从JFrame关闭JPanel窗口[Java]

Question

So, i want to make a Menu screen with a JPanel, and I got this to work, but when I press the Start button, it does not close the Menu Window, and it just makes a new window, how do I either, keep it on the same window, without closing/opening the menu window, OR I would like to close the menu window and open up the Game window(JPanel), when i press the start button.

Here is the MainClass.java

    package bombermangame;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class MainClass extends JFrame{
    private static final long serialVersionUID = 1L;

    public static int WIDTH = 870, HEIGHT = 800;
    public static JPanel menu = new Menu();
    public static Listener keys = new Listener();

    public MainClass(){
        setContentPane(menu);
        pack();
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setTitle("BomberMan V0.3");
        setSize(WIDTH, HEIGHT);
        setLocationRelativeTo(null);
        setVisible(true);
    }

    public static void main(String[] args) {
        new MainClass();
    }
}

Here is the Menu.java class

 package bombermangame;

    import java.awt.Color;
    import java.awt.Dimension;
    import java.awt.Graphics;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import java.util.Timer;
    import java.util.TimerTask;

    import javax.swing.JButton;
    import javax.swing.JFrame;
    import javax.swing.JPanel;

    public class Menu extends JPanel implements ActionListener {
        private static final long serialVersionUID = 1L;

        private JButton startButton = new JButton("Play");

        private int x = 0, y = 500;

        private boolean down = false;
        private boolean up = true;

        private Timer timer = new Timer();

        public Menu() {
            setBackground(Color.blue);
            startButton = new JButton("Start");
            startButton.setBounds(0,0, 100, 40);
            startButton.setPreferredSize(new Dimension(100, 40));
            startButton.addActionListener(this);
            startButton.setFocusPainted(true);
            this.add(startButton);


        public void actionPerformed(ActionEvent ae) {
            Object a = ae.getSource();
            Game game = new Game();
            MainClass frm = new MainClass();
            Listener keys = new Listener();

            if (a == startButton) {
                timer.cancel();
                frm.getContentPane().remove(new Menu());
                frm.addKeyListener(keys);
                frm.setContentPane(game);
                frm.revalidate();
                frm.repaint();
                game.setBackground(Color.BLACK);
                game.setDoubleBuffered(true);
                game.setBounds(0, 0, WIDTH, HEIGHT);
                Game.running = true;
            }
        }

    }

EDIT: Thanks to the help of @whiskeyspider I learned that I made 2 frames and did not reference them right. But now that I got that solved, there is a problem with my Listener, when I fixed this, my Jpanel won't work with my Listener. I have tried adding the Listener straight to my Game JPanel and to my MainClass JFrame, but neither will work.

Here is some of my Menu class,

    public void actionPerformed(ActionEvent ae) {
    Object a = ae.getSource();
    JPanel game = new Game();
    Listener keys = new Listener();

    if (a == startButton) {
        timer.cancel();
        MainClass.frame.getContentPane().remove(this);
        MainClass.frame.setContentPane(game);
        MainClass.frame.addKeyListener(keys);
        game.addKeyListener(keys);
        game.setBackground(Color.BLACK);
        game.setDoubleBuffered(true);
        game.setBounds(0, 0, WIDTH, HEIGHT);
        Game.running = true;
    }
}

Answer 1

You created a MainClass here:

public static void main(String[] args) {
    new MainClass();
}

... and again here...

public void actionPerformed(ActionEvent ae) {
    Object a = ae.getSource();
    JPanel game = new Game();
    JFrame frm = new MainClass();

Then when you try to remove the Menu, instead of giving it a reference to the existing Menu, you created a new one:

frm.getContentPane().remove(new Menu());

You need to rethink your design a little and make sure you are referencing the right (already existing) objects. That is, you are creating new objects when you be referencing existing ones.