在基本Java动画中更改颜色

Question

我正在用Java处理图形（特别是通过Swing和AWT libs），我想在屏幕上移动一个圆圈（生成线条），当线条到达画布边界时，将x或y坐标设置回0。我对这部分代码没有问题。

然而，现在我试图在坐标均为0时清除屏幕（使用super.paint（图形））并更改颜色。屏幕清除工作正常，但我不能更改颜色。似乎，当坐标均为0时，在（0,0）位置的条纹下会出现指定颜色的椭圆形。然而，我希望条纹的颜色改变，直到坐标重置为（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);
        }
    }
}

Answer 1

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

用于此油漆的Graphics2D实例即将被处理。现在换颜色没用。

相反，可以更改组件本身的BG或FG颜色（即JPanel），然后在每次绘制之前设置图形对象的颜色。

Answer 2

请注意更改和评论：

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 paintComponent(Graphics g) { //for custom painting override paint componenet

        Graphics2D g2d = (Graphics2D) g;
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        if (x == 0 && y == 0)
        {
            // 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;
            }

            g2d.setColor(rand_color);
            g2d.fillRect(0, 0, getWidth(), getHeight()); //clear screen 
        }

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

private final Color[] colors = {Color.black, Color.red, Color.green, Color.blue, Color.gray};

要使代码更简洁，请执行以下操作：

@Override
public void paintComponent(Graphics g) { //for custom painting override paint componenet

    Graphics2D g2d = (Graphics2D) g;
    g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

    if (x == 0 && y == 0)
    {
        int rand_val = new Random().nextInt(colors.length);
        Color rand_color = colors[rand_val];
        g2d.setColor(rand_color);
        g2d.fillRect(0, 0, getWidth(), getHeight()); //clear screen
    }

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