循环跳出--只打印 x = 0 一次；

    public static void main(String[] args)
    {
        System.out.println("main");
        w: for (int x = 0; x < 3; x++)
        {
            for (int y = 0; y < 4; y++)
            {
                System.out.println("x=" + x);
                break w;
            }
        }
    }