碰撞检测处理，
刷新代码：public void update(Graphics g) {
if(offScreenImage == null) {
offScreenImage = this.createImage(GAME_WIDTH, GAME_HEIGHT);
}
Graphics gOffScreen = offScreenImage.getGraphics();
Color c = gOffScreen.getColor();
gOffScreen.setColor(Color.GREEN);
gOffScreen.fillRect(0, 0, GAME_WIDTH, GAME_HEIGHT);
gOffScreen.setColor(c);
paint(gOffScreen);
g.drawImage(offScreenImage, 0, 0, null);
}
