) {
+    LOGGER.info("create user: " + user.getUsername());
+    var id = -1;
+    try (var session = factory.openSession()) {
+      var tx = session.beginTransaction();
+      id = (Integer) session.save(user);
+      tx.commit();
+    } catch (HibernateException e) {
+      LOGGER.debug("fail to create user", e);
+    }
+    LOGGER.info("create user " + user.getUsername() + " at " + id);
+    return id;
+  }
+
+  /**
+   * Update user.
+   * @param id user id
+   * @param user new user entity
+   */
+  public void updateUser(Integer id, User user) {
+    LOGGER.info("update user at " + id);
+    try (var session = factory.openSession()) {
+      var tx = session.beginTransaction();
+      user.setId(id);
+      session.update(user);
+      tx.commit();
+    } catch (HibernateException e) {
+      LOGGER.debug("fail to update user", e);
+    }
+  }
+
+  /**
+   * Delete user.
+   * @param id user id
+   */
+  public void deleteUser(Integer id) {
+    LOGGER.info("delete user at: " + id);
+    try (var session = factory.openSession()) {
+      var tx = session.beginTransaction();
+      var user = session.get(User.class, id);
+      session.delete(user);
+      tx.commit();
+    } catch (HibernateException e) {
+      LOGGER.debug("fail to delete user", e);
+    }
+  }
+
+  /**
+   * Get user.
+   * @param id user id
+   * @return deleted user
+   */
+  public User getUser(Integer id) {
+    LOGGER.info("get user at: " + id);
+    User user = null;
+    try (var session = factory.openSession()) {
+      var tx = session.beginTransaction();
+      user = session.get(User.class, id);
+      tx.commit();
+    } catch (HibernateException e) {
+      LOGGER.debug("fail to get user", e);
+    }
+    return user;
+  }
+
+  /**
+   * Close hibernate.
+   */
+  public void close() {
+    HibernateUtil.shutdown();
+  }
+}

metadata-mapping/src/main/java/com/iluwatar/metamapping/utils/DatabaseUtil.java

@@ -0,0 +1,39 @@
+package com.iluwatar.metamapping.utils;
+
+import java.sql.SQLException;
+import lombok.extern.slf4j.Slf4j;
+import org.h2.jdbcx.JdbcDataSource;
+
+/**
+ * Create h2 database.
+ */
+@Slf4j
+public class DatabaseUtil {
+  private static final String DB_URL = "jdbc:h2:mem:metamapping";
+  private static final String CREATE_SCHEMA_SQL = "DROP TABLE IF EXISTS `user`;"
+      + "CREATE TABLE `user` (\n"
+      + "  `id` int(11) NOT NULL AUTO_INCREMENT,\n"
+      + "  `username` varchar(255) NOT NULL,\n"
+      + "  `password` varchar(255) NOT NULL,\n"
+      + "  PRIMARY KEY (`id`)\n"
+      + ");";
+
+  /**
+   * Hide constructor.
+   */
+  private DatabaseUtil() {}
+
+  /**
+   * Create database.
+   */
+  static {
+    LOGGER.info("create h2 database");
+    var source = new JdbcDataSource();
+    source.setURL(DB_URL);
+    try (var statement = source.getConnection().createStatement()) {
+      statement.execute(CREATE_SCHEMA_SQL);
+    } catch (SQLException e) {
+      LOGGER.error("unable to create h2 data source", e);
+    }
+  }
+}

metadata-mapping/src/main/java/com/iluwatar/metamapping/utils/HibernateUtil.java

@@ -0,0 +1,45 @@
+package com.iluwatar.metamapping.utils;
+
+import lombok.extern.slf4j.Slf4j;
+import org.hibernate.SessionFactory;
+import org.hibernate.cfg.Configuration;
+
+/**
+ * Manage hibernate.
+ */
+@Slf4j
+public class HibernateUtil {
+
+  private static final SessionFactory sessionFactory = buildSessionFactory();
+
+  /**
+   * Hide constructor.
+   */
+  private HibernateUtil() {}
+
+  /**
+   * Build session factory.
+   * @return session factory
+   */
+  private static SessionFactory buildSessionFactory() {
+    // Create the SessionFactory from hibernate.cfg.xml
+    return new Configuration().configure().buildSessionFactory();
+  }
+
+  /**
+   * Get session factory.
+   * @return session factory
+   */
+  public static SessionFactory getSessionFactory() {
+    return sessionFactory;
+  }
+
+  /**
+   * Close session factory.
+   */
+  public static void shutdown() {
+    // Close caches and connection pools
+    getSessionFactory().close();
+  }
+
+}

metadata-mapping/src/main/resources/com/iluwatar/metamapping/model/User.hbm.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE hibernate-mapping PUBLIC
+    "-//Hibernate/Hibernate Mapping DTD//EN"
+    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
+
+<hibernate-mapping>
+  <class name="com.iluwatar.metamapping.model.User" table="user">
+    <id name="id" type="java.lang.Integer" column="id">
+      <generator class="native"/>
+    </id>
+    <property name="username" column="username" type="java.lang.String"/>
+    <property name="password" column="password" type="java.lang.String"/>
+  </class>
+</hibernate-mapping>

metadata-mapping/src/main/resources/hibernate.cfg.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE hibernate-configuration PUBLIC
+    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
+    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
+<hibernate-configuration>
+  <session-factory>
+    <!-- JDBC Database connection settings -->
+    <property name="connection.url">jdbc:h2:mem:metamapping</property>
+    <property name="connection.driver_class">org.h2.Driver</property>
+    <!-- JDBC connection pool settings ... using built-in test pool -->
+    <property name="connection.pool_size">1</property>
+    <!-- Select our SQL dialect -->
+    <property name="dialect">org.hibernate.dialect.H2Dialect</property>
+    <!-- Echo the SQL to stdout -->
+    <property name="show_sql">false</property>
+    <!-- Drop and re-create the database schema on startup -->
+    <property name="hbm2ddl.auto">create-drop</property>
+    <mapping resource="com/iluwatar/metamapping/model/User.hbm.xml" />
+  </session-factory>
+</hibernate-configuration>

metadata-mapping/src/test/java/com/iluwatar/metamapping/AppTest.java

@@ -0,0 +1,20 @@
+package com.iluwatar.metamapping;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+/**
+ * Tests that metadata mapping example runs without errors.
+ */
+class AppTest {
+  /**
+   * Issue: Add at least one assertion to this test case.
+   *
+   * Solution: Inserted assertion to check whether the execution of the main method in {@link App#main(String[])}
+   * throws an exception.
+   */
+  @Test
+  void shouldExecuteMetaMappingWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+  }
+}

pom.xml

@@ -75,6 +75,7 @@
     <license-maven-plugin.version>3.0</license-maven-plugin.version>
     <urm-maven-plugin.version>1.4.8</urm-maven-plugin.version>
     <commons-io.version>2.7</commons-io.version>
+    <istack-commons-runtime.version>4.0.1</istack-commons-runtime.version>
     <!-- SonarCloud -->
     <sonar.host.url>https://sonarcloud.io</sonar.host.url>
     <sonar.organization>iluwatar</sonar.organization>
@@ -227,6 +228,7 @@
     <module>lockable-object</module>
     <module>fanout-fanin</module>
     <module>domain-model</module>
+    <module>metadata-mapping</module>
   </modules>
   <repositories>
     <repository>
@@ -377,6 +379,11 @@
         <artifactId>commons-io</artifactId>
         <version>${commons-io.version}</version>
       </dependency>
+      <dependency>
+        <groupId>com.sun.istack</groupId>
+        <artifactId>istack-commons-runtime</artifactId>
+        <version>${istack-commons-runtime.version}</version>
+      </dependency>
     </dependencies>
   </dependencyManagement>
   <dependencies>

thread-pool/README.md

@@ -18,11 +18,11 @@ threads and eliminating the latency of creating new threads.
 
