java swing panel问题

Question

我在将面板添加到主框架并立即隐藏它时遇到了问题，只在按下按钮时才使其可见。这是我的密码。寻找任何关于问题所在的见解。此外，我试图添加到面板上的标签也没有出现

/*
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 * and open the template in the editor.
 */
package cis2430_a4;


import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JMenuBar;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.GridLayout;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
/**
 *
 * @author Tristan
 */
public class MainWindow extends JFrame implements ActionListener{
    public static final int WIDTH = 600;
    public static final int HEIGHT = 700;

    private JPanel addPanel;

public MainWindow()
{
    super("Day Planner");
    setSize(WIDTH, HEIGHT);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLayout(new BorderLayout());


    JLabel intro1 = new JLabel("Welcome to your Day Planner", JLabel.CENTER);
    add(intro1, BorderLayout.NORTH);

    JLabel intro2 = new JLabel("Please choose an option from the menu bar above.", JLabel.CENTER);
    add(intro2, BorderLayout.CENTER);

    JMenu commands = new JMenu("Commands");

    JMenuItem addOption = new JMenuItem("Add");
    addOption.addActionListener(this);
    commands.add(addOption);

    JMenuItem searchOption = new JMenuItem("Search");
    searchOption.addActionListener(this);
    commands.add(searchOption);

    JMenuBar menuBar = new JMenuBar();
    menuBar.add(commands);
    setJMenuBar(menuBar);

    JButton button = new JButton("Add");
    button.addActionListener(this);
    add(button, BorderLayout.SOUTH);

    //add panel
    addPanel = new JPanel();
    addPanel.setLayout(new BorderLayout());
    addPanel.setSize(600,400);
    addPanel.setBackground(Color.CYAN);
    addPanel.add(new JLabel("add panel"), BorderLayout.CENTER);
    add(addPanel, BorderLayout.CENTER);
    addPanel.setVisible(false);


}

 @Override
 public void actionPerformed(ActionEvent ae)
 {
     /*String menuChoice = ae.getActionCommand();

     if (menuChoice.equals("Add")){
         addPanel.setVisible(true);
     }*/
     add(addPanel);
     //addPanel.setVisible(true);
 }
}