Proper Progress Bar Technique

Question

Hi,
I have been having a few strange issues lately and have done alot of research, but I am confused about the proper way to implement a progress bar that updates as a specific task is being performed. Here is some example code before I explain where my confusion lies...
public class Test extends JFrame {
public JFrame DisplayFrame;
public StatusBar statusBar;
public JButton button;
public static void main( String args[] ) {
     Test t = new Test();
public Test(){
     init();
public void init(){
     DisplayFrame = new JFrame( "Test Window" );
     DisplayFrame.setSize( 600, 600 );
     statusBar = new StatusBar();
     statusBar.setPreferredSize( new Dimension( 300, 100 ) );
     statusBar.setVisible(true);
     button = new JButton("go");
     button.addActionListener( new ActionListener() {
          public void actionPerformed( ActionEvent e ) {
          testProgressBar();     
     final Container cont = getContentPane();
     cont.removeAll();
     cont.add( button, BorderLayout.CENTER );
     cont.add( statusBar, BorderLayout.SOUTH );
     DisplayFrame.setContentPane( cont );          
     DisplayFrame.setVisible( true );
public void testProgressBar(){
     statusBar.progressBar.setMaximum(200);
     statusBar.progressBar.setValue(0);
     for ( int i = 0; i < 200; i++ ){
     for( int j=0; j<1000000; j++ ) {
     statusBar.progressBar.setValue(i);
     statusBar.progressBar.update(statusBar.progressBar.getGraphics());
class StatusBar extends JPanel {
public JProgressBar progressBar;
public StatusBar() {
     progressBar = new JProgressBar();
     progressBar.setVisible(true);
     progressBar.setStringPainted(true);
     add( progressBar );     
Now, when I run this code it seems to work fine, and as you would expect it to. The ProgressBar is updated at the appropriate intervals...However, I have read that updating a part of a swing GUI such as this, should not update at regular intervals but instead should hang until the Event Dispatch Thread has finished and releases its control.
I am confused as to why this actually works. I have a much more elaborate system with many progress bars where I have been updating them similar to the above example code. I have never had any problems whatsoever, but I am convinced, that I am just getting lucky, and this is a poor way to solve this problem. Anyone have any suggestions to help alleviate my confusion? I would really like to know why this works when it looks as if it should not update unless the progress bar was capable of interupting the EDT in order to facilitate its changes....

Answer 1

Your code works fine because all of it - excluding main() of course - is running on the EDT.
But what happens
- if you overlay your frame with another application?
- if you try to change the size of your frame?
The EDT will not be able to process events and your GUI will not be painted and/or layout until your loop finishes.
Only your progressBar will be painted because you're doing it explicitly.
If you want to do it right, take a look at SwingWorker or http://spin.sourceforge.net .
Sven