双缓存

private Image offImage = new BufferedImage(800, 700, BufferedImage.TYPE_INT_RGB);//在内存中new一张图片出来

public void paint(Graphics g) {
        Graphics offG = offImage.getGraphics();//得到内存图片的画笔
        super.paint(offG);
        for (Shape shape : shapes) {
            if (shape != null) {
                shape.draw(offG);//把图形画到内存图片上
            }
        }
        g.drawImage(offImage,0,0, null);//把内存中的图片画到屏幕上
    }