 ## Explanation
 
-Real world example
+Real-world example
 
 > We have a large number of relatively short tasks at hand. We need to peel huge amounts of potatoes 
-> and serve mighty amount of coffee cups. Creating a new thread for each task would be a waste so we 
-> establish a thread pool.       
+> and serve a mighty amount of coffee cups. Creating a new thread for each task would be a waste so 
+> we establish a thread pool.       
 
 In plain words
 
@@ -99,7 +99,7 @@ public class PotatoPeelingTask extends Task {
 }
 ```
 
-Next we present a runnable `Worker` class that the thread pool will utilize to handle all the potato 
+Next, we present a runnable `Worker` class that the thread pool will utilize to handle all the potato 
 peeling and coffee making.
 
 ```java

throttling/README.md

@@ -16,10 +16,12 @@ Ensure that a given client is not able to access service resources more than the
 
 ## Explanation
 
-Real world example
+Real-world example
 
-> A large multinational corporation offers API to its customers. The API is rate-limited and each 
-> customer can only make certain amount of calls per second.      
+> A young human and an old dwarf walk into a bar. They start ordering beers from the bartender.
+> The bartender immediately sees that the young human shouldn't consume too many drinks too fast
+> and refuses to serve if enough time has not passed. For the old dwarf, the serving rate can
+> be higher.
 
 In plain words
 
@@ -33,30 +35,25 @@ In plain words
 
 **Programmatic Example**
 
-Tenant class presents the clients of the API. CallsCount tracks the number of API calls per tenant.
+`BarCustomer` class presents the clients of the `Bartender` API. `CallsCount` tracks the number of 
+calls per `BarCustomer`.
 
 ```java
-public class Tenant {
+public class BarCustomer {
 
-  private final String name;
-  private final int allowedCallsPerSecond;
+    @Getter
+    private final String name;
+    @Getter
+    private final int allowedCallsPerSecond;
 
-  public Tenant(String name, int allowedCallsPerSecond, CallsCount callsCount) {
-    if (allowedCallsPerSecond < 0) {
-      throw new InvalidParameterException("Number of calls less than 0 not allowed");
+    public BarCustomer(String name, int allowedCallsPerSecond, CallsCount callsCount) {
+        if (allowedCallsPerSecond < 0) {
+            throw new InvalidParameterException("Number of calls less than 0 not allowed");
+        }
+        this.name = name;
+        this.allowedCallsPerSecond = allowedCallsPerSecond;
+        callsCount.addTenant(name);
     }
-    this.name = name;
-    this.allowedCallsPerSecond = allowedCallsPerSecond;
-    callsCount.addTenant(name);
-  }
-
-  public String getName() {
-    return name;
-  }
-
-  public int getAllowedCallsPerSecond() {
-    return allowedCallsPerSecond;
-  }
 }
 
 @Slf4j
@@ -76,14 +73,14 @@ public final class CallsCount {
   }
 
   public void reset() {
-    LOGGER.debug("Resetting the map.");
     tenantCallsCount.replaceAll((k, v) -> new AtomicLong(0));
+    LOGGER.info("reset counters");
   }
 }
 ```
 
-Next we introduce the service that the tenants are calling. To track the call count we use the 
-throttler timer.
+Next, the service that the tenants are calling is introduced. To track the call count, a throttler 
+timer is used.
 
 ```java
 public interface Throttler {
@@ -111,71 +108,103 @@ public class ThrottleTimerImpl implements Throttler {
     }, 0, throttlePeriod);
   }
 }
+```
 
-class B2BService {
+`Bartender` offers the `orderDrink` service to the `BarCustomer`s. The customers probably don't
+know that the beer serving rate is limited by their appearances.
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(B2BService.class);
-  private final CallsCount callsCount;
+```java
+class Bartender {
 
-  public B2BService(Throttler timer, CallsCount callsCount) {
-    this.callsCount = callsCount;
-    timer.start();
-  }
+    private static final Logger LOGGER = LoggerFactory.getLogger(Bartender.class);
+    private final CallsCount callsCount;
 
-  public int dummyCustomerApi(Tenant tenant) {
-    var tenantName = tenant.getName();
-    var count = callsCount.getCount(tenantName);
-    LOGGER.debug("Counter for {} : {} ", tenant.getName(), count);
-    if (count >= tenant.getAllowedCallsPerSecond()) {
-      LOGGER.error("API access per second limit reached for: {}", tenantName);
-      return -1;
+    public Bartender(Throttler timer, CallsCount callsCount) {
+        this.callsCount = callsCount;
+        timer.start();
     }
-    callsCount.incrementCount(tenantName);
-    return getRandomCustomerId();
-  }
 
-  private int getRandomCustomerId() {
-    return ThreadLocalRandom.current().nextInt(1, 10000);
-  }
+    public int orderDrink(BarCustomer barCustomer) {
+        var tenantName = barCustomer.getName();
+        var count = callsCount.getCount(tenantName);
+        if (count >= barCustomer.getAllowedCallsPerSecond()) {
+            LOGGER.error("I'm sorry {}, you've had enough for today!", tenantName);
+            return -1;
+        }
+        callsCount.incrementCount(tenantName);
+        LOGGER.debug("Serving beer to {} : [{} consumed] ", barCustomer.getName(), count+1);
+        return getRandomCustomerId();
+    }
+
+    private int getRandomCustomerId() {
+        return ThreadLocalRandom.current().nextInt(1, 10000);
+    }
 }
 ```
 
-Now we are ready to see the full example in action. Tenant Adidas is rate-limited to 5 calls per 
-second and Nike to 6.
+Now it is possible to see the full example in action. `BarCustomer` young human is rate-limited to 2 
+calls per second and the old dwarf to 4.
 
 ```java
-  public static void main(String[] args) {
+public static void main(String[] args) {
     var callsCount = new CallsCount();
-    var adidas = new Tenant("Adidas", 5, callsCount);
-    var nike = new Tenant("Nike", 6, callsCount);
+    var human = new BarCustomer("young human", 2, callsCount);
+    var dwarf = new BarCustomer("dwarf soldier", 4, callsCount);
 
     var executorService = Executors.newFixedThreadPool(2);
-    executorService.execute(() -> makeServiceCalls(adidas, callsCount));
-    executorService.execute(() -> makeServiceCalls(nike, callsCount));
-    executorService.shutdown();
-    
-    try {
-      executorService.awaitTermination(10, TimeUnit.SECONDS);
-    } catch (InterruptedException e) {
-      LOGGER.error("Executor Service terminated: {}", e.getMessage());
-    }
-  }
 
-  private static void makeServiceCalls(Tenant tenant, CallsCount callsCount) {
-    var timer = new ThrottleTimerImpl(10, callsCount);
-    var service = new B2BService(timer, callsCount);
+    executorService.execute(() -> makeServiceCalls(human, callsCount));
+    executorService.execute(() -> makeServiceCalls(dwarf, callsCount));
+
+    executorService.shutdown();
+    try {
+        executorService.awaitTermination(10, TimeUnit.SECONDS);
+    } catch (InterruptedException e) {
+        LOGGER.error("Executor service terminated: {}", e.getMessage());
+    }
+}
+
+private static void makeServiceCalls(BarCustomer barCustomer, CallsCount callsCount) {
+    var timer = new ThrottleTimerImpl(1000, callsCount);
+    var service = new Bartender(timer, callsCount);
     // Sleep is introduced to keep the output in check and easy to view and analyze the results.
-    IntStream.range(0, 20).forEach(i -> {
-      service.dummyCustomerApi(tenant);
-      try {
-        Thread.sleep(1);
-      } catch (InterruptedException e) {
-        LOGGER.error("Thread interrupted: {}", e.getMessage());
-      }
+    IntStream.range(0, 50).forEach(i -> {
+        service.orderDrink(barCustomer);
+        try {
+            Thread.sleep(100);
+        } catch (InterruptedException e) {
+            LOGGER.error("Thread interrupted: {}", e.getMessage());
+        }
     });
-  }
+}
 ```
 
