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Merge pull request iluwatar#290 from hoswey/master
implements Twin design pattern iluwatar#63
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‎pom.xml‎

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<module>multiton</module>
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<module>resource-acquisition-is-initialization</module>
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<module>thread-pool</module>
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<module>twin</module>
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<module>private-class-data</module>
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<module>object-pool</module>
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<module>dependency-injection</module>

‎twin/.gitignore‎

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/target/

‎twin/etc/twin.png‎

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‎twin/etc/twin.ucls‎

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
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realizations="true" associations="true" dependencies="false" nesting-relationships="true">
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<class id="1" language="java" name="com.iluwatar.twin.BallThread" project="twin"
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file="/twin/src/main/java/com/iluwatar/twin/BallThread.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="550" y="316"/>
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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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<class id="2" language="java" name="com.iluwatar.twin.BallItem" project="twin"
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file="/twin/src/main/java/com/iluwatar/twin/BallItem.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="285" y="315"/>
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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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<class id="3" language="java" name="com.iluwatar.twin.GameItem" project="twin"
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file="/twin/src/main/java/com/iluwatar/twin/GameItem.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="285" y="84"/>
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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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<class id="4" language="java" name="java.lang.Thread" project="twin"
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file="/Library/Java/JavaVirtualMachines/jdk1.8.0_65.jdk/Contents/Home/jre/lib/rt.jar" binary="true"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="550" y="96"/>
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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="false" package="false" protected="false" private="false" static="true"/>
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<operations public="false" package="false" protected="false" private="false" static="true"/>
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</display>
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</class>
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<generalization id="5">
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<end type="SOURCE" refId="2"/>
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<end type="TARGET" refId="3"/>
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</generalization>
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<association id="6">
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<end type="SOURCE" refId="2" navigable="false">
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<attribute id="7" name="twin"/>
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<multiplicity id="8" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="1" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="9">
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<end type="SOURCE" refId="4" navigable="false">
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<attribute id="10" name="threadQ"/>
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<multiplicity id="11" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="4" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="12">
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<end type="SOURCE" refId="1" navigable="false">
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<attribute id="13" name="twin"/>
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<multiplicity id="14" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="2" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<generalization id="15">
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<end type="SOURCE" refId="1"/>
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<end type="TARGET" refId="4"/>
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</generalization>
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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>

‎twin/index.md‎

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---
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layout: pattern
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title: Twin
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folder: twin
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permalink: /patterns/twin/
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categories: Creational
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tags: Java
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---
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**Intent:** Twin pattern is a design pattern which provides a standard solution to simulate multiple
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inheritance in java
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![alt text](./etc/twin.png "Twin")
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**Applicability:** Use the Twin idiom when
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* to simulate multiple inheritance in a language that does not support this feature.
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* to avoid certain problems of multiple inheritance such as name clashes.
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**Credits:**
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* [Twin – A Design Pattern for Modeling Multiple Inheritance](http://www.ssw.uni-linz.ac.at/Research/Papers/Moe99/Paper.pdf)

‎twin/pom.xml‎

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<?xml version="1.0"?>
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<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
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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>1.8.0-SNAPSHOT</version>
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</parent>
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<artifactId>twin</artifactId>
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<dependencies>
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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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</dependencies>
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</project>

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

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package com.iluwatar.twin;
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/**
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* Twin pattern is a design pattern which provides a standard solution to simulate multiple
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* inheritance in java.
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* <p>
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* In this example, the essence of the Twin pattern is the {@link BallItem} class and
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* {@link BallThread} class represent the twin objects to coordinate with each other(via the twin
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* reference) like a single class inheriting from {@link GameItem} and {@link Thread}.
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*/
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public class App {
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/**
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* Program entry point
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*
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* @param args command line args
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*/
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public static void main(String[] args) throws Exception {
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BallItem ballItem = new BallItem();
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BallThread ballThread = new BallThread();
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ballItem.setTwin(ballThread);
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ballThread.setTwin(ballItem);
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ballThread.start();
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waiting();
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ballItem.click();
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waiting();
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ballItem.click();
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waiting();
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// exit
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ballThread.stopMe();
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}
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private static void waiting() throws Exception {
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Thread.sleep(2500);
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}
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}

‎twin/src/main/java/com/iluwatar/twin/BallItem.java‎

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package com.iluwatar.twin;
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/**
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* This class represents a Ball which extends {@link GameItem} and implements the logic for ball
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* item, like move and draw. It hold a reference of {@link BallThread} to delegate the suspend and
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* resume task.
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*/
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public class BallItem extends GameItem {
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private boolean isSuspended = false;
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private BallThread twin;
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public void setTwin(BallThread twin) {
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this.twin = twin;
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}
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@Override
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public void doDraw() {
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System.out.println("doDraw");
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}
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public void move() {
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System.out.println("move");
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}
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@Override
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public void click() {
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isSuspended = !isSuspended;
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if (isSuspended) {
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twin.suspendMe();
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} else {
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twin.resumeMe();
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}
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}
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}
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‎twin/src/main/java/com/iluwatar/twin/BallThread.java‎

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package com.iluwatar.twin;
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/**
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* This class is a UI thread for drawing the {@link BallItem}, and provide the method for suspend
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* and resume. It hold the reference of {@link BallItem} to delegate the draw task.
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*
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*/
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public class BallThread extends Thread {
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private BallItem twin;
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private volatile boolean isSuspended;
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private volatile boolean isRunning = true;
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public void setTwin(BallItem twin) {
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this.twin = twin;
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}
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public void run() {
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while (isRunning) {
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while (!isSuspended) {
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twin.draw();
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twin.move();
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try {
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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throw new RuntimeException(e);
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}
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}
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}
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}
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public void suspendMe() {
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isSuspended = true;
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System.out.println("Begin to suspend BallThread");
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}
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public void resumeMe() {
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isSuspended = false;
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System.out.println("Begin to resume BallThread");
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}
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public void stopMe() {
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this.isRunning = false;
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this.isSuspended = true;
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}
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}
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‎twin/src/main/java/com/iluwatar/twin/GameItem.java‎

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package com.iluwatar.twin;
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/**
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* GameItem is a common class which provides some common methods for game object.
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*/
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public abstract class GameItem {
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/**
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* Template method, do some common logic before draw
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*
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* @param other
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* @return
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*/
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public void draw() {
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System.out.println("draw");
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doDraw();
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}
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public abstract void doDraw();
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public abstract void click();
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}

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