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;
      }  
   }   
 }
}