+An excerpt from the example's console output:
+
+```
+18:46:36.218 [Timer-0] INFO com.iluwatar.throttling.CallsCount - reset counters
+18:46:36.218 [Timer-1] INFO com.iluwatar.throttling.CallsCount - reset counters
+18:46:36.242 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [1 consumed] 
+18:46:36.242 [pool-1-thread-1] DEBUG com.iluwatar.throttling.Bartender - Serving beer to young human : [1 consumed] 
+18:46:36.342 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [2 consumed] 
+18:46:36.342 [pool-1-thread-1] DEBUG com.iluwatar.throttling.Bartender - Serving beer to young human : [2 consumed] 
+18:46:36.443 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:36.443 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [3 consumed] 
+18:46:36.544 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:36.544 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [4 consumed] 
+18:46:36.645 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
+18:46:36.645 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:36.745 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:36.745 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
+18:46:36.846 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:36.846 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
+18:46:36.947 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
+18:46:36.947 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:37.048 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
+18:46:37.048 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:37.148 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
+18:46:37.148 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
+```
 
 ## Class diagram
 
@@ -185,7 +214,7 @@ second and Nike to 6.
 
 The Throttling pattern should be used:
 
-* When a service access needs to be restricted to not have high impacts on the performance of the service.
+* When service access needs to be restricted not to have high impact on the performance of the service.
 * When multiple clients are consuming the same service resources and restriction has to be made according to the usage per client.
 
 ## Credits

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

@@ -34,11 +34,11 @@ import lombok.extern.slf4j.Slf4j;
  * complete service by users or a particular tenant. This can allow systems to continue to function
  * and meet service level agreements, even when an increase in demand places load on resources.
  * <p>
- * In this example we have ({@link App}) as the initiating point of the service. This is a time
+ * In this example there is a {@link Bartender} serving beer to {@link BarCustomer}s. This is a time
  * based throttling, i.e. only a certain number of calls are allowed per second.
  * </p>
- * ({@link Tenant}) is the Tenant POJO class with which many tenants can be created ({@link
- * B2BService}) is the service which is consumed by the tenants and is throttled.
+ * ({@link BarCustomer}) is the service tenant class having a name and the number of calls allowed.
+ * ({@link Bartender}) is the service which is consumed by the tenants and is throttled.
  */
 @Slf4j
 public class App {
@@ -50,33 +50,35 @@ public class App {
    */
   public static void main(String[] args) {
     var callsCount = new CallsCount();
-    var adidas = new Tenant("Adidas", 5, callsCount);
-    var nike = new Tenant("Nike", 6, callsCount);
+    var human = new BarCustomer("young human", 2, callsCount);
+    var dwarf = new BarCustomer("dwarf soldier", 4, callsCount);
 
     var executorService = Executors.newFixedThreadPool(2);
 
-    executorService.execute(() -> makeServiceCalls(adidas, callsCount));
-    executorService.execute(() -> makeServiceCalls(nike, callsCount));
+    executorService.execute(() -> makeServiceCalls(human, callsCount));
+    executorService.execute(() -> makeServiceCalls(dwarf, callsCount));
 
     executorService.shutdown();
     try {
-      executorService.awaitTermination(10, TimeUnit.SECONDS);
+      if (!executorService.awaitTermination(10, TimeUnit.SECONDS)) {
+        executorService.shutdownNow();
+      }
     } catch (InterruptedException e) {
-      LOGGER.error("Executor Service terminated: {}", e.getMessage());
+      executorService.shutdownNow();
     }
   }
 
   /**
-   * Make calls to the B2BService dummy API.
+   * Make calls to the bartender.
    */
-  private static void makeServiceCalls(Tenant tenant, CallsCount callsCount) {
-    var timer = new ThrottleTimerImpl(10, callsCount);
-    var service = new B2BService(timer, callsCount);
+  private static void makeServiceCalls(BarCustomer barCustomer, CallsCount callsCount) {
+    var timer = new ThrottleTimerImpl(1000, callsCount);
+    var service = new Bartender(timer, callsCount);
     // Sleep is introduced to keep the output in check and easy to view and analyze the results.
-    IntStream.range(0, 20).forEach(i -> {
-      service.dummyCustomerApi(tenant);
+    IntStream.range(0, 50).forEach(i -> {
+      service.orderDrink(barCustomer);
       try {
-        Thread.sleep(1);
+        Thread.sleep(100);
       } catch (InterruptedException e) {
         LOGGER.error("Thread interrupted: {}", e.getMessage());
       }

throttling/src/main/java/com/iluwatar/throttling/BarCustomer.java

@@ -25,22 +25,26 @@ package com.iluwatar.throttling;
 
 import java.security.InvalidParameterException;
 
-/**
- * A Pojo class to create a basic Tenant with the allowed calls per second.
- */
-public class Tenant {
+import lombok.Getter;
 
+/**
+ * BarCustomer is a tenant with a name and a number of allowed calls per second.
+ */
+public class BarCustomer {
+
+  @Getter
   private final String name;
+  @Getter
   private final int allowedCallsPerSecond;
 
   /**
    * Constructor.
    *
-   * @param name                  Name of the tenant
-   * @param allowedCallsPerSecond The number of calls allowed for a particular tenant.
+   * @param name Name of the BarCustomer
+   * @param allowedCallsPerSecond The number of calls allowed for this particular tenant.
    * @throws InvalidParameterException If number of calls is less than 0, throws exception.
    */
-  public Tenant(String name, int allowedCallsPerSecond, CallsCount callsCount) {
+  public BarCustomer(String name, int allowedCallsPerSecond, CallsCount callsCount) {
     if (allowedCallsPerSecond < 0) {
       throw new InvalidParameterException("Number of calls less than 0 not allowed");
     }
@@ -48,12 +52,4 @@ public class Tenant {
     this.allowedCallsPerSecond = allowedCallsPerSecond;
     callsCount.addTenant(name);
   }
-
-  public String getName() {
-    return name;
-  }
-
-  public int getAllowedCallsPerSecond() {
-    return allowedCallsPerSecond;
-  }
 }

throttling/src/main/java/com/iluwatar/throttling/Bartender.java

@@ -29,33 +29,32 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 /**
- * A service which accepts a tenant and throttles the resource based on the time given to the
- * tenant.
+ * Bartender is a service which accepts a BarCustomer (tenant) and throttles
+ * the resource based on the time given to the tenant.
  */
-class B2BService {
+class Bartender {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(B2BService.class);
+  private static final Logger LOGGER = LoggerFactory.getLogger(Bartender.class);
   private final CallsCount callsCount;
 
