我试图让按钮在按下时至少执行一些东西，BlueJ没有显示任何错误，但当我执行程序并尝试按下按钮时，什么也没有发生。我真的不知道为什么会这样。我会很感激任何帮助！

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
class MainMenu
{
JFrame frame= new JFrame();
JButton button = new JButton("Singleplayer");
JButton button2 = new JButton("Multiplayer");
MainMenu(){
    prepareGUI();
}  

public void prepareGUI(){
    frame.setTitle("Game");
    frame.getContentPane().setLayout(null);
    frame.add(button);
    frame.add(button2);
    button.setBounds(100,200,100,40);
    button2.setBounds(200,200,100,40);
    frame.setVisible(true);
    frame.setBounds(200,200,400,400);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public void setUpButtonListeners(){
    ActionListener buttonlistener = new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                frame.getContentPane().setBackground(Color.green);
                System.out.println("Singleplayer Selected");
            }
        };

    ActionListener buttonlistener2 = new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                frame.getContentPane().setBackground(Color.red);
                System.out.println("Multiplayer Selected");
            }
        };
    button.addActionListener(buttonlistener);
    button2.addActionListener(buttonlistener2);
}
public class MainClass {
    public void main(String args[] )
    { 
        new MainMenu();
    }
}
}