How to make an immutable singleton in Java?

Question

An immutable object is initialized by its constuctor only, while a singleton is instantiated by a static method. How to make an immutable singleton in Java?

Answer 1

The solution pointed out by Sean is a good way of initializing singletons if their creation is not expensive. If you want to "lazy loading" capability, look into the initialization on demand holder idiom.

 // from wikipedia entry
 public class Singleton {

   // Private constructor prevents instantiation from other classes
   private Singleton() {
   }

   /**
    * SingletonHolder is loaded on the first execution of Singleton.getInstance() 
    * or the first access to SingletonHolder.INSTANCE, not before.
    */
   private static class SingletonHolder { 
     public static final Singleton INSTANCE = new Singleton();
   }

   public static Singleton getInstance() {
     return SingletonHolder.INSTANCE;
   }

 }