循环跳出--只打印 x = 0 一次；

 1     public static void main(String[] args)
 2     {
 3         System.out.println("main");
 4         w: for (int x = 0; x < 3; x++)
 5         {
 6             for (int y = 0; y < 4; y++)
 7             {
 8                 System.out.println("x=" + x);
 9                 break w;
10             }
11         }
12     }