Java : singleton with example

A singleton is a class that can be instantiated once, and only once. The restriction on the singleton is that there can be only one instance of a singleton created by the Java Virtual Machine (JVM). Often in designing a system, we want to control how an object is used, and prevent others (ourselves included) from making copies of it or creating new instances. For example, a central configuration object that stores setup information should have one and one only instance - a global copy accessible from any part of the application, including any threads that are running.

public class SingletonObject {
   //marked as private so that class can't be directly instantiated
   private SingletonObject() {
      // no code req'd
   }

   //add synchronized keyword to prevent thread making a new copy
   public static synchronized SingletonObject getSingletonObject() {
      if (ref == null) {
         // it's ok, we can call this constructor
         ref = new SingletonObject();
      }
      return ref;
   }

   //overwrite default clone method so that this class can't be cloned
   public Object clone()
   throws CloneNotSupportedException {
      throw new CloneNotSupportedException();
      // that'll teach 'em
   }

   private static SingletonObject ref;
}

reference : http://www.javacoffeebreak.com/articles/designpatterns/index.html