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Merge pull request #330 from mikulucky/master

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Delegation Pattern
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Ilkka Seppälä 2016-01-01 16:49:01 +02:00
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<?xml version="1.0" encoding="UTF-8"?>
<class-diagram version="1.1.9" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
realizations="true" associations="true" dependencies="false" nesting-relationships="true" router="FAN">
<class id="1" language="java" name="com.iluwatar.delegation.simple.printers.HpPrinter" project="delegation"
file="/delegation/src/main/java/com/iluwatar/delegation/simple/printers/HpPrinter.java" binary="false"
corner="BOTTOM_RIGHT">
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
sort-features="false" accessors="true" visibility="true">
<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="2" language="java" name="com.iluwatar.delegation.simple.printers.EpsonPrinter" project="delegation"
file="/delegation/src/main/java/com/iluwatar/delegation/simple/printers/EpsonPrinter.java" binary="false"
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<class id="3" language="java" name="com.iluwatar.delegation.simple.printers.CanonPrinter" project="delegation"
file="/delegation/src/main/java/com/iluwatar/delegation/simple/printers/CanonPrinter.java" binary="false"
corner="BOTTOM_RIGHT">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
<operations public="true" package="true" protected="true" private="true" static="true"/>
</display>
</class>
<interface id="4" language="java" name="com.iluwatar.delegation.simple.Printer" project="delegation"
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</interface>
<class id="5" language="java" name="com.iluwatar.delegation.simple.PrinterController" project="delegation"
file="/delegation/src/main/java/com/iluwatar/delegation/simple/PrinterController.java" binary="false"
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<association id="7">
<end type="SOURCE" refId="5" navigable="false">
<attribute id="8" name="printer"/>
<multiplicity id="9" minimum="0" maximum="1"/>
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<display labels="true" multiplicity="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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<association-display labels="true" multiplicity="true"/>
</class-diagram>

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---
layout: pattern
title: Delegation
folder: delegation
permalink: /patterns/delegation/
categories: Behavioral
tags:
- Java
- Difficulty-Beginner
---
**Also known as:** Proxy Pattern
**Intent:** It is a technique where an object expresses certain behavior to the outside but in
reality delegates responsibility for implementing that behaviour to an associated object.
![alt text](./etc/delegation.png "Delegate")
**Applicability:** Use the Delegate pattern in order to achieve the following
* Reduce the coupling of methods to their class
* Components that behave identically, but realize that this situation can change in the future.
**Credits**
* [Delegate Pattern: Wikipedia ](https://en.wikipedia.org/wiki/Delegation_pattern)
* [Proxy Pattern: Wikipedia ](https://en.wikipedia.org/wiki/Proxy_pattern)

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<?xml version="1.0" encoding="UTF-8"?>
<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>1.9.0-SNAPSHOT</version>
</parent>
<modelVersion>4.0.0</modelVersion>
<artifactId>delegation</artifactId>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.github.stefanbirkner</groupId>
<artifactId>system-rules</artifactId>
<scope>test</scope>
</dependency>
</dependencies>
</project>

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delegation/src/main/java/com/iluwatar/delegation/simple/App.java Normal file
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package com.iluwatar.delegation.simple;
import com.iluwatar.delegation.simple.printers.CanonPrinter;
import com.iluwatar.delegation.simple.printers.EpsonPrinter;
import com.iluwatar.delegation.simple.printers.HpPrinter;
/**
* The delegate pattern provides a mechanism to abstract away the implementation and control of the desired action.
* The class being called in this case {@link PrinterController} is not responsible for the actual desired action,
* but is actually delegated to a helper class either {@link CanonPrinter}, {@link EpsonPrinter} or {@link HpPrinter}.
* The consumer does not have or require knowledge of the actual class carrying out the action, only the
* container on which they are calling.
*
* In this example the delegates are {@link EpsonPrinter}, {@link HpPrinter} and {@link CanonPrinter} they all implement
* {@link Printer}. The {@link PrinterController} class also implements {@link Printer}. However neither provide the
* functionality of {@link Printer} by printing to the screen, they actually call upon the instance of {@link Printer}
* that they were instantiated with. Therefore delegating the behaviour to another class.
*/
public class App {
public static final String MESSAGE_TO_PRINT = "hello world";
/**
* Program entry point
*
* @param args command line args
*/
public static void main(String[] args) {
PrinterController hpPrinterController = new PrinterController(new HpPrinter());
PrinterController canonPrinterController = new PrinterController(new CanonPrinter());
PrinterController epsonPrinterController = new PrinterController(new EpsonPrinter());
hpPrinterController.print(MESSAGE_TO_PRINT);
canonPrinterController.print(MESSAGE_TO_PRINT);
epsonPrinterController.print(MESSAGE_TO_PRINT);
}
}

