Added Null Object pattern.

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);
+	}
+}