JCheckBox自动取消选择

Question

当我选择JCheckBox时，它会自动取消选择...

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

public class Math extends JFrame 
{
    private JPanel panel2 = new JPanel();
    private JCheckBox cb = new JCheckBox("Record Answers");

    //Constructor
    public Math()
    {
        setSize(300,300);
        setTitle("Math");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        panel2.add(cb);

        //add ActionListners
        cb.addItemListener(new listenerCb());

        add(panel2, BorderLayout.SOUTH);            
        setVisible(true);
    }

    //itemListner for cb
    public  class listenerCb implements ItemListener
    {
        public void itemStateChanged(ItemEvent e)
        {       
            if(cb.isSelected())
            {
                JOptionPane.showMessageDialog(null,"Example");

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

    }

 }

每当我尝试选择cb时，它都会弹出JOPtionPane并取消选择。如果我移除JOptionPane，它就能正常工作。