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delegation/src/main/java/com/iluwatar/delegation/simple/Printer.java Normal file
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package com.iluwatar.delegation.simple;
import com.iluwatar.delegation.simple.printers.CanonPrinter;
import com.iluwatar.delegation.simple.printers.EpsonPrinter;
import com.iluwatar.delegation.simple.printers.HpPrinter;
/**
* Interface that both the Controller and the Delegate will implement.
*
* @see CanonPrinter
* @see EpsonPrinter
* @see HpPrinter
*/
public interface Printer {
/**
* Method that takes a String to print to the screen. This will be implemented on both the
* controller and the delegate allowing the controller to call the same method on the delegate class.
*
* @param message to be printed to the screen
*/
void print(final String message);
}

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delegation/src/main/java/com/iluwatar/delegation/simple/PrinterController.java Normal file
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package com.iluwatar.delegation.simple;
public class PrinterController implements Printer {
private final Printer printer;
public PrinterController(Printer printer) {
this.printer = printer;
}
/**
* This method is implemented from {@link Printer} however instead on providing an
* implementation, it instead calls upon the class passed through the constructor. This is the delegate,
* hence the pattern. Therefore meaning that the caller does not care of the implementing class only the owning
* controller.
*
* @param message to be printed to the screen
*/
@Override
public void print(String message) {
printer.print(message);
}
}

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delegation/src/main/java/com/iluwatar/delegation/simple/printers/CanonPrinter.java Normal file
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package com.iluwatar.delegation.simple.printers;
import com.iluwatar.delegation.simple.Printer;
/**
* Specialised Implementation of {@link Printer} for a Canon Printer, in
* this case the message to be printed is appended to "Canon Printer : "
*
* @see Printer
*/
public class CanonPrinter implements Printer {
/**
* {@inheritDoc}
*/
@Override
public void print(String message) {
System.out.print("Canon Printer : " + message);
}
}

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delegation/src/main/java/com/iluwatar/delegation/simple/printers/EpsonPrinter.java Normal file
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package com.iluwatar.delegation.simple.printers;
import com.iluwatar.delegation.simple.Printer;
/**
* Specialised Implementation of {@link Printer} for a Epson Printer, in
* this case the message to be printed is appended to "Epson Printer : "
*
* @see Printer
*/
public class EpsonPrinter implements Printer {
/**
* {@inheritDoc}
*/
@Override
public void print(String message) {
System.out.print("Epson Printer : " + message);
}
}

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delegation/src/main/java/com/iluwatar/delegation/simple/printers/HpPrinter.java Normal file
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package com.iluwatar.delegation.simple.printers;
import com.iluwatar.delegation.simple.Printer;
/**
* Specialised Implementation of {@link Printer} for a HP Printer, in
* this case the message to be printed is appended to "HP Printer : "
*
* @see Printer
*/
public class HpPrinter implements Printer {
/**
* {@inheritDoc}
*/
@Override
public void print(String message) {
System.out.print("HP Printer : " + message);
}
}

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delegation/src/test/java/com/iluwatar/delegation/simple/AppTest.java Normal file
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package com.iluwatar.delegation.simple;
import org.junit.Test;
public class AppTest {
@Test
public void test() {
String[] args = {};
App.main(args);
}
}

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delegation/src/test/java/com/iluwatar/delegation/simple/DelegateTest.java Normal file
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package com.iluwatar.delegation.simple;
import com.iluwatar.delegation.simple.printers.CanonPrinter;
import com.iluwatar.delegation.simple.printers.EpsonPrinter;
import com.iluwatar.delegation.simple.printers.HpPrinter;
import org.junit.Rule;
import org.junit.Test;
import org.junit.contrib.java.lang.system.SystemOutRule;
import static org.junit.Assert.assertEquals;
public class DelegateTest {
private static final String MESSAGE = "Test Message Printed";
@Rule
public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();
@Test
public void testCanonPrinter() throws Exception {
PrinterController printerController = new PrinterController(new CanonPrinter());
printerController.print(MESSAGE);
assertEquals("Canon Printer : Test Message Printed", systemOutRule.getLog());
}
@Test
public void testHpPrinter() throws Exception {
PrinterController printerController = new PrinterController(new HpPrinter());
printerController.print(MESSAGE);
assertEquals("HP Printer : Test Message Printed", systemOutRule.getLog());
}
@Test
public void testEpsonPrinter() throws Exception {
PrinterController printerController = new PrinterController(new EpsonPrinter());
printerController.print(MESSAGE);
assertEquals("Epson Printer : Test Message Printed", systemOutRule.getLog());
}
}

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<camel.version>2.15.3</camel.version>
<log4j.version>1.2.17</log4j.version>
<guava.version>18.0</guava.version>
<systemrules.version>1.14.0</systemrules.version>
</properties>
<modules>
<module>abstract-factory</module>
@ -88,6 +89,7 @@
<module>reactor</module>
<module>caching</module>
<module>publish-subscribe</module>
<module>delegation</module>
</modules>
<dependencyManagement>
@ -154,6 +156,12 @@
<artifactId>guava</artifactId>
<version>${guava.version}</version>
</dependency>
<dependency>
<groupId>com.github.stefanbirkner</groupId>
<artifactId>system-rules</artifactId>
<version>${systemrules.version}</version>
<scope>test</scope>
</dependency>
</dependencies>
</dependencyManagement>