JPanel repainting

public class Test
{
    public static void main(String args[])
    {
        JFrame f = new JFrame("test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            f.add(new Circle());
            f.pack();
            f.setVisible(true);
            f.setLocationRelativeTo(null);  //o מיקום החלון במרכז המסך
    }
}

public class Circle extends JPanel
{
    public JPanel()
    {
         setPreferredSize(new Dimension(500,500));
    }


    @override
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;

        g2d.drawOval(200, 200, 100, 100);

        //If you want to call it again
        //repaint();
    }

}