Switching JFrame's contentPane

Question

Hi folks,
I've had a problem trying to switch the contentPane of a JFrame. Basically, I'd created a class to manage my application, that at construction time sets up the application's main window ( JFrame mainWindow ), and then have a method to switch the contentPane
public void switchContentPane( JPanel newContent )  throws Exception {
  newContent.setOpaque( true );
  mainWindow.setContentPane( newContent );
}The problem was that every time the switchContentPane() were invoked, the GUI would be updated only if the window were resized. I've tried many things to solve this problem:
- call mainWindow.repaint() after setContentPane();
- call mainWindow.revalidate() after setContentPane();
- invoke switchContentPane using SwingUtilities.invokeLater();
None of this worked for me. The same behavior was ocurring. What worked for me was to invoke mainWindow.getRootPane().updateUI() after mainWindow.setContentPane(). This solution was just a guessing. I don't have the minor idea of why this works, adn would like to know if someone could explain this. I would really appreciate it.
Thanks,
Alexandre

Answer 1

I've tried many things to solve this problem:
- call mainWindow.repaint() after setContentPane();
- call mainWindow.revalidate() after setContentPane();
- invoke switchContentPane using SwingUtilities.invokeLater();
The solution is to call validate() and then repaint(), as with any time when you modify an on-screen component hierarchy. The code below works perfectly.
JFrame doesn't even have a revalidate() method so I'm not sure how you managed to try that one.
I haven't tried this, and I'm no Swing expert, but I haven't seen content panes being switched like that before.
It's a perfectly decent way of doing it, and I would suggest a better way - why have an arbitrarily-configured panel sitting there doing nothing except ruining the semantics of placing your content into a frame?
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import javax.swing.AbstractAction;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class TestCP implements Runnable
     private JFrame frame = null;
     private int swaps = 0;
     public static void main(String[] args)
          SwingUtilities.invokeLater(new TestCP());
     public void run()
          this.frame = new JFrame();
          updateContent();
          this.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          this.frame.pack();
          this.frame.setLocationRelativeTo(null);
          this.frame.setVisible(true);
     private void updateContent()
          JLabel l = new JLabel("Swaps: " + this.swaps, JLabel.CENTER);
          JButton b = new JButton(new Swap());
          JPanel p = new JPanel(new BorderLayout());
          p.add(l, BorderLayout.CENTER);
          p.add(b, BorderLayout.SOUTH);
          this.frame.setContentPane(p);
          this.frame.validate();
          this.frame.repaint();
     private class Swap extends AbstractAction
          private Swap()
               putValue(NAME, "Swap");
          public void actionPerformed(ActionEvent e)
               swaps ++;
               updateContent();
}