Question about java transperancy

I'm trying to make it so that my JPanel is transparent and shows the frame behind it (it's background is green). But when i start the program the JPanel is there and i see no green. Here is my code

public static void main (String [] args)

{

frame = new JFrame("FrameDemo");

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

JPanel panel = new JPanel(new GridBagLayout());

panel.setOpaque(false);

frame.getContentPane().add(panel, BorderLayout.CENTER);

frame.pack();

frame.setVisible(true);

frame.setBackground(Color.green);

}

[604 byte] By [P.F.a] at [2007-11-27 8:35:46]