Refactor and add explanation for value object

* docs: update README.md [skip ci]

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

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

.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/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),
+        baelish,
-        new LordVarys(kingsHand),
+        varys,
-        new Scout(kingsHand)
+        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) {
+  public KingsHand(EventObserver obs, Event e) {
-    super(obs);
+    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) {
+  public LordBaelish(EventObserver obs, Event e) {
-    super(obs);
+    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) {
+  public LordVarys(EventObserver obs, Event e) {
-    super(obs);
+    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) {
+  public Scout(EventObserver obs, Event e) {
-    super(obs);
+    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(observer1, event);
-    emitter.registerObserver(observer2);
+    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);
+    final var emitter = this.factoryWithDefaultObserver.apply(defaultObserver, event);
-    emitter.registerObserver(observer1);
+    emitter.registerObserver(observer1, event);
-    emitter.registerObserver(observer2);
+    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);
+
   }
 
 }

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>

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.Value;
-import lombok.Getter;
-import lombok.RequiredArgsConstructor;
-import lombok.ToString;
 
 /**
  * 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
+@Value(staticConstructor = "valueOf")
-@ToString
+class HeroStat {
-@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.
 
+  int strength;
+  int intelligence;
+  int luck;
 }