Add DCL and Initializing on demand holder idiom.

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();
+	}
+
+}