写一个懒汉式延迟加载单例设计示例（面试）

必背

class Single
{
  private static Single s =null;
  private Single(){}

  public static Single getInstance()
  {
    if(s==null)
    {
      synchronized(Single.class)
      {
        if(s==null)
          --->A
          s = new Single();
      }
    }
    return s;
  }
}