如何在Java中隐藏JFrame窗口上的默认最小化/最大化和关闭按钮？

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class CustomFrame extends JFrame {
    private JPanel titleBarPanel;
    private JLabel titleLabel;
    private JButton minimizeButton;
    private JButton maximizeButton;
    private JButton closeButton;

    public CustomFrame() {
        setUndecorated(true);

        titleBarPanel = new JPanel();
        titleBarPanel.setLayout(null);
        titleBarPanel.setBackground(Color.GRAY);

        titleLabel = new JLabel("Custom Frame");
        titleLabel.setForeground(Color.WHITE);
        titleLabel.setBounds(10, 5, 150, 20);

        minimizeButton = new JButton("-");
        minimizeButton.setBounds(170, 5, 20, 20);
        minimizeButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                setExtendedState(JFrame.ICONIFIED);
            }
        });

        maximizeButton = new JButton("□");
        maximizeButton.setBounds(195, 5, 20, 20);
        maximizeButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if (getExtendedState() == JFrame.MAXIMIZED_BOTH) {
                    setExtendedState(JFrame.NORMAL);
                } else {
                    setExtendedState(JFrame.MAXIMIZED_BOTH);
                }
            }
        });

        closeButton = new JButton("X");
        closeButton.setBounds(220, 5, 20, 20);
        closeButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                System.exit(0);
            }
        });

        titleBarPanel.add(titleLabel);
        titleBarPanel.add(minimizeButton);
        titleBarPanel.add(maximizeButton);
        titleBarPanel.add(closeButton);

        titleBarPanel.addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                getComponentAt(e.getPoint());
                Point origin = new Point(e.getXOnScreen(), e.getYOnScreen());
                SwingUtilities.convertPointFromScreen(origin, titleBarPanel);
                setLocation(origin);
            }
        });

        addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });

        setLayout(null);
        titleBarPanel.setBounds(0, 0, getWidth(), 30);
        add(titleBarPanel);
    }

    public static void main(String[] args) {
        CustomFrame frame = new CustomFrame();
        frame.setSize(400, 300);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}