请看代码，怎样可以让用户按下NO的时候窗口不关闭？该如何解决

class MyWindowAdapter extends WindowAdapter        {            public void windowClosing(WindowEvent e)            {                int flag = JOptionPane.showConfirmDialog(MainFrame.this, "Are you sure that you want to exit? " +                     "Any unsaved work will be lost! ", "Sure to exit?", JOptionPane.YES_NO_OPTION);                                if (flag == 0)    // User selected YES                    setVisible(false);                else if (flag == 1)    // User selected NO                    // ...            }        }                addWindowListener(new MyWindowAdapter());

import java.awt.*;import java.awt.event.*;import javax.swing.*;public class Test extends JFrame{    public Test()    {        setSize(200, 200);        addWindowListener(new MyWindowAdapter());    }        public static void main(String[] args)    {        Test t = new Test();        t.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);//你肯定落了这句        t.setVisible(true);    }        class MyWindowAdapter extends WindowAdapter    {        public void windowClosing(WindowEvent e)        {            int flag = JOptionPane.showConfirmDialog(Test.this, "Are you sure that you want to exit? " +                         "Any unsaved work will be lost! ", "Sure to exit?", JOptionPane.YES_NO_OPTION);            if (flag == JOptionPane.YES_OPTION)                Test.this.setVisible(false);        }    }}