Funny JFrame behaviour

[Steffen Reinhard]

Hi everybody!

I wrote some code overwriting JPanel's paintComponent() method and put it in a convenience JFrame.

    
package toyz.lotto.gui;
 
import gui.containers.BasicWindowMonitor;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.geom.Rectangle2D;
import javax.swing.JPanel;
 
/**
 *
 * @author vfriedmann
 */
public class ScalableLottoPanelImage extends JPanel
{
    public static void main (String[] args)
    {
        ScalableLottoPanelImage lottoimage = new ScalableLottoPanelImage();
        
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.gridwidth = 1;
        gbc.gridheight = 1;
        BasicWindowMonitor monitor = new BasicWindowMonitor("Testing " + lottoimage.getClass ().getName ());
        monitor.getContentPane ().add (lottoimage, new GridBagConstraints());
    }
    
    public ScalableLottoPanelImage ()
    {
        setSize(200,200);
        setMinimumSize (getSize());
        setMaximumSize (getSize ());
        setPreferredSize (getSize ());
        
    }
    
    protected void paintComponent (Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setPaint (Color.WHITE);
        Rectangle2D rect = new Rectangle2D.Double(0D, 0D, (double)getWidth () -10, (double)getHeight ()-10);
        g2.fill (rect);
        g2.setPaint (Color.RED);
        
        Rectangle2D field = new Rectangle2D.Double(2d, 2d, 25d, 25d);
        g2.draw (field);
        
    }
    
}
 

When I run it the Rectangle drawn in paintComponent() is sometimes shown and sometimes it isn't. If it isn't I have to force a repaint by changing JFrame's size and then it is shown ... bizarre! Any Ideas, anybody?

Thanks in advance for any hint on how to solve this problem.

[Craig Wood]

When you add a component to a container after it (s top-level container) has been realized
you have to let it know so it can do a new layout. We usually do this with the Container
method validate or the JComponent method revalidate. Sometimes you may need
to follow this (validation) with a call to repaint, but not always.

import java.awt.*;
import java.awt.geom.Rectangle2D;
import javax.swing.*;
 
public class SLPI extends JPanel
{
    public static void main (String[] args)
    {
        SLPI lottoimage = new SLPI();
        BasicWindowMonitor monitor = new BasicWindowMonitor("Testing " +
                                             lottoimage.getClass().getName());
        monitor.getContentPane().add(lottoimage, new GridBagConstraints());
        monitor.validate();
    }
 
    public SLPI()
    {
        setPreferredSize(new Dimension(200,200));
    }
 
    protected void paintComponent (Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setPaint (Color.WHITE);
        Rectangle2D rect = new Rectangle2D.Double(0D, 0D, (double)getWidth () -10,
                                                          (double)getHeight ()-10);
        g2.fill (rect);
        g2.setPaint (Color.RED);
        Rectangle2D field = new Rectangle2D.Double(2d, 2d, 25d, 25d);
        g2.draw (field);
    }
}
 
class BasicWindowMonitor extends JFrame
{
    BasicWindowMonitor(String id)
    {
        super(id);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        getContentPane().setLayout(new GridBagLayout());
        setSize(300,300);
        setLocation(200,200);
        setVisible(true);
    }
}