单例

public class Singleton{
    private Singleton(){}
    private static final Singleton instance =new Singleton();
    public Singleton getInstance(){
        return instance;
    }
}
public class Singleton{
    private Singleton(){}
    private static final class SingletonInstance(){
        private static final Singleton instance=new Singleton();
    }
    public Singleton getInstance(){
        return SingletonInstance.instance;
    }
}
public class Singleton{
    private Singleton();
    private volatile static  Singleton=null;
    public Singleton getIntance(){
      if(instance==null)
      {
        sychronized(Singleton.class)
        {
            if(instance==null){
            //return new Singleton();
            instance=new Singleton();
            }
         }
      }
      return instance;
    }
}