Question

Why this code never prints “Hello2” ?

public class Test4 {

    public static void main(String[] args) {
        JFrame f = new JFrame();
        JPanel p = new JPanel();
        f.getContentPane().add(p);

        JLabel x = new JLabel("Hello");
        p.add(x);

        p.addComponentListener(new ComponentListener() {

            public void componentResized(ComponentEvent arg0) {
                 System.err.println("Hello1");
            }

            public void componentMoved(ComponentEvent arg0) {
            }

            public void componentShown(ComponentEvent arg0) {
                System.err.println("Hello2");
            }

            public void componentHidden(ComponentEvent arg0) {
            }
        });

        f.setVisible(true);
        f.pack();
    }
}

Answer 1

I would guess that it’s called when the visibility state of the actual object changes.
in this case, you change the visibility of the Frame, not of the Panel.
(by default, Frame starts hidden, but panels are visible)
try to add the listener to the frame.