更改基本Java动画中的颜色

Question

我正在弄乱Java中的图形（特别是通过Swing和AWT库），并且想在屏幕上移动一个圆圈（画线），并且当x或y坐标设置为0时，线击中了画布的边界。我对这部分代码没有问题。

但是，现在我试图在坐标均为0（使用super.paint(Graphics)）时清除屏幕并更改颜色。屏幕清除效果很好，但是我无法更改颜色。看起来，当坐标都为0时，指定颜色的椭圆形出现在[0，0]位置的条纹。但是，我希望条纹的颜色可以更改颜色，直到将坐标重置为（0，0）。这是我的代码：

@SuppressWarnings("serial")
public class Game extends JPanel
{
    static final int SCREEN_X = 300;
    static final int SCREEN_Y = 400;
    int x = 0;
    int y = 0;

    private void moveBall()
    {
        x += 1;
        y += 1;

        if (x >= SCREEN_X) x = 0;
        if (y >= SCREEN_Y) y = 0;
    }

    @Override
    public void paint(Graphics g)
    {
        Graphics2D g2d = (Graphics2D) g;

        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        if (x == 0 && y == 0)
        {
            super.paint(g);

            // Generate random value in [0, 5)
            int rand_val = (new Random()).nextInt(5);

            // Init random color variable
            Color rand_color = Color.black;

            // Pick a random color
            switch (rand_val)
            {
            case 0:
                rand_color = Color.black;
                break;

            case 1:
                rand_color = Color.red;
                break;

            case 2:
                rand_color = Color.green;
                break;

            case 3:
                rand_color = Color.blue;
                break;

            case 4:
                rand_color = Color.gray;
                break;
            }

            // I'm assuming I need to do something more here?
            // This is likely where my mistake is...
            g2d.setColor(rand_color);
        }

        g2d.fillOval(x, y, 30, 30);
    }

    public static void main(String[] args) throws InterruptedException
    {
        JFrame frame = new JFrame("Mini Tennis");
        Game game = new Game();

        frame.add(game);
        frame.setSize(SCREEN_X, SCREEN_Y);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        while (true)
        {
            game.moveBall();
            game.repaint();
            Thread.sleep(10);
        }
    }
}

我正在弄乱Java中的图形（特别是通过Swing和AWT库），并想在屏幕上移动一个圆（画线），并在该行将x或y坐标设置回0时... 

Answer 1

// I'm assuming I need to do something more here?
// This is likely where my mistake is...
g2d.setColor(rand_color);