Singleton Factory method

/**
 * An interface defining objects that can create Singleton
 * instances.
 */
public interface SingletonFactoryFunctor {
   /**
    * @return An instance of the Singleton.
    */
   public Singleton makeInstance();
}

public static synchronized Singleton getInstance()
{
   // since whole method is synchronized only 1 thread can
   // enter the following block ensuring only one instance
   // is ever created. however we pay a synchronization
   // penalty every time this method is called.
   if(mInstance==null) {
      mInstance=new Singleton();
   }
   return mInstance;
}

// no synchronization penalty, but penalty for eager init
private static Singleton mInstance = new Singleton();

public static Singleton getInstance()
{
    return mInstance;
}

private volatile static Singleton mInstance;

private Singleton(){}

public static Singleton getInstance()
{
   if(mInstance==null) {
      synchronized (Singleton.class) {
          // only pay synchronization penalty once
          if(mInstance==null){
              mInstance=new Singleton();
          }
      }
   }
   return mInstance;
}

public class Singleton {

private static Singleton mInstance;

private Singleton(){}

public static Singleton getInstance()
{
   if(mInstance==null){
      mInstance=new Singleton();
   }
return mInstance;
}

}