Dear experts,
i'm a student from Italy and i'm approaching java language.
I have a curious problem with the Swing components (Java 1.5).
I've wrote this class for a customizable progress monitor frame (very
simple):

public class ProgressMonitor extends JFrame{
   /**
    *
    */
    private static final long serialVersionUID = 1L;
    public final static int ONE_SECOND = 1000;

   private JPanel panel;
   public static JProgressBar progressBar;
   public static int BAR_MAX;
   private JButton stopButton;

   public ProgressMonitor(String title) {
       super(title);
       panel = createPanel();

       setPreferredSize(new Dimension(310, 100));
       setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));
       setLocation(350, 200);
       add(panel, BorderLayout.CENTER);
       pack();
       setVisible(true);
       validate();
   }

   protected JPanel createPanel() {
       panel = new JPanel();
       stopButton = new JButton("Stop!");

       BAR_MAX = AnalisiParagrafi.getNumberOfParags() - 1;
       if(BAR_MAX == -1) BAR_MAX = 0;
       progressBar = new JProgressBar(0, BAR_MAX);
       progressBar.setPreferredSize(new Dimension(300, 20));
       progressBar.setValue(0);
       progressBar.setStringPainted(true);
       panel.add(progressBar, BorderLayout.CENTER);
       stopButton.setEnabled(true);
       stopButton.setVisible(true);
       panel.add(stopButton, BorderLayout.SOUTH);
        return panel;
   }

   /**
    * Create the GUI and show it.  For thread safety,
    * this method should be invoked from the
    * event-dispatching thread.
    */

    public void updateProgressBar(int i){

        Rectangle progressRect = progressBar.getBounds();
        progressRect.x = 0;
        progressRect.y = 0;
        progressBar.setValue(i);
        progressBar.paintImmediately(progressRect);
        if(i == BAR_MAX) this.setVisible(false);

    }

}

In this frame i've put a progress bar (updated by an index 'i') and a
Stop Button.
The problem is that, when i invoke this class during main program's
elaboration, it displays the main frame, the progress bar (an
appropriate external method udpate it) but there is no button in the
window!
I can't really understand why...the strange thing is that if i call
this class with this simple line of code (out of the main program):
ProgressMonitor pm = new ProgressMonitor("Program is running...");
...it works!

Does anybody know this problem?
Thanks in advance for all your help!

Sergio.