Adding initialization on demand holder idiom.

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

@@ -27,22 +27,24 @@ package com.iluwatar.singleton;
  */
 public final class IvoryTower {
 
-  /**
-   * Static to class instance of the class.
-   */
-  private static final IvoryTower INSTANCE = new IvoryTower();
-
   /**
    * Private constructor so nobody can instantiate the class.
    */
   private IvoryTower() {}
 
+  private static class IvoryTowerHolder {
+    /**
+   * Static to class instance of the class.
+   */
+    private static final IvoryTower INSTANCE = new IvoryTower();
+  }
+
   /**
    * To be called by user to obtain instance of the class.
    *
    * @return instance of the singleton.
    */
   public static IvoryTower getInstance() {
-    return INSTANCE;
+    return IvoryTowerHolder.INSTANCE;
   }
 }