如何用箭头键移动圆？

Question

我想使用左右箭头键在黑色背景上移动白色圆圈。 KeyListener正在运行（使用System.out.print），但是当按下键时圆圈不会移动。我认为这与x和y位置（xPos，yPos）有关。

private static final int SPEED = 50, BALL_SIZE = 50;
private int dx;
private int xPos, yPos;

public Background() {

    addKeyListener(this);

    setBackground(Color.BLACK);

    // the starting point of ball
    xPos = 100; 
    yPos = 700;

    dx = SPEED;
    }

@Override
public void actionPerformed(ActionEvent e) {
    // screen size
    int width = getWidth();

    xPos += dx;

    // boundaries
    if (xPos < 100) {
        xPos = 100;
        dx = SPEED;
    }
    else if (xPos > width - BALL_SIZE) {
        xPos = width - BALL_SIZE;
        dx = -SPEED;
    }
    repaint();
}

@Override
public void keyPressed(KeyEvent e) {

    if (e.getKeyCode() == KeyEvent.VK_RIGHT) { // right arrow key
        dx++;
    }
    else if (e.getKeyCode() == KeyEvent.VK_LEFT) { // left arrow key
        dx--;
    }
    else if (e.getKeyCode() == KeyEvent.VK_SPACE) { // space bar
        // shoot laser!
    }

    System.out.println("After");
}

public void paintComponent(Graphics g) {
    super.paintComponent(g);

    g.setColor(Color.WHITE);
    g.fillOval(xPos, yPos, BALL_SIZE, BALL_SIZE);
}