Swing - paintComponent method not being called

Question

i simply implemented class that inherits JPanel like below

public class Orpanel extends JPanel {

....
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D)g;
        g2d.setPaint(tpaint);
        g2d.fill(rect); 
    }
....
}

class Orpanel is loading image and adjusting it's own size.

here's question.

Calling JFrame's setContentpane(Orpanel's instance) makes it works fine but when i attach Orpanel to JFrame calling add() method instead of setContentpane (i know setcontentpane is not meaning attach.. anyway), it dosen't work.

finally figured out when i used add() method, Component that was added to JFrame doesn't call paintComponent() method. even though i call repaint() method manually, still paintComponent() method is not called.

did i miss something? any help will be appreciated!

thx in advance. Jaeyong shin.

---

i added extra code.

public Test(OwPanel op) 
{
    super();
    Dimension monitor = Toolkit.getDefaultToolkit().getScreenSize();
    op.setBackground(Color.white);
    this.setBackground(Color.white);        
    this.setBounds(monitor.width / 2 - 200 , monitor.height / 2 - 200, 400, 400);       
    this.setDefaultCloseOperation(EXIT_ON_CLOSE);
    this.setTitle("test");      
    this.setLayout(null);
    this.getContentPane().add(op);
    //this.setContentPane(op);
    this.setVisible(true);
    this.validate();
}

    public static void main(String[] args) {
    // TODO Auto-generated method stub

    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            OwPanel op = new OwPanel("d:\\java_workplace\\img\\img1.jpg", 100, 100);
            OwLabel ol = new OwLabel("d:\\java_workplace\\img\\img2.jpg", 300, 50);
            Test tst =  new Test(op);
            tst.add(ol);
        }
    });

still doesn't work if setContentpane() method replaced to getContentpane().add(). don't be confused. Owpanel and Orpanel is same :)