public class panel1 extends JPanel implements ActionListener(){
    private panel2 p2=new panel2();
    private JButton button;
    public panel1(){
    button=new JButton("open panel2");
    add(button,BorderLayout.BEFORE_FIRST_LINE);
    button.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent ae) {
            add(p2);
        }
    });
  }
}