How does one properly handle keypresses and repainting of a JComponent in the context of moving a ball around a screen?

Question

I thought I would try and write a program that would paint a ball, and when the arrow keys are pressed, would move the ball around the screen in the direction pressed. First

Answer 1

You can use the KeyboardFocusManager to listen for keystrokes of the arrow keys and update the position of the ball. Here is the reworked code:

class BallFrame extends JFrame {
    private static final int DEFAULT_WIDTH = 500;
    private static final int DEFAULT_HEIGHT = 500;
    final BallComponent ball = new BallComponent();

    public BallFrame() {
        super.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        super.setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
        super.setResizable(false);
        super.add(ball);
        super.setVisible(true);

        KeyboardFocusManager.getCurrentKeyboardFocusManager()
                .addKeyEventDispatcher(new KeyEventDispatcher() {
                    @Override
                    public boolean dispatchKeyEvent(KeyEvent e) {
                        if (e.getKeyCode() == KeyEvent.VK_UP)
                            ball.move(0, -10);
                        if (e.getKeyCode() == KeyEvent.VK_RIGHT)
                            ball.move(10, 0);
                        if (e.getKeyCode() == KeyEvent.VK_LEFT)
                            ball.move(-10, 0);
                        if (e.getKeyCode() == KeyEvent.VK_DOWN)
                            ball.move(0, 10);
                        return false;
                    }
                });

    }
}

And for BallComponent class:

static class BallComponent extends JComponent {
    private double x = 225;
    private double y = 225;
    private Ellipse2D.Double ellipse = new Ellipse2D.Double(x, y, 50, 50);

    public void move(int dX, int dY) {
        x += dX;
        y += dY;
        ellipse = new Ellipse2D.Double(x, y, 50, 50);
        repaint();
    }

    public void paintComponent(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;
        g2d.fill(ellipse);
    }
}