-  public B2BService(Throttler timer, CallsCount callsCount) {
+  public Bartender(Throttler timer, CallsCount callsCount) {
     this.callsCount = callsCount;
     timer.start();
   }
 
   /**
-   * Calls dummy customer api.
-   *
+   * Orders a drink from the bartender.
    * @return customer id which is randomly generated
    */
-  public int dummyCustomerApi(Tenant tenant) {
-    var tenantName = tenant.getName();
+  public int orderDrink(BarCustomer barCustomer) {
+    var tenantName = barCustomer.getName();
     var count = callsCount.getCount(tenantName);
-    LOGGER.debug("Counter for {} : {} ", tenant.getName(), count);
-    if (count >= tenant.getAllowedCallsPerSecond()) {
-      LOGGER.error("API access per second limit reached for: {}", tenantName);
+    if (count >= barCustomer.getAllowedCallsPerSecond()) {
+      LOGGER.error("I'm sorry {}, you've had enough for today!", tenantName);
       return -1;
     }
     callsCount.incrementCount(tenantName);
+    LOGGER.debug("Serving beer to {} : [{} consumed] ", barCustomer.getName(), count + 1);
     return getRandomCustomerId();
   }
 

throttling/src/main/java/com/iluwatar/throttling/CallsCount.java

@@ -69,7 +69,7 @@ public final class CallsCount {
    * Resets the count of all the tenants in the map.
    */
   public void reset() {
-    LOGGER.debug("Resetting the map.");
     tenantCallsCount.replaceAll((k, v) -> new AtomicLong(0));
+    LOGGER.info("reset counters");
   }
 }

throttling/src/test/java/com/iluwatar/throttling/BarCustomerTest.java

@@ -23,20 +23,21 @@
 
 package com.iluwatar.throttling;
 
-import static org.junit.jupiter.api.Assertions.assertThrows;
+import org.junit.jupiter.api.Test;
 
 import java.security.InvalidParameterException;
-import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 /**
  * TenantTest to test the creation of Tenant with valid parameters.
  */
-public class TenantTest {
+public class BarCustomerTest {
 
   @Test
   void constructorTest() {
     assertThrows(InvalidParameterException.class, () -> {
-      new Tenant("FailTenant", -1, new CallsCount());
+      new BarCustomer("sirBrave", -1, new CallsCount());
     });
   }
 }

throttling/src/test/java/com/iluwatar/throttling/BartenderTest.java

@@ -32,19 +32,18 @@ import org.junit.jupiter.api.Test;
 /**
  * B2BServiceTest class to test the B2BService
  */
-public class B2BServiceTest {
+public class BartenderTest {
 
   private final CallsCount callsCount = new CallsCount();
 
   @Test
   void dummyCustomerApiTest() {
-    var tenant = new Tenant("testTenant", 2, callsCount);
+    var tenant = new BarCustomer("pirate", 2, callsCount);
     // In order to assure that throttling limits will not be reset, we use an empty throttling implementation
-    var timer = (Throttler) () -> {
-    };
-    var service = new B2BService(timer, callsCount);
+    var timer = (Throttler) () -> {};
+    var service = new Bartender(timer, callsCount);
 
-    IntStream.range(0, 5).mapToObj(i -> tenant).forEach(service::dummyCustomerApi);
+    IntStream.range(0, 5).mapToObj(i -> tenant).forEach(service::orderDrink);
     var counter = callsCount.getCount(tenant.getName());
     assertEquals(2, counter, "Counter limit must be reached");
   }

trampoline/README.md

@@ -17,19 +17,19 @@ and to interleave the execution of functions without hard coding them together.
 ## Explanation
 
 Recursion is a frequently adopted technique for solving algorithmic problems in a divide and conquer
-style. For example calculating fibonacci accumulating sum and factorials. In these kinds of problems 
-recursion is more straightforward than their loop counterpart. Furthermore recursion may need less 
-code and looks more concise. There is a saying that every recursion problem can be solved using 
-a loop with the cost of writing code that is more difficult to understand.
+style. For example, calculating Fibonacci accumulating sum and factorials. In these kinds of 
+problems, recursion is more straightforward than its loop counterpart. Furthermore, recursion may 
+need less code and looks more concise. There is a saying that every recursion problem can be solved 
+using a loop with the cost of writing code that is more difficult to understand.
 
-However recursion type solutions have one big caveat. For each recursive call it typically needs 
+However, recursion-type solutions have one big caveat. For each recursive call, it typically needs 
 an intermediate value stored and there is a limited amount of stack memory available. Running out of 
 stack memory creates a stack overflow error and halts the program execution.
 
-Trampoline pattern is a trick that allows us define recursive algorithms in Java without blowing the 
+Trampoline pattern is a trick that allows defining recursive algorithms in Java without blowing the 
 stack. 
 
-Real world example
+Real-world example
 
 > A recursive Fibonacci calculation without the stack overflow problem using the Trampoline pattern.       
 
@@ -105,24 +105,26 @@ public interface Trampoline<T> {
 Using the `Trampoline` to get Fibonacci values.
 
 ```java
-  public static Trampoline<Integer> loop(int times, int prod) {
+public static void main(String[] args) {
+    LOGGER.info("Start calculating war casualties");
+    var result = loop(10, 1).result();
+    LOGGER.info("The number of orcs perished in the war: {}", result);
+}
+
+public static Trampoline<Integer> loop(int times, int prod) {
     if (times == 0) {
-      return Trampoline.done(prod);
+        return Trampoline.done(prod);
     } else {
-      return Trampoline.more(() -> loop(times - 1, prod * times));
+        return Trampoline.more(() -> loop(times - 1, prod * times));
     }
-  }
-  
-  log.info("start pattern");
-  var result = loop(10, 1).result();
-  log.info("result {}", result);
+}
 ```
 
 Program output:
 
 ```
-start pattern
-result 3628800
+19:22:24.462 [main] INFO com.iluwatar.trampoline.TrampolineApp - Start calculating war casualties
+19:22:24.472 [main] INFO com.iluwatar.trampoline.TrampolineApp - The number of orcs perished in the war: 3628800
 ```
 
 ## Class diagram
@@ -133,8 +135,8 @@ result 3628800
 
 Use the Trampoline pattern when
 
-* For implementing tail recursive function. This pattern allows to switch on a stackless operation.
-* For interleaving the execution of two or more functions on the same thread.
+* For implementing tail-recursive functions. This pattern allows to switch on a stackless operation.
+* For interleaving execution of two or more functions on the same thread.
 
 ## Known uses
 

trampoline/src/main/java/com/iluwatar/trampoline/Trampoline.java

@@ -107,6 +107,4 @@ public interface Trampoline<T> {
       }
     };
   }
-
-
 }

trampoline/src/main/java/com/iluwatar/trampoline/TrampolineApp.java

@@ -39,9 +39,9 @@ public class TrampolineApp {
    * Main program for showing pattern. It does loop with factorial function.
    */
   public static void main(String[] args) {
-    LOGGER.info("start pattern");
+    LOGGER.info("Start calculating war casualties");
     var result = loop(10, 1).result();
-    LOGGER.info("result {}", result);
+    LOGGER.info("The number of orcs perished in the war: {}", result);
 
   }
 
@@ -55,5 +55,4 @@ public class TrampolineApp {
       return Trampoline.more(() -> loop(times - 1, prod * times));
     }
   }
-
 }

unit-of-work/README.md

@@ -12,20 +12,20 @@ tags:
 
 ## Intent
 
-When a business transaction is completed, all the the updates are sent as one big unit of work to be 
+When a business transaction is completed, all the updates are sent as one big unit of work to be 
 persisted in one go to minimize database round-trips. 
 
