Add DCL and Initializing on demand holder idiom.

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;
+	}
+}