java单例模式（类只能创建唯一对象）

 //饿汉式
 class  Single
 {
    private static final Single s= new Single();
    private Single(){}
    public static Single getInstance()
       {
           return s;
       }
 }
 //懒汉式
 class  Single
 {
    private static Single s=null;
    private Single(){}
    public static Single getInstance()
  {
       if(s==null)//解决效率问题
    {
       synchronized(single.class)//解决线程安全问题
       {
          if(s==null)
          s=new Single();
          return s;
       }
    }
  }
 }