 ## Explanation
 
-Real world example
+Real-world example
 
-> We have a database containing student information. Administrators all over the country are 
-> constantly updating this information and it causes high load on the database server. To make the 
+> Arms dealer has a database containing weapon information. Merchants all over the town are 
+> constantly updating this information and it causes a high load on the database server. To make the 
 > load more manageable we apply to Unit of Work pattern to send many small updates in batches.       
 
 In plain words
 
-> Unit of Work merges many small database updates in single batch to optimize the number of 
+> Unit of Work merges many small database updates in a single batch to optimize the number of 
 > round-trips. 
 
 [MartinFowler.com](https://martinfowler.com/eaaCatalog/unitOfWork.html) says
@@ -35,37 +35,20 @@ In plain words
 
 **Programmatic Example**
 
-Here's the `Student` entity that is being persisted to the database.
+Here's the `Weapon` entity that is being persisted in the database.
 
 ```java
-public class Student {
-  private final Integer id;
-  private final String name;
-  private final String address;
-
-  public Student(Integer id, String name, String address) {
-    this.id = id;
-    this.name = name;
-    this.address = address;
-  }
-
-  public String getName() {
-    return name;
-  }
-
-  public Integer getId() {
-    return id;
-  }
-
-  public String getAddress() {
-    return address;
-  }
+@Getter
+@RequiredArgsConstructor
+public class Weapon {
+    private final Integer id;
+    private final String name;
 }
 ```
 
-The essence of the implementation is the `StudentRepository` implementing the Unit of Work pattern. 
+The essence of the implementation is the `ArmsDealer` implementing the Unit of Work pattern. 
 It maintains a map of database operations (`context`) that need to be done and when `commit` is 
-called it applies them in single batch.
+called it applies them in a single batch.
 
 ```java
 public interface IUnitOfWork<T> {
@@ -84,96 +67,117 @@ public interface IUnitOfWork<T> {
 }
 
 @Slf4j
-public class StudentRepository implements IUnitOfWork<Student> {
+@RequiredArgsConstructor
+public class ArmsDealer implements IUnitOfWork<Weapon> {
 
-  private final Map<String, List<Student>> context;
-  private final StudentDatabase studentDatabase;
+    private final Map<String, List<Weapon>> context;
+    private final WeaponDatabase weaponDatabase;
 
-  public StudentRepository(Map<String, List<Student>> context, StudentDatabase studentDatabase) {
-    this.context = context;
-    this.studentDatabase = studentDatabase;
-  }
-
-  @Override
-  public void registerNew(Student student) {
-    LOGGER.info("Registering {} for insert in context.", student.getName());
-    register(student, IUnitOfWork.INSERT);
-  }
-
-  @Override
-  public void registerModified(Student student) {
-    LOGGER.info("Registering {} for modify in context.", student.getName());
-    register(student, IUnitOfWork.MODIFY);
-
-  }
-
-  @Override
-  public void registerDeleted(Student student) {
-    LOGGER.info("Registering {} for delete in context.", student.getName());
-    register(student, IUnitOfWork.DELETE);
-  }
-
-  private void register(Student student, String operation) {
-    var studentsToOperate = context.get(operation);
-    if (studentsToOperate == null) {
-      studentsToOperate = new ArrayList<>();
-    }
-    studentsToOperate.add(student);
-    context.put(operation, studentsToOperate);
-  }
-
-  @Override
-  public void commit() {
-    if (context == null || context.size() == 0) {
-      return;
-    }
-    LOGGER.info("Commit started");
-    if (context.containsKey(IUnitOfWork.INSERT)) {
-      commitInsert();
+    @Override
+    public void registerNew(Weapon weapon) {
+        LOGGER.info("Registering {} for insert in context.", weapon.getName());
+        register(weapon, UnitActions.INSERT.getActionValue());
     }
 
-    if (context.containsKey(IUnitOfWork.MODIFY)) {
-      commitModify();
-    }
-    if (context.containsKey(IUnitOfWork.DELETE)) {
-      commitDelete();
-    }
-    LOGGER.info("Commit finished.");
-  }
+    @Override
+    public void registerModified(Weapon weapon) {
+        LOGGER.info("Registering {} for modify in context.", weapon.getName());
+        register(weapon, UnitActions.MODIFY.getActionValue());
 
-  private void commitInsert() {
-    var studentsToBeInserted = context.get(IUnitOfWork.INSERT);
-    for (var student : studentsToBeInserted) {
-      LOGGER.info("Saving {} to database.", student.getName());
-      studentDatabase.insert(student);
     }
-  }
 
-  private void commitModify() {
-    var modifiedStudents = context.get(IUnitOfWork.MODIFY);
-    for (var student : modifiedStudents) {
-      LOGGER.info("Modifying {} to database.", student.getName());
-      studentDatabase.modify(student);
+    @Override
+    public void registerDeleted(Weapon weapon) {
+        LOGGER.info("Registering {} for delete in context.", weapon.getName());
+        register(weapon, UnitActions.DELETE.getActionValue());
     }
-  }
 
-  private void commitDelete() {
-    var deletedStudents = context.get(IUnitOfWork.DELETE);
-    for (var student : deletedStudents) {
-      LOGGER.info("Deleting {} to database.", student.getName());
-      studentDatabase.delete(student);
+    private void register(Weapon weapon, String operation) {
+        var weaponsToOperate = context.get(operation);
+        if (weaponsToOperate == null) {
+            weaponsToOperate = new ArrayList<>();
+        }
+        weaponsToOperate.add(weapon);
+        context.put(operation, weaponsToOperate);
+    }
+
+    /**
+     * All UnitOfWork operations are batched and executed together on commit only.
+     */
+    @Override
+    public void commit() {
+        if (context == null || context.size() == 0) {
+            return;
+        }
+        LOGGER.info("Commit started");
+        if (context.containsKey(UnitActions.INSERT.getActionValue())) {
+            commitInsert();
+        }
+
+        if (context.containsKey(UnitActions.MODIFY.getActionValue())) {
+            commitModify();
+        }
+        if (context.containsKey(UnitActions.DELETE.getActionValue())) {
+            commitDelete();
+        }
+        LOGGER.info("Commit finished.");
+    }
+
+    private void commitInsert() {
+        var weaponsToBeInserted = context.get(UnitActions.INSERT.getActionValue());
+        for (var weapon : weaponsToBeInserted) {
+            LOGGER.info("Inserting a new weapon {} to sales rack.", weapon.getName());
+            weaponDatabase.insert(weapon);
+        }
+    }
+
+    private void commitModify() {
+        var modifiedWeapons = context.get(UnitActions.MODIFY.getActionValue());
+        for (var weapon : modifiedWeapons) {
+            LOGGER.info("Scheduling {} for modification work.", weapon.getName());
+            weaponDatabase.modify(weapon);
+        }
+    }
+
+    private void commitDelete() {
+        var deletedWeapons = context.get(UnitActions.DELETE.getActionValue());
+        for (var weapon : deletedWeapons) {
+            LOGGER.info("Scrapping {}.", weapon.getName());
+            weaponDatabase.delete(weapon);
+        }
     }
-  }
 }
 ```
 
