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Merge pull request #362 from JuhoKang/master
Value object pattern #349
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‎pom.xml

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<module>delegation</module>
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<module>event-driven-architecture</module>
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<module>feature-toggle</module>
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<module>value-object</module>
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</modules>
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<dependencyManagement>
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<build>
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<pluginManagement>
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<plugins>
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<!-- This plugin's configuration is used to store Eclipse m2e settings
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only. It has no influence on the Maven build itself. TODO: Remove when the
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<!-- This plugin's configuration is used to store Eclipse m2e settings
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only. It has no influence on the Maven build itself. TODO: Remove when the
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m2e plugin can correctly bind to Maven lifecycle -->
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<plugin>
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<groupId>org.eclipse.m2e</groupId>
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<groupId>org.jacoco</groupId>
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<artifactId>jacoco-maven-plugin</artifactId>
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<version>${jacoco.version}</version>
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<!-- The following exclude configuration was added because error occurred
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<!-- The following exclude configuration was added because error occurred
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when executing "mvn clean test jacoco:report coveralls:report" -->
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<!-- [ERROR] Failed to execute goal org.eluder.coveralls:coveralls-maven-plugin:3.1.0:report
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(default-cli) on project java-design-patterns: I/O operation failed: No source
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<!-- [ERROR] Failed to execute goal org.eluder.coveralls:coveralls-maven-plugin:3.1.0:report
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(default-cli) on project java-design-patterns: I/O operation failed: No source
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found for domainapp/dom/modules/simple/QSimpleObject.java -> [Help 1] -->
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<configuration>
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<excludes>
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<excludeFromFailureFile>exclude-pmd.properties</excludeFromFailureFile>
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</configuration>
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</execution>
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</executions>
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</executions>
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</plugin>
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<plugin>
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</plugin>
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</plugins>
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</reporting>
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</project>

‎value-object/etc/value-object.png

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‎value-object/etc/value-object.ucls

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.9" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
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realizations="true" associations="true" dependencies="true" nesting-relationships="true" router="FAN">
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<class id="1" language="java" name="com.iluwatar.value.object.HeroStat" project="value-object"
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file="/value-object/src/main/java/com/iluwatar/value/object/HeroStat.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="520" y="337"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</classifier-display>
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<association-display labels="true" multiplicity="true"/>
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</class-diagram>

‎value-object/index.md

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---
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layout: pattern
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title: Value Object
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folder: value-object
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permalink: /patterns/value-object/
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categories: Creational
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tags:
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- Java
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- Difficulty-Beginner
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---
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## Intent
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Provide objects which follow value semantics rather than reference semantics.
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This means value objects' equality are not based on identity. Two value objects are
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equal when they have the same value, not necessarily being the same object.
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![alt text](./etc/value-object.png "Value Object")
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## Applicability
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Use the Value Object when
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* you need to measure the objects' equality based on the objects' value
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## Real world examples
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* [java.util.Optional](https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html)
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* [java.time.LocalDate](https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html)
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* [joda-time, money, beans](http://www.joda.org/)
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## Credits
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* [Patterns of Enterprise Application Architecture](http://www.martinfowler.com/books/eaa.html)
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* [VALJOs - Value Java Objects : Stephen Colebourne's blog](http://blog.joda.org/2014/03/valjos-value-java-objects.html)
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* [Value Object : Wikipedia](https://en.wikipedia.org/wiki/Value_object)

‎value-object/pom.xml

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<?xml version="1.0"?>
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<project
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
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xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.11.0-SNAPSHOT</version>
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</parent>
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<artifactId>value-object</artifactId>
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<dependencies>
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<dependency>
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<groupId>com.google.guava</groupId>
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<artifactId>guava-testlib</artifactId>
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<version>19.0</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.mockito</groupId>
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<artifactId>mockito-core</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>

