public class Menu extends JPanel implements ActionListener {

protected JFrame frame;
JButton btnStart = new JButton();

public Menu(JFrame j) {

    super();
    frame = j;

    //creating and styling button

    btnStart.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(java.awt.event.ActionEvent evt) {
            StartGame();
        }
    });



}

private void StartGame(){
    frame.getContentPane().removeAll();
    frame.getContentPane().add(new Main(frame));
    frame.repaint();

}