-Finally, here's how we use the `StudentRepository` and `commit` the transaction.
+Here is how the whole app is put together.
 
 ```java
-    studentRepository.registerNew(ram);
-    studentRepository.registerModified(shyam);
-    studentRepository.registerDeleted(gopi);
-    studentRepository.commit();
+// create some weapons
+var enchantedHammer = new Weapon(1, "enchanted hammer");
+var brokenGreatSword = new Weapon(2, "broken great sword");
+var silverTrident = new Weapon(3, "silver trident");
+
+// create repository
+var weaponRepository = new ArmsDealer(new HashMap<String, List<Weapon>>(), new WeaponDatabase());
+
+// perform operations on the weapons
+weaponRepository.registerNew(enchantedHammer);
+weaponRepository.registerModified(silverTrident);
+weaponRepository.registerDeleted(brokenGreatSword);
+weaponRepository.commit();
+```
+
+Here is the console output.
+
+```
+21:39:21.984 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Registering enchanted hammer for insert in context.
+21:39:21.989 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Registering silver trident for modify in context.
+21:39:21.989 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Registering broken great sword for delete in context.
+21:39:21.989 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Commit started
+21:39:21.989 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Inserting a new weapon enchanted hammer to sales rack.
+21:39:21.989 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Scheduling silver trident for modification work.
+21:39:21.989 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Scrapping broken great sword.
+21:39:21.989 [main] INFO com.iluwatar.unitofwork.ArmsDealer - Commit finished.
 ```
 
 ## Class diagram
@@ -186,7 +190,7 @@ Use the Unit Of Work pattern when
 
 * To optimize the time taken for database transactions.
 * To send changes to database as a unit of work which ensures atomicity of the transaction.
-* To reduce number of database calls.
+* To reduce the number of database calls.
 
 ## Tutorials
 

unit-of-work/etc/unit-of-work.ucls

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <class-diagram version="1.2.1" 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.unitofwork.StudentDatabase" project="unit-of-work" 
+  <class id="1" language="java" name="com.iluwatar.unitofwork.WeaponDatabase" project="unit-of-work"
     file="/unit-of-work/src/main/java/com/iluwatar/unitofwork/StudentDatabase.java" binary="false" corner="BOTTOM_RIGHT">    
     <position height="-1" width="-1" x="170" y="406"/>    
     <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
@@ -28,7 +28,7 @@
       <operations public="true" package="true" protected="true" private="true" static="true"/>    
     </display>  
   </class>  
-  <class id="4" language="java" name="com.iluwatar.unitofwork.StudentRepository" project="unit-of-work" 
+  <class id="4" language="java" name="com.iluwatar.unitofwork.ArmsDealer" project="unit-of-work"
     file="/unit-of-work/src/main/java/com/iluwatar/unitofwork/StudentRepository.java" binary="false" 
     corner="BOTTOM_RIGHT">    
     <position height="-1" width="-1" x="377" y="166"/>    
@@ -38,7 +38,7 @@
       <operations public="true" package="true" protected="true" private="true" static="true"/>    
     </display>  
   </class>  
-  <class id="5" language="java" name="com.iluwatar.unitofwork.Student" project="unit-of-work" 
+  <class id="5" language="java" name="com.iluwatar.unitofwork.Weapon" project="unit-of-work"
     file="/unit-of-work/src/main/java/com/iluwatar/unitofwork/Student.java" binary="false" corner="BOTTOM_RIGHT">    
     <position height="-1" width="-1" x="696" y="130"/>    
     <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 

unit-of-work/src/main/java/com/iluwatar/unitofwork/App.java

@@ -27,7 +27,7 @@ import java.util.HashMap;
 import java.util.List;
 
 /**
- * {@link App} Application for managing student data.
+ * {@link App} Application demonstrating unit of work pattern.
  */
 public class App {
   /**
@@ -37,17 +37,19 @@ public class App {
    */
 
   public static void main(String[] args) {
-    var ram = new Student(1, "Ram", "Street 9, Cupertino");
-    var shyam = new Student(2, "Shyam", "Z bridge, Pune");
-    var gopi = new Student(3, "Gopi", "Street 10, Mumbai");
+    // create some weapons
+    var enchantedHammer = new Weapon(1, "enchanted hammer");
+    var brokenGreatSword = new Weapon(2, "broken great sword");
+    var silverTrident = new Weapon(3, "silver trident");
 
-    var context = new HashMap<String, List<Student>>();
-    var studentDatabase = new StudentDatabase();
-    var studentRepository = new StudentRepository(context, studentDatabase);
+    // create repository
+    var weaponRepository = new ArmsDealer(new HashMap<String, List<Weapon>>(),
+            new WeaponDatabase());
 
-    studentRepository.registerNew(ram);
-    studentRepository.registerModified(shyam);
-    studentRepository.registerDeleted(gopi);
-    studentRepository.commit();
+    // perform operations on the weapons
+    weaponRepository.registerNew(enchantedHammer);
+    weaponRepository.registerModified(silverTrident);
+    weaponRepository.registerDeleted(brokenGreatSword);
+    weaponRepository.commit();
   }
 }

unit-of-work/src/main/java/com/iluwatar/unitofwork/ArmsDealer.java

@@ -30,41 +30,41 @@ import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 
 /**
- * {@link StudentRepository} Student database repository. supports unit of work for student data.
+ * {@link ArmsDealer} Weapon repository that supports unit of work for weapons.
  */
 @Slf4j
 @RequiredArgsConstructor
-public class StudentRepository implements IUnitOfWork<Student> {
+public class ArmsDealer implements IUnitOfWork<Weapon> {
 
-  private final Map<String, List<Student>> context;
-  private final StudentDatabase studentDatabase;
+  private final Map<String, List<Weapon>> context;
+  private final WeaponDatabase weaponDatabase;
 
   @Override
-  public void registerNew(Student student) {
-    LOGGER.info("Registering {} for insert in context.", student.getName());
-    register(student, UnitActions.INSERT.getActionValue());
+  public void registerNew(Weapon weapon) {
+    LOGGER.info("Registering {} for insert in context.", weapon.getName());
+    register(weapon, UnitActions.INSERT.getActionValue());
   }
 
   @Override
-  public void registerModified(Student student) {
-    LOGGER.info("Registering {} for modify in context.", student.getName());
-    register(student, UnitActions.MODIFY.getActionValue());
+  public void registerModified(Weapon weapon) {
+    LOGGER.info("Registering {} for modify in context.", weapon.getName());
+    register(weapon, UnitActions.MODIFY.getActionValue());
 
   }
 
   @Override
-  public void registerDeleted(Student student) {
-    LOGGER.info("Registering {} for delete in context.", student.getName());
-    register(student, UnitActions.DELETE.getActionValue());
+  public void registerDeleted(Weapon weapon) {
+    LOGGER.info("Registering {} for delete in context.", weapon.getName());
+    register(weapon, UnitActions.DELETE.getActionValue());
   }
 
