设计模式之Singleton

 class Singleton {

     private Singleton() {
     }

     private static Singleton instance;

     // v0.1
 //    public static Singleton getInstance(){
 //        if(instance==null) instance=new Singleton();
 //        return instance;
 //    }

     // v0.2
 //    //Double-check singleton
 //    public static Singleton getInstance() {
 //        if(instance==null) {
 //            synchronized (Singleton.class) {
 //                if(instance==null) instance = new Singleton();
 //            }
 //        }
 //        return instance;
 //    }

     // v0.3
     private static class SingletonHolder{
         static final Singleton INSTANCE=new Singleton();
     }

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

 }