private Image offScreenImage = null;         public void update(Graphics g) {        if(offScreenImage == null)            offScreenImage = this.createImage(Constant.GAME_WIDTH,Constant.GAME_HEIGHT);//这是游戏窗口的宽度和高度                 Graphics gOff = offScreenImage.getGraphics();        paint(gOff);        g.drawImage(offScreenImage, 0, 0, null);