-  private void register(Student student, String operation) {
-    var studentsToOperate = context.get(operation);
-    if (studentsToOperate == null) {
-      studentsToOperate = new ArrayList<>();
+  private void register(Weapon weapon, String operation) {
+    var weaponsToOperate = context.get(operation);
+    if (weaponsToOperate == null) {
+      weaponsToOperate = new ArrayList<>();
     }
-    studentsToOperate.add(student);
-    context.put(operation, studentsToOperate);
+    weaponsToOperate.add(weapon);
+    context.put(operation, weaponsToOperate);
   }
 
   /**
@@ -90,26 +90,26 @@ public class StudentRepository implements IUnitOfWork<Student> {
   }
 
   private void commitInsert() {
-    var studentsToBeInserted = context.get(UnitActions.INSERT.getActionValue());
-    for (var student : studentsToBeInserted) {
-      LOGGER.info("Saving {} to database.", student.getName());
-      studentDatabase.insert(student);
+    var weaponsToBeInserted = context.get(UnitActions.INSERT.getActionValue());
+    for (var weapon : weaponsToBeInserted) {
+      LOGGER.info("Inserting a new weapon {} to sales rack.", weapon.getName());
+      weaponDatabase.insert(weapon);
     }
   }
 
   private void commitModify() {
-    var modifiedStudents = context.get(UnitActions.MODIFY.getActionValue());
-    for (var student : modifiedStudents) {
-      LOGGER.info("Modifying {} to database.", student.getName());
-      studentDatabase.modify(student);
+    var modifiedWeapons = context.get(UnitActions.MODIFY.getActionValue());
+    for (var weapon : modifiedWeapons) {
+      LOGGER.info("Scheduling {} for modification work.", weapon.getName());
+      weaponDatabase.modify(weapon);
     }
   }
 
   private void commitDelete() {
-    var deletedStudents = context.get(UnitActions.DELETE.getActionValue());
-    for (var student : deletedStudents) {
-      LOGGER.info("Deleting {} to database.", student.getName());
-      studentDatabase.delete(student);
+    var deletedWeapons = context.get(UnitActions.DELETE.getActionValue());
+    for (var weapon : deletedWeapons) {
+      LOGGER.info("Scrapping {}.", weapon.getName());
+      weaponDatabase.delete(weapon);
     }
   }
 }

unit-of-work/src/main/java/com/iluwatar/unitofwork/Weapon.java

@@ -27,14 +27,12 @@ import lombok.Getter;
 import lombok.RequiredArgsConstructor;
 
 /**
- * {@link Student} is an entity.
+ * {@link Weapon} is an entity.
  */
 @Getter
 @RequiredArgsConstructor
-public class Student {
+public class Weapon {
 
   private final Integer id;
   private final String name;
-  private final String address;
-
 }

unit-of-work/src/main/java/com/iluwatar/unitofwork/WeaponDatabase.java

@@ -24,19 +24,19 @@
 package com.iluwatar.unitofwork;
 
 /**
- * Act as Database for student records.
+ * Act as database for weapon records.
  */
-public class StudentDatabase {
+public class WeaponDatabase {
 
-  public void insert(Student student) {
+  public void insert(Weapon weapon) {
     //Some insert logic to DB
   }
 
-  public void modify(Student student) {
+  public void modify(Weapon weapon) {
     //Some modify logic to DB
   }
 
-  public void delete(Student student) {
+  public void delete(Weapon weapon) {
     //Some delete logic to DB
   }
 }

unit-of-work/src/test/java/com/iluwatar/unitofwork/ArmsDealerTest.java

@@ -36,102 +36,102 @@ import java.util.Map;
 import org.junit.jupiter.api.Test;
 
 /**
- * tests {@link StudentRepository}
+ * tests {@link ArmsDealer}
  */
 
-class StudentRepositoryTest {
-  private final Student student1 = new Student(1, "Ram", "street 9, cupertino");
-  private final Student student2 = new Student(1, "Sham", "Z bridge, pune");
+class ArmsDealerTest {
+  private final Weapon weapon1 = new Weapon(1, "battle ram");
+  private final Weapon weapon2 = new Weapon(1, "wooden lance");
 
-  private final Map<String, List<Student>> context = new HashMap<>();
-  private final StudentDatabase studentDatabase = mock(StudentDatabase.class);
-  private final StudentRepository studentRepository = new StudentRepository(context, studentDatabase);;
+  private final Map<String, List<Weapon>> context = new HashMap<>();
+  private final WeaponDatabase weaponDatabase = mock(WeaponDatabase.class);
+  private final ArmsDealer armsDealer = new ArmsDealer(context, weaponDatabase);;
 
   @Test
   void shouldSaveNewStudentWithoutWritingToDb() {
-    studentRepository.registerNew(student1);
-    studentRepository.registerNew(student2);
+    armsDealer.registerNew(weapon1);
+    armsDealer.registerNew(weapon2);
 
     assertEquals(2, context.get(UnitActions.INSERT.getActionValue()).size());
-    verifyNoMoreInteractions(studentDatabase);
+    verifyNoMoreInteractions(weaponDatabase);
   }
 
   @Test
   void shouldSaveDeletedStudentWithoutWritingToDb() {
-    studentRepository.registerDeleted(student1);
-    studentRepository.registerDeleted(student2);
+    armsDealer.registerDeleted(weapon1);
+    armsDealer.registerDeleted(weapon2);
 
     assertEquals(2, context.get(UnitActions.DELETE.getActionValue()).size());
-    verifyNoMoreInteractions(studentDatabase);
+    verifyNoMoreInteractions(weaponDatabase);
   }
 
   @Test
   void shouldSaveModifiedStudentWithoutWritingToDb() {
-    studentRepository.registerModified(student1);
-    studentRepository.registerModified(student2);
+    armsDealer.registerModified(weapon1);
+    armsDealer.registerModified(weapon2);
 
     assertEquals(2, context.get(UnitActions.MODIFY.getActionValue()).size());
-    verifyNoMoreInteractions(studentDatabase);
+    verifyNoMoreInteractions(weaponDatabase);
   }
 
   @Test
   void shouldSaveAllLocalChangesToDb() {
-    context.put(UnitActions.INSERT.getActionValue(), List.of(student1));
-    context.put(UnitActions.MODIFY.getActionValue(), List.of(student1));
-    context.put(UnitActions.DELETE.getActionValue(), List.of(student1));
+    context.put(UnitActions.INSERT.getActionValue(), List.of(weapon1));
+    context.put(UnitActions.MODIFY.getActionValue(), List.of(weapon1));
+    context.put(UnitActions.DELETE.getActionValue(), List.of(weapon1));
 
-    studentRepository.commit();
+    armsDealer.commit();
 
-    verify(studentDatabase, times(1)).insert(student1);
-    verify(studentDatabase, times(1)).modify(student1);
-    verify(studentDatabase, times(1)).delete(student1);
+    verify(weaponDatabase, times(1)).insert(weapon1);
+    verify(weaponDatabase, times(1)).modify(weapon1);
+    verify(weaponDatabase, times(1)).delete(weapon1);
   }
 
   @Test
   void shouldNotWriteToDbIfContextIsNull() {
-    var studentRepository = new StudentRepository(null, studentDatabase);
+    var weaponRepository = new ArmsDealer(null, weaponDatabase);
 
-    studentRepository.commit();
+    weaponRepository.commit();
 
-    verifyNoMoreInteractions(studentDatabase);
+    verifyNoMoreInteractions(weaponDatabase);
   }
 
