JComponent size issue

Question

I have a JComponent subclass that I am using to draw shapes onto my screen. In the constructor, I am trying to set ballX and ballY to half of the X and Y size values of the JComponent, and I think I am doing it wrong. I have looked this up a lot now, and cannot find a remedy. The code is as follows. Please bear in mind that this is my first real Swing/Graphics2D venture.

public class PongCanvas extends JComponent {
//Vars to hold XY values and Dimension values.

    private int batXDim, batYDim;
    private int b1X, b1Y;
    private int b2X, b2Y;
    private int ballRad, ballX, ballY;

    public PongCanvas() {//Instantiate vars.
        batXDim = 20;
        batYDim = 100;

        b1X = 0;
        b1Y = 0;

        b2X = 0;
        b2Y = 0;

        ballRad = 20;
        ballX = getWidth() / 2;
        ballY = getHeight() / 2;
    }

    public void paint(Graphics g) {//Main paint Method.
        //Cast Graphics to Graphics2D.
        Graphics2D g2 = (Graphics2D) g;
        //Enable antialiasing.
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        //Draw background.
        g2.setPaint(Color.black);
        g2.fillRect(0, 0, getWidth(), getHeight());
        //Draw ball.
        g2.setPaint(Color.white);
        g2.fillOval(ballX, ballY, ballRad, ballRad);
        //Draw bat 1.
        g2.fillRect(b1X, b1Y, batXDim, batYDim);
        //Draw bat 2.
        g2.fillRect(b2X, b2Y, batXDim, batYDim);
    }
}

Answer 1

Override getPreferredSize() in your JComponent to return your preferred size, and start with half the width and height of that Dimension. To the same end, this KineticModel invokes setPreferredSize() in DisplayPanel.

Addendum: By way of explanation, your current approach fails because the results from getWidth() and getHeight() are invalid until validate() has been called on the enclosing container, typically as the result of pack().

Answer 2

I agree with trashgod's answer. (+1)

Move the ballX and ballY in the paintComponent(g) like this

if (ballX==0 && ballY==0) {
    ballX = getWidth()/2;
    ballY = getHeight()/2;
}