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

@@ -1776,6 +1776,15 @@
       "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-195-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/>
@@ -325,6 +325,7 @@ This project is licensed under the terms of the MIT license.
     <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>
 

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 a mighty amount of coffee cups. Creating a new thread for each task would be a waste so 
-> we establish a thread pool.       
+> and serve 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

@@ -16,12 +16,10 @@ Ensure that a given client is not able to access service resources more than the
 
 ## Explanation
 
-Real-world example
+Real world example
 
-> 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.
+> 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.      
 
 In plain words
 
@@ -35,25 +33,30 @@ In plain words
 
 **Programmatic Example**
 
-`BarCustomer` class presents the clients of the `Bartender` API. `CallsCount` tracks the number of 
-calls per `BarCustomer`.
+Tenant class presents the clients of the API. CallsCount tracks the number of API calls per tenant.
 
 ```java
-public class BarCustomer {
+public class Tenant {
 
-    @Getter
-    private final String name;
-    @Getter
-    private final int allowedCallsPerSecond;
+  private final String name;
+  private final int allowedCallsPerSecond;
 
-    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);
+  public Tenant(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);
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public int getAllowedCallsPerSecond() {
+    return allowedCallsPerSecond;
+  }
 }
 
 @Slf4j
@@ -73,14 +76,14 @@ public final class CallsCount {
   }
 
   public void reset() {
+    LOGGER.debug("Resetting the map.");
     tenantCallsCount.replaceAll((k, v) -> new AtomicLong(0));
-    LOGGER.info("reset counters");
   }
 }
 ```
 
-Next, the service that the tenants are calling is introduced. To track the call count, a throttler 
-timer is used.
+Next we introduce the service that the tenants are calling. To track the call count we use the 
+throttler timer.
 
 ```java
 public interface Throttler {
@@ -108,103 +111,71 @@ public class ThrottleTimerImpl implements Throttler {
     }, 0, throttlePeriod);
   }
 }
-```
 
-`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.
+class B2BService {
 
-```java
-class Bartender {
+  private static final Logger LOGGER = LoggerFactory.getLogger(B2BService.class);
+  private final CallsCount callsCount;
 
-    private static final Logger LOGGER = LoggerFactory.getLogger(Bartender.class);
-    private final CallsCount callsCount;
+  public B2BService(Throttler timer, CallsCount callsCount) {
+    this.callsCount = callsCount;
+    timer.start();
+  }
 
-    public Bartender(Throttler timer, CallsCount callsCount) {
-        this.callsCount = callsCount;
-        timer.start();
+  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;
     }
+    callsCount.incrementCount(tenantName);
+    return getRandomCustomerId();
+  }
 
-    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);
-    }
+  private int getRandomCustomerId() {
+    return ThreadLocalRandom.current().nextInt(1, 10000);
+  }
 }
 ```
 