   @Test
   void shouldNotWriteToDbIfNothingToCommit() {
-    var studentRepository = new StudentRepository(new HashMap<>(), studentDatabase);
+    var weaponRepository = new ArmsDealer(new HashMap<>(), weaponDatabase);
 
-    studentRepository.commit();
+    weaponRepository.commit();
 
-    verifyNoMoreInteractions(studentDatabase);
+    verifyNoMoreInteractions(weaponDatabase);
   }
 
   @Test
   void shouldNotInsertToDbIfNoRegisteredStudentsToBeCommitted() {
-    context.put(UnitActions.MODIFY.getActionValue(), List.of(student1));
-    context.put(UnitActions.DELETE.getActionValue(), List.of(student1));
+    context.put(UnitActions.MODIFY.getActionValue(), List.of(weapon1));
+    context.put(UnitActions.DELETE.getActionValue(), List.of(weapon1));
 
-    studentRepository.commit();
+    armsDealer.commit();
 
-    verify(studentDatabase, never()).insert(student1);
+    verify(weaponDatabase, never()).insert(weapon1);
   }
 
   @Test
   void shouldNotModifyToDbIfNotRegisteredStudentsToBeCommitted() {
-    context.put(UnitActions.INSERT.getActionValue(), List.of(student1));
-    context.put(UnitActions.DELETE.getActionValue(), List.of(student1));
+    context.put(UnitActions.INSERT.getActionValue(), List.of(weapon1));
+    context.put(UnitActions.DELETE.getActionValue(), List.of(weapon1));
 
-    studentRepository.commit();
+    armsDealer.commit();
 
-    verify(studentDatabase, never()).modify(student1);
+    verify(weaponDatabase, never()).modify(weapon1);
   }
 
   @Test
   void shouldNotDeleteFromDbIfNotRegisteredStudentsToBeCommitted() {
-    context.put(UnitActions.INSERT.getActionValue(), List.of(student1));
-    context.put(UnitActions.MODIFY.getActionValue(), List.of(student1));
+    context.put(UnitActions.INSERT.getActionValue(), List.of(weapon1));
+    context.put(UnitActions.MODIFY.getActionValue(), List.of(weapon1));
 
-    studentRepository.commit();
+    armsDealer.commit();
 
-    verify(studentDatabase, never()).delete(student1);
+    verify(weaponDatabase, never()).delete(weapon1);
   }
 }

value-object/README.md

@@ -10,19 +10,80 @@ tags:
 ---
 
 ## Intent
+
 Provide objects which follow value semantics rather than reference semantics.
-This means value objects' equality are not based on identity. Two value objects are
+This means value objects' equality is not based on identity. Two value objects are
 equal when they have the same value, not necessarily being the same object.
 
+## Explanation
+
+Real-world example
+
+> There is a class for hero statistics in a role-playing game. The statistics contain attributes
+> such as strength, intelligence, and luck. The statistics of different heroes should be equal
+> when all the attributes are equal.
+
+In plain words
+
+> Value objects are equal when their attributes have the same value
+
+Wikipedia says
+
+> In computer science, a value object is a small object that represents a simple entity whose 
+> equality is not based on identity: i.e. two value objects are equal when they have the same 
+> value, not necessarily being the same object.
+
+**Programmatic Example**
+
+Here is the `HeroStat` class that is the value object. Notice the use of 
+[Lombok's `@Value`](https://projectlombok.org/features/Value) annotation.
+
+```java
+@Value(staticConstructor = "valueOf")
+class HeroStat {
+
+    int strength;
+    int intelligence;
+    int luck;
+}
+```
+
+The example creates three different `HeroStat`s and compares their equality.
+
+```java
+var statA = HeroStat.valueOf(10, 5, 0);
+var statB = HeroStat.valueOf(10, 5, 0);
+var statC = HeroStat.valueOf(5, 1, 8);
+
+LOGGER.info(statA.toString());
+LOGGER.info(statB.toString());
+LOGGER.info(statC.toString());
+
+LOGGER.info("Is statA and statB equal : {}", statA.equals(statB));
+LOGGER.info("Is statA and statC equal : {}", statA.equals(statC));
+```
+
+Here's the console output.
+
+```
+20:11:12.199 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=10, intelligence=5, luck=0)
+20:11:12.202 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=10, intelligence=5, luck=0)
+20:11:12.202 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=5, intelligence=1, luck=8)
+20:11:12.202 [main] INFO com.iluwatar.value.object.App - Is statA and statB equal : true
+20:11:12.203 [main] INFO com.iluwatar.value.object.App - Is statA and statC equal : false
+```
+
 ## Class diagram
+
 ![alt text](./etc/value-object.png "Value Object")
 
 ## Applicability
+
 Use the Value Object when
 
-* You need to measure the objects' equality based on the objects' value
+* The object's equality needs to be based on the object's value
 
-## Real world examples
+## Known uses
 
 * [java.util.Optional](https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html)
 * [java.time.LocalDate](https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html)
@@ -31,6 +92,7 @@ Use the Value Object when
 ## Credits
 
 * [Patterns of Enterprise Application Architecture](http://www.martinfowler.com/books/eaa.html)
+* [ValueObject](https://martinfowler.com/bliki/ValueObject.html)
 * [VALJOs - Value Java Objects : Stephen Colebourne's blog](http://blog.joda.org/2014/03/valjos-value-java-objects.html)
 * [Value Object : Wikipedia](https://en.wikipedia.org/wiki/Value_object)
 * [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=f27d2644fbe5026ea448791a8ad09c94)

value-object/src/main/java/com/iluwatar/value/object/App.java

@@ -43,7 +43,7 @@ import lombok.extern.slf4j.Slf4j;
 public class App {
 
   /**
-   * This practice creates three HeroStats(Value object) and checks equality between those.
+   * This example creates three HeroStats (value objects) and checks equality between those.
    */
   public static void main(String[] args) {
     var statA = HeroStat.valueOf(10, 5, 0);
@@ -51,6 +51,8 @@ public class App {
     var statC = HeroStat.valueOf(5, 1, 8);
 
     LOGGER.info(statA.toString());
+    LOGGER.info(statB.toString());
+    LOGGER.info(statC.toString());
 
     LOGGER.info("Is statA and statB equal : {}", statA.equals(statB));
     LOGGER.info("Is statA and statC equal : {}", statA.equals(statC));

value-object/src/main/java/com/iluwatar/value/object/HeroStat.java

@@ -23,10 +23,7 @@
 
 package com.iluwatar.value.object;
 
-import lombok.EqualsAndHashCode;
-import lombok.Getter;
-import lombok.RequiredArgsConstructor;
-import lombok.ToString;
+import lombok.Value;
 
 /**
  * HeroStat is a value object.
@@ -35,23 +32,10 @@ import lombok.ToString;
  *     http://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html
  *     </a>
  */
-@Getter
-@ToString
-@EqualsAndHashCode
-@RequiredArgsConstructor
-public class HeroStat {
-
-  // Stats for a hero
-
-  private final int strength;
-  private final int intelligence;
-  private final int luck;
-
-  // Static factory method to create new instances.
-  public static HeroStat valueOf(int strength, int intelligence, int luck) {
-    return new HeroStat(strength, intelligence, luck);
-  }
-
-  // The clone() method should not be public. Just don't override it.
+@Value(staticConstructor = "valueOf")
+class HeroStat {
 
+  int strength;
+  int intelligence;
+  int luck;
 }