Updated decorator implementation and class diagram. Added comments.

decorator/src/main/java/com/iluwatar/App.java

 package com.iluwatar;
 
+/**
+ * 
+ * Decorator pattern is more flexible alternative to
+ * subclassing. The decorator class implements the same
+ * interface as the target and uses composition to
+ * "decorate" calls to the target.
+ *
+ */
 public class App 
 {
     public static void main( String[] args )
     {
     	
     	System.out.println("A simple looking troll approaches.");
-    	Troll troll = new Troll();
+    	Hostile troll = new Troll();
     	troll.attack();
     	troll.fleeBattle();
     	
     	System.out.println("\nA smart looking troll surprises you.");
-    	Troll smart = new SmartTroll(new Troll());
+    	Hostile smart = new SmartTroll(new Troll());
     	smart.attack();
     	smart.fleeBattle();
     }

decorator/src/main/java/com/iluwatar/Hostile.java

+package com.iluwatar;
+
+public interface Hostile {
+
+	void attack();
+	void fleeBattle();
+	
+}

decorator/src/main/java/com/iluwatar/SmartTroll.java

 package com.iluwatar;
 
-public class SmartTroll extends Troll {
+public class SmartTroll implements Hostile {
 
-	private Troll decorated;
+	private Hostile decorated;
 
-	public SmartTroll(Troll decorated) {
+	public SmartTroll(Hostile decorated) {
 		this.decorated = decorated;
 	}
 	

decorator/src/main/java/com/iluwatar/Troll.java

 package com.iluwatar;
 
-public class Troll {
+public class Troll implements Hostile {
 
 	public void attack() {
 		System.out.println("The troll swings at you with a club!");