Null object as a singleton #50

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

@@ -18,14 +18,14 @@ public class App
     	Node root = new NodeImpl("1",
     			new NodeImpl("11",
     					new NodeImpl("111",
-    							new NullNode(),
-    							new NullNode()),
-    					new NullNode()), 
+    							NullNode.getInstance(),
+    							NullNode.getInstance()),
+    					NullNode.getInstance()), 
     			new NodeImpl("12",
-    					new NullNode(),
+    					NullNode.getInstance(),
     					new NodeImpl("122",
-    							new NullNode(),
-    							new NullNode())));
+    							NullNode.getInstance(),
+    							NullNode.getInstance())));
 
     	root.walk();
     }

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

@@ -1,33 +1,44 @@
-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() {
-	}
-}
+package com.iluwatar;
+
+/**
+ * 
+ * Null Object implementation for binary tree node.
+ * 
+ * Implemented as Singleton, since all the NullNodes are the same.
+ *
+ */
+public class NullNode implements Node {
+
+	private static NullNode instance = new NullNode();
+	
+	private NullNode() {
+	}
+	
+	public static NullNode getInstance() {
+		return instance;
+	}
+	
+	@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() {
+	}
+}