Merge pull request #330 from mikulucky/master

Delegation Pattern

delegation/etc/delegation.ucls

@@ -0,0 +1,83 @@
+<?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">    
+    <position height="-1" width="-1" x="764" y="272"/>    
+    <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" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="511" y="270"/>    
+    <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="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">    
+    <position height="-1" width="-1" x="252" y="275"/>    
+    <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>  
+  <interface id="4" language="java" name="com.iluwatar.delegation.simple.Printer" project="delegation" 
+    file="/delegation/src/main/java/com/iluwatar/delegation/simple/Printer.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="524" y="89"/>    
+    <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>  
+  </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" 
+    corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="112" y="90"/>    
+    <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>  
+  <realization id="6">    
+    <end type="SOURCE" refId="2"/>    
+    <end type="TARGET" refId="4"/>  
+  </realization>  
+  <association id="7">    
+    <end type="SOURCE" refId="5" navigable="false">      
+      <attribute id="8" name="printer"/>      
+      <multiplicity id="9" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="4" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <realization id="10">    
+    <end type="SOURCE" refId="5"/>    
+    <end type="TARGET" refId="4"/>  
+  </realization>  
+  <realization id="11">    
+    <end type="SOURCE" refId="3"/>    
+    <end type="TARGET" refId="4"/>  
+  </realization>  
+  <realization id="12">    
+    <end type="SOURCE" refId="1"/>    
+    <end type="TARGET" refId="4"/>  
+  </realization>  
+  <classifier-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"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

delegation/index.md

@@ -0,0 +1,27 @@
+---
+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)

delegation/pom.xml

@@ -0,0 +1,27 @@
+<?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>

delegation/src/main/java/com/iluwatar/delegation/simple/App.java

@@ -0,0 +1,38 @@
+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);
+  }
+
+}

delegation/src/main/java/com/iluwatar/delegation/simple/Printer.java

@@ -0,0 +1,23 @@
+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);
+}

delegation/src/main/java/com/iluwatar/delegation/simple/PrinterController.java

@@ -0,0 +1,23 @@
+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);
+  }
+}

delegation/src/main/java/com/iluwatar/delegation/simple/printers/CanonPrinter.java

@@ -0,0 +1,21 @@
+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);
+  }
+
+}

delegation/src/main/java/com/iluwatar/delegation/simple/printers/EpsonPrinter.java

@@ -0,0 +1,21 @@
+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);
+  }
+
+}

delegation/src/main/java/com/iluwatar/delegation/simple/printers/HpPrinter.java

@@ -0,0 +1,21 @@
+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);
+  }
+
+}

delegation/src/test/java/com/iluwatar/delegation/simple/AppTest.java

@@ -0,0 +1,13 @@
+package com.iluwatar.delegation.simple;
+
+import org.junit.Test;
+
+public class AppTest {
+
+  @Test
+  public void test() {
+    String[] args = {};
+    App.main(args);
+  }
+
+}

delegation/src/test/java/com/iluwatar/delegation/simple/DelegateTest.java

@@ -0,0 +1,43 @@
+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());
+  }
+
+}

pom.xml

@@ -22,6 +22,7 @@
 		<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>