How can i add panel to jframe when form opned?

See more:

Java

JavaSE6

Eclipse

    public Container() {
    initGui();
}
void initGui(){

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 600, 600);

    contentPane = new JPanel();
    contentPane.setBackground(Color.LIGHT_GRAY);
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    GroupLayout gl_contentPane = new GroupLayout(contentPane);
    gl_contentPane.setHorizontalGroup(
        gl_contentPane.createParallelGroup(Alignment.LEADING)
            .addGap(0, 574, Short.MAX_VALUE)
    );
    gl_contentPane.setVerticalGroup(
        gl_contentPane.createParallelGroup(Alignment.LEADING)
            .addGap(0, 551, Short.MAX_VALUE)
    );

    contentPane.setLayout(gl_contentPane);

    this.addWindowListener(new WindowAdapter() {
        @Override
        public void windowOpened(WindowEvent ev) {
            formWindowOpened(ev);
        }
    });
    windowPosition();
}
private void formWindowOpened(WindowEvent e){
    panelMenu=new MainMenu();
    this.getContentPane().add(panelMenu);
    panelMenu.setVisible(true);

}

What I have tried:

I have a trouble with GUI in eclipse when I want JFrame display Jpanel when form opened. For Jpanel I create another that extends Jpanel and then I add that class to Main Fram. But it Does not display Jpanel. So What should I do?