问Java swing方法setLayout null不起作用

EN

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

class MenuPanel extends JPanel {
    JButton setTextColor;
    JSlider setMobNumber;
    MenuPanel() {
        setLayout(null);
        setBackground(Color.LIGHT_GRAY);

        setTextColor = new JButton("Change Text Color");
        setMobNumber = new JSlider(1, 10);

        setTextColor.setBounds(10, 40, 20, 10);
        setMobNumber.setBounds(10, 80, 20, 10);

        add(setTextColor);
        add(setMobNumber);

        setTextColor.setFocusable(false);
        setMobNumber.setFocusable(false);
    }
}

class GameBoard extends JFrame {
    MenuPanel menuPanel = null;
    GameBoard() {
        setTitle("MyGame");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        menuPanel = new MenuPanel();
        add(menuPanel, BorderLayout.WEST);

        setSize(600, 500);
        setVisible(true);
    }
}

public class MyGame {
    public static void main(String args[]) {
        new GameBoard();
    }
}