Drawing on aJPanel

[Djangolo]

I've built a GUI using Netbeans. The GUI includes a JPanel and I want to draw graphics on this panel. The instance of JPanel is called jPanel1 and is set to preferred size 604x310.

If I create a bit of code like this -

  public JPanel createContentPane () {
     JPanel redPanel = new JPanel();
        redPanel.setBackground(Color.red);
        redPanel.setLocation(10, 10);
        redPanel.setSize(50,50);
        return redPanel;
}

and then add this to jPanel1 like this -

 jPanel1.add(createContentPane());

the red square appears on the panel. Simple stuff.
But if I do this -

  public JPanel createContentPane (){
        
       JPanel mainPanel = new JPanel(new GridLayout(2, 4, 10, 30));
       mainPanel.setPreferredSize(new Dimension(604, 310));
        
        JPanel tempPanel = new JPanel();
        tempPanel.setBackground(Color.blue);
        tempPanel.setMinimumSize(new Dimension(50, 50));
        tempPanel.setMaximumSize(new Dimension(50, 50));
        tempPanel.setPreferredSize(new Dimension(50, 50));
        mainPanel.add(tempPanel);
        return mainPanel;

and then try and add it to jPanel1, nothing is displayed.

Can anyone tell me why this happens, 'cos I don't understand what's wrong.

Thanks for any help.

DJ

[hardwired]

import java.awt.*;
import javax.swing.*;

public class Test {
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setContentPane(getContentPane());
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    private static JPanel createContentPane() {
        JPanel redPanel = new JPanel();
        redPanel.setBackground(Color.red);
        redPanel.setLocation(10, 10);
        redPanel.setSize(50,50);
        return redPanel;
    }

    private static JPanel getContentPane(){
        // GridLayout tends to ignore the size hints,
        // creates a grid of the available space and
        // tries to size the components to fill the
        // space of the grid cells.
        JPanel mainPanel = new JPanel(new GridLayout(2, 4, 10, 30));
        mainPanel.setPreferredSize(new Dimension(604, 310));

        JPanel tempPanel = new JPanel();
        tempPanel.setBackground(Color.blue);
        tempPanel.setMinimumSize(new Dimension(50, 50));
        tempPanel.setMaximumSize(new Dimension(50, 50));
        tempPanel.setPreferredSize(new Dimension(50, 50));
        mainPanel.add(tempPanel);
        mainPanel.add(createContentPane());
        mainPanel.add(createContentPane());
        mainPanel.add(createContentPane());
        return mainPanel;
    }
}