Singleton Design Pattern.

When to use Singleton: Singleton is used there must be one instance of the class. And it must be accessable to clients from global point.

Example usage of Singleton: Logger object, Configuration object, Cache object, Properties Object, Factory methods are used Single ton, etc..

Problems of singleton:      
          1) Multi-thread safe. ( method level it self)
          2) Lazy installation using double  locking machanism, (Synchronized inside the getInstalnce() method)
          3) Early installtion method.
          4) Serialization. (serializable(), readResolve())