Add DCL and Initializing on demand holder idiom.

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

@@ -21,6 +21,15 @@ public class App {
 				.getInstance();
 		System.out.println("threadSafeIvoryTower1=" + threadSafeIvoryTower1);
 		System.out.println("threadSafeIvoryTower2=" + threadSafeIvoryTower2);
-
+		
+		InitializingOnDemandHolderIdiom demandHolderIdiom = InitializingOnDemandHolderIdiom.getInstance();
+		System.out.println(demandHolderIdiom);
+		InitializingOnDemandHolderIdiom demandHolderIdiom2 = InitializingOnDemandHolderIdiom.getInstance();
+		System.out.println(demandHolderIdiom2);
+		
+		ThreadSafeDoubleCheckLocking dcl1 = ThreadSafeDoubleCheckLocking.getInstance();
+		System.out.println(dcl1);
+		ThreadSafeDoubleCheckLocking dcl2 = ThreadSafeDoubleCheckLocking.getInstance();
+		System.out.println(dcl2);
 	}
 }

singleton/src/main/java/com/iluwatar/InitializingOnDemandHolderIdiom.java

@@ -0,0 +1,29 @@
+package com.iluwatar;
+
+import java.io.Serializable;
+
+/**
+ * 
+ * @author mortezaadi@gmail.com
+ *
+ */
+public class InitializingOnDemandHolderIdiom implements Serializable{
+
+	private static final long serialVersionUID = 1L;
+
+	private static class HelperHolder {
+		public static final InitializingOnDemandHolderIdiom INSTANCE = new InitializingOnDemandHolderIdiom();
+	}
+
+	public static InitializingOnDemandHolderIdiom getInstance() {
+		return HelperHolder.INSTANCE;
+	}
+
+	private InitializingOnDemandHolderIdiom() {
+	}
+
+	protected Object readResolve() {
+		return getInstance();
+	}
+
+}

singleton/src/main/java/com/iluwatar/ThreadSafeDoubleCheckLocking.java

@@ -0,0 +1,36 @@
+package com.iluwatar;
+
+/**
+ * Broken under Java 1.4.
+ * @author mortezaadi@gmail.com
+ *
+ */
+public class ThreadSafeDoubleCheckLocking {
+	
+	private static volatile ThreadSafeDoubleCheckLocking INSTANCE;
+
+	/**
+	 * private constructor to prevent client from instantiating.
+	 * 
+	 */
+	private ThreadSafeDoubleCheckLocking() {
+		//to prevent instantiating by Reflection call 
+		if(INSTANCE != null)
+			throw new IllegalStateException("Already initialized.");
+	}
+	
+	public static ThreadSafeDoubleCheckLocking getInstance() {
+		//local variable increases performance by 25 percent 
+		//Joshua Bloch "Effective Java, Second Edition", p. 283-284
+		ThreadSafeDoubleCheckLocking result = INSTANCE;
+		if (result == null) {
+			synchronized (ThreadSafeDoubleCheckLocking.class) {
+				result = INSTANCE;
+				if (result == null) {
+					INSTANCE = result = new ThreadSafeDoubleCheckLocking();
+				}
+			}
+		}
+		return result;
+	}
+}