java窗口询问关闭

f.addWindowListener( new WindowAdapter() { public void windowClosing( WindowEvent e ) { cls(); System.exit(0); } }); 关闭窗口 怎么里面的方法没有作用啊，没有弹出询问窗口，而令建个按钮这方法确正常能用

import java.awt.event.WindowEvent;

import javax.swing.JFrame;
import javax.swing.JOptionPane;

public class TestCloseFrame extends JFrame {

TestCloseFrame() {
this.setTitle("测试窗体");

this.setBounds(30, 30, 300, 200);
this.setVisible(true);
}

public static void main(String a[]) {
new TestCloseFrame();
}

protected void processWindowEvent(WindowEvent e) {

//super.processWindowEvent(e);

if (e.getID() == WindowEvent.WINDOW_CLOSING) {

int index = JOptionPane.showConfirmDialog(this, "您好，您确定要关闭窗体吗？",
"确认对话框", JOptionPane.YES_NO_OPTION,
JOptionPane.QUESTION_MESSAGE);

if (index == 0) {//如果点击的是确认

System.exit(0);
} else { //否则什么也不做

}

}

}
}