解决Frame闪烁问题（双缓冲）

与paint方法同级，加上这段代码就OK了

private Image offScreenImage = null;
  public void update(Graphics g) {
    if(offScreenImage == null) {
      //这是游戏窗口的宽度和高度
      offScreenImage = this.createImage(Constant.GAME_WIDTH, Constant.GAME_HEIGHT);
    }

    Graphics gOff = offScreenImage.getGraphics();
    paint(gOff);
    g.drawImage(offScreenImage, 0, 0, null);
  }