java中如何设置Dialog关闭

import java.awt.*;
import java.awt.event.*;
public class TestWindowClosing{
public static void main(String[] args) {
new FrameWindow("HELLO");
}
}
class FrameWindow extends Frame {
public static int i = 1; //设置计数器，计算窗口的数量，当界面没有显示窗口时退出系统
FrameWindow(String s) {
super(s);
setLayout(new FlowLayout());
setBackground(Color.yellow);
setBounds(300,300,400, 600);
setVisible(true);
Button bt = new Button("点击弹出窗口");
this.add(bt);
bt.addActionListener(new ButtonActionL());
this.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
i--;//每调用一次关闭事件，窗口数量减少1
setVisible(false);
if(i == 0) {
System.exit(0);
} else {
}
}
});
}
class ButtonActionL implements ActionListener {
public void actionPerformed(ActionEvent e) {
i++; //每调用一次，窗口数增1
FrameWindow c = new FrameWindow("子窗口");
c.setBounds(200,200,200,200);
c.setBackground(Color.blue);
c.setVisible(true);
Dialog a = new Dialog(c,true);
a.setVisible(true);
a.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
setVisible(false);
//System.exit(-1);
}
});
}
}
}
为什么无法关闭Dialog，请高手指点。