Newbie: Update of window

Hi!

I am new to Java and Swing. I have tried to search the net (including
comp.lang.java.gui FAQ), but I cannot figure out what is wrong with
the following test of swing. The window only updates when I am
changing the size of the window. How do I get the program to update
the window all by itself?

The program genereates a random number of lines, and is adding a
random number of lines when pu****ng a button.

Thanks in advance for any help!

public class Test implements ActionListener {

    Box box;
    int lines;
    Random generator = new Random();

    Test() {
        JFrame jfrm = new JFrame("Random Lines Test");
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        box = Box.createVerticalBox();
        int random = generator.nextInt(9)+1;
        for (int i = 1; i <= random; i++) {
            box.add(new JLabel("Line number " + i));
        }
        lines = random;
        JButton jbRandom = new JButton("Add a random (1-9) number of
lines");
        jbRandom.addActionListener(this);
        jfrm.add(box);
        jfrm.add(jbRandom, BorderLayout.SOUTH);
        jfrm.pack();
        jfrm.setExtendedState(Frame.MAXIMIZED_BOTH);
        jfrm.setVisible(true);
    }

    public void actionPerformed(ActionEvent ae) {
        int random = generator.nextInt(9)+1;
        for (int i = lines; i < lines+random; i++) {
            box.add(new JLabel("Line number " + i));
        }
        lines+=random;
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new Test();
            }
        });
    }

}