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

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package com.iluwatar.value.object;
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/**
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* A Value Object are objects which follow value semantics rather than reference semantics. This
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* means value objects' equality are not based on identity. Two value objects are equal when they
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* have the same value, not necessarily being the same object..
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*
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* Value Objects must override equals(), hashCode() to check the equality with values.
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* Value Objects should be immutable so declare members final.
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* Obtain instances by static factory methods.
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* The elements of the state must be other values, including primitive types.
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* Provide methods, typically simple getters, to get the elements of the state.
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* A Value Object must check equality with equals() not ==
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*
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* For more specific and strict rules to implement value objects check the rules from Stephen
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* Colebourne's term VALJO : http://blog.joda.org/2014/03/valjos-value-java-objects.html
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*/
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public class App {
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/**
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* This practice creates three HeroStats(Value object) and checks equality between those.
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*/
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public static void main(String[] args) {
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HeroStat statA = HeroStat.valueOf(10, 5, 0);
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HeroStat statB = HeroStat.valueOf(10, 5, 0);
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HeroStat statC = HeroStat.valueOf(5, 1, 8);
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System.out.println(statA.toString());
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System.out.println("Is statA and statB equal : " + statA.equals(statB));
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System.out.println("Is statA and statC equal : " + statA.equals(statC));
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}
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}

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

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package com.iluwatar.value.object;
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/**
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* HeroStat is a value object
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*
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* {@link http://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html}
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*/
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public class HeroStat {
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// Stats for a hero
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private final int strength;
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private final int intelligence;
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private final int luck;
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// All constructors must be private.
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private HeroStat(int strength, int intelligence, int luck) {
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super();
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this.strength = strength;
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this.intelligence = intelligence;
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this.luck = luck;
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}
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// Static factory method to create new instances.
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public static HeroStat valueOf(int strength, int intelligence, int luck) {
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return new HeroStat(strength, intelligence, luck);
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}
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public int getStrength() {
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return strength;
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}
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public int getIntelligence() {
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return intelligence;
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}
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public int getLuck() {
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return luck;
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}
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/*
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* Recommended to provide a static factory method capable of creating an instance from the formal
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* string representation declared like this. public static HeroStat parse(String string) {}
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*/
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// toString, hashCode, equals
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@Override
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public String toString() {
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return "HeroStat [strength=" + strength + ", intelligence=" + intelligence
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+ ", luck=" + luck + "]";
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}
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@Override
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public int hashCode() {
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final int prime = 31;
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int result = 1;
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result = prime * result + intelligence;
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result = prime * result + luck;
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result = prime * result + strength;
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return result;
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}
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@Override
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public boolean equals(Object obj) {
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if (this == obj) {
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return true;
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}
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if (obj == null) {
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return false;
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}
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if (getClass() != obj.getClass()) {
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return false;
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}
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HeroStat other = (HeroStat) obj;
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if (intelligence != other.intelligence) {
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return false;
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}
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if (luck != other.luck) {
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return false;
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}
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if (strength != other.strength) {
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return false;
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}
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return true;
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}
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// The clone() method should not be public. Just don't override it.
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}

‎value-object/src/test/java/com/iluwatar/value/object/AppTest.java

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package com.iluwatar.value.object;
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import org.junit.Test;
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/**
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* Application test
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*/
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public class AppTest {
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@Test
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public void test() {
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String[] args = {};
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App.main(args);
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}
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}

‎value-object/src/test/java/com/iluwatar/value/object/HeroStatTest.java

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package com.iluwatar.value.object;
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import static org.hamcrest.CoreMatchers.is;
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import static org.hamcrest.CoreMatchers.not;
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import static org.junit.Assert.assertThat;
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import com.google.common.testing.EqualsTester;
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import org.junit.Test;
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/**
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* Unit test for HeroStat.
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*/
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public class HeroStatTest {
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/**
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* Tester for equals() and hashCode() methods of a class. Using guava's EqualsTester.
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*
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* @see http://static.javadoc.io/com.google.guava/guava-testlib/19.0/com/google/common/testing/
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* EqualsTester.html
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*/
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@Test
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public void testEquals() {
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HeroStat heroStatA = HeroStat.valueOf(3, 9, 2);
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HeroStat heroStatB = HeroStat.valueOf(3, 9, 2);
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new EqualsTester().addEqualityGroup(heroStatA, heroStatB).testEquals();
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}
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/**
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* The toString() for two equal values must be the same. For two non-equal values it must be
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* different.
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*/
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@Test
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public void testToString() {
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HeroStat heroStatA = HeroStat.valueOf(3, 9, 2);
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HeroStat heroStatB = HeroStat.valueOf(3, 9, 2);
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HeroStat heroStatC = HeroStat.valueOf(3, 9, 8);
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assertThat(heroStatA.toString(), is(heroStatB.toString()));
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assertThat(heroStatA.toString(), is(not(heroStatC.toString())));
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}
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}

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