Added Null Object pattern.

null-object/etc/test.ucls

@@ -0,0 +1,61 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true" 
+  realizations="true" associations="true" dependencies="false" nesting-relationships="true">  
+  <interface id="1" language="java" name="com.iluwatar.Node" project="null-object" 
+    file="/null-object/src/main/java/com/iluwatar/Node.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="535" y="139"/>    
+    <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="2" language="java" name="com.iluwatar.NodeImpl" project="null-object" 
+    file="/null-object/src/main/java/com/iluwatar/NodeImpl.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="857" y="348"/>    
+    <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.NullNode" project="null-object" 
+    file="/null-object/src/main/java/com/iluwatar/NullNode.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="535" y="408"/>    
+    <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>  
+  <association id="4">    
+    <end type="SOURCE" refId="2" navigable="false">      
+      <attribute id="5" name="left"/>      
+      <multiplicity id="6" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="1" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="7">    
+    <end type="SOURCE" refId="2" navigable="false">      
+      <attribute id="8" name="right"/>      
+      <multiplicity id="9" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="1" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <realization id="10">    
+    <end type="SOURCE" refId="2"/>    
+    <end type="TARGET" refId="1"/>  
+  </realization>  
+  <realization id="11">    
+    <end type="SOURCE" refId="3"/>    
+    <end type="TARGET" refId="1"/>  
+  </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>

null-object/pom.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<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"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>null-object</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

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

@@ -0,0 +1,32 @@
+package com.iluwatar;
+
+/**
+ * 
+ * Null Object pattern replaces null values with neutral objects.
+ * Many times this simplifies algorithms since no extra null checks
+ * are needed.
+ * 
+ * In this example we build a binary tree where the nodes are either
+ * normal or Null Objects. No null values are used in the tree making
+ * the traversal easy.
+ *
+ */
+public class App 
+{
+    public static void main( String[] args ) {
+    	
+    	Node root = new NodeImpl("1",
+    			new NodeImpl("11",
+    					new NodeImpl("111",
+    							new NullNode(),
+    							new NullNode()),
+    					new NullNode()), 
+    			new NodeImpl("12",
+    					new NullNode(),
+    					new NodeImpl("122",
+    							new NullNode(),
+    							new NullNode())));
+
+    	root.walk();
+    }
+}

null-object/src/main/java/com/iluwatar/Node.java

@@ -0,0 +1,15 @@
+package com.iluwatar;
+
+/**
+ * 
+ * Interface for binary tree node.
+ *
+ */
+public interface Node {
+
+	String getName();
+	int getTreeSize();
+	Node getLeft();
+	Node getRight();
+	void walk();
+}

null-object/src/main/java/com/iluwatar/NodeImpl.java

@@ -0,0 +1,50 @@
+package com.iluwatar;
+
+/**
+ * 
+ * Implementation for binary tree's normal nodes.
+ *
+ */
+public class NodeImpl implements Node {
+
+	private final String name;
+	private final Node left;
+	private final Node right;
+	
+	public NodeImpl(String name, Node left, Node right) {
+		this.name = name;
+		this.left = left;
+		this.right = right;
+	}
+	
+	@Override
+	public int getTreeSize() {
+		return 1 + left.getTreeSize() + right.getTreeSize();
+	}
+
+	@Override
+	public Node getLeft() {
+		return left;
+	}
+
+	@Override
+	public Node getRight() {
+		return right;
+	}
+
+	@Override
+	public String getName() {
+		return name;
+	}
+
+	@Override
+	public void walk() {
+		System.out.println(name);
+		if (left.getTreeSize() > 0) {
+			left.walk();
+		}
+		if (right.getTreeSize() > 0) {
+			right.walk();
+		}
+	}
+}

null-object/src/main/java/com/iluwatar/NullNode.java

@@ -0,0 +1,33 @@
+package com.iluwatar;
+
+/**
+ * 
+ * Null Object implementation for binary tree node.
+ *
+ */
+public class NullNode implements Node {
+
+	@Override
+	public int getTreeSize() {
+		return 0;
+	}
+
+	@Override
+	public Node getLeft() {
+		return null;
+	}
+
+	@Override
+	public Node getRight() {
+		return null;
+	}
+
+	@Override
+	public String getName() {
+		return null;
+	}
+
+	@Override
+	public void walk() {
+	}
+}

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

@@ -0,0 +1,12 @@
+package com.iluwatar;
+
+import org.junit.Test;
+
+public class AppTest {
+
+	@Test
+	public void test() {
+		String[] args = {};
+		App.main(args);
+	}
+}

pom.xml

@@ -1,6 +1,5 @@
 <?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">
+<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">
 	<modelVersion>4.0.0</modelVersion>
 
 	<groupId>com.iluwatar</groupId>
@@ -39,7 +38,8 @@
 		<module>double-checked-locking</module>
 		<module>servant</module>
 		<module>service-locator</module>
-	</modules>
+    <module>null-object</module>
+  </modules>
 
 	<dependencyManagement>
 		<dependencies>