-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.
+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.
 
 ```java
-public static void main(String[] args) {
+  public static void main(String[] args) {
     var callsCount = new CallsCount();
-    var human = new BarCustomer("young human", 2, callsCount);
-    var dwarf = new BarCustomer("dwarf soldier", 4, callsCount);
+    var adidas = new Tenant("Adidas", 5, callsCount);
+    var nike = new Tenant("Nike", 6, callsCount);
 
     var executorService = Executors.newFixedThreadPool(2);
-
-    executorService.execute(() -> makeServiceCalls(human, callsCount));
-    executorService.execute(() -> makeServiceCalls(dwarf, callsCount));
-
+    executorService.execute(() -> makeServiceCalls(adidas, callsCount));
+    executorService.execute(() -> makeServiceCalls(nike, callsCount));
     executorService.shutdown();
+    
     try {
-        executorService.awaitTermination(10, TimeUnit.SECONDS);
+      executorService.awaitTermination(10, TimeUnit.SECONDS);
     } catch (InterruptedException e) {
-        LOGGER.error("Executor service terminated: {}", e.getMessage());
+      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);
+  private static void makeServiceCalls(Tenant tenant, CallsCount callsCount) {
+    var timer = new ThrottleTimerImpl(10, callsCount);
+    var service = new B2BService(timer, callsCount);
     // Sleep is introduced to keep the output in check and easy to view and analyze the results.
-    IntStream.range(0, 50).forEach(i -> {
-        service.orderDrink(barCustomer);
-        try {
-            Thread.sleep(100);
-        } catch (InterruptedException e) {
-            LOGGER.error("Thread interrupted: {}", e.getMessage());
-        }
+    IntStream.range(0, 20).forEach(i -> {
+      service.dummyCustomerApi(tenant);
+      try {
+        Thread.sleep(1);
+      } 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
 
@@ -214,7 +185,7 @@ An excerpt from the example's console output:
 
 The Throttling pattern should be used:
 
-* When service access needs to be restricted not to have high impact on the performance of the service.
+* When a service access needs to be restricted to not have high impacts 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 there is a {@link Bartender} serving beer to {@link BarCustomer}s. This is a time
+ * In this example we have ({@link App}) as the initiating point of the service. This is a time
  * based throttling, i.e. only a certain number of calls are allowed per second.
  * </p>
- * ({@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.
+ * ({@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.
  */
 @Slf4j
 public class App {
@@ -50,35 +50,33 @@ public class App {
    */
   public static void main(String[] args) {
     var callsCount = new CallsCount();
-    var human = new BarCustomer("young human", 2, callsCount);
-    var dwarf = new BarCustomer("dwarf soldier", 4, callsCount);
+    var adidas = new Tenant("Adidas", 5, callsCount);
+    var nike = new Tenant("Nike", 6, callsCount);
 
     var executorService = Executors.newFixedThreadPool(2);
 
-    executorService.execute(() -> makeServiceCalls(human, callsCount));
-    executorService.execute(() -> makeServiceCalls(dwarf, callsCount));
+    executorService.execute(() -> makeServiceCalls(adidas, callsCount));
+    executorService.execute(() -> makeServiceCalls(nike, callsCount));
 
     executorService.shutdown();
     try {
-      if (!executorService.awaitTermination(10, TimeUnit.SECONDS)) {
-        executorService.shutdownNow();
-      }
+      executorService.awaitTermination(10, TimeUnit.SECONDS);
     } catch (InterruptedException e) {
-      executorService.shutdownNow();
+      LOGGER.error("Executor Service terminated: {}", e.getMessage());
     }
   }
 
   /**
-   * Make calls to the bartender.
+   * Make calls to the B2BService dummy API.
    */
-  private static void makeServiceCalls(BarCustomer barCustomer, CallsCount callsCount) {
-    var timer = new ThrottleTimerImpl(1000, callsCount);
-    var service = new Bartender(timer, callsCount);
+  private static void makeServiceCalls(Tenant tenant, CallsCount callsCount) {
+    var timer = new ThrottleTimerImpl(10, callsCount);
+    var service = new B2BService(timer, callsCount);
     // Sleep is introduced to keep the output in check and easy to view and analyze the results.
-    IntStream.range(0, 50).forEach(i -> {
-      service.orderDrink(barCustomer);
+    IntStream.range(0, 20).forEach(i -> {
+      service.dummyCustomerApi(tenant);
       try {
-        Thread.sleep(100);
+        Thread.sleep(1);
       } catch (InterruptedException e) {
         LOGGER.error("Thread interrupted: {}", e.getMessage());
       }

throttling/src/main/java/com/iluwatar/throttling/B2BService.java

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

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

throttling/src/test/java/com/iluwatar/throttling/B2BServiceTest.java

@@ -32,18 +32,19 @@ import org.junit.jupiter.api.Test;
 /**
  * B2BServiceTest class to test the B2BService
  */
-public class BartenderTest {
+public class B2BServiceTest {
 
   private final CallsCount callsCount = new CallsCount();
 
   @Test
   void dummyCustomerApiTest() {
-    var tenant = new BarCustomer("pirate", 2, callsCount);
+    var tenant = new Tenant("testTenant", 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 Bartender(timer, callsCount);
+    var timer = (Throttler) () -> {
+    };
+    var service = new B2BService(timer, callsCount);
 
-    IntStream.range(0, 5).mapToObj(i -> tenant).forEach(service::orderDrink);
+    IntStream.range(0, 5).mapToObj(i -> tenant).forEach(service::dummyCustomerApi);
     var counter = callsCount.getCount(tenant.getName());
     assertEquals(2, counter, "Counter limit must be reached");
   }

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

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

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;
 }