import java.awt.event.WindowEvent;

import java.awt.event.WindowStateListener;

import javax.swing.JFrame;

public class Demo extends JFrame {

public Demo() {

addWindowFocusListener(new MyWindowStateListener());

setSize(200, 200);// 设置大小

setDefaultCloseOperation(EXIT_ON_CLOSE);// 关闭窗口退出程序

setVisible(true);// 窗体可以显示

}

private class MyWindowStateListener implements WindowStateListener {

public void windowStateChanged(WindowEvent e) {

int oldState = e.getOldState();// 获得窗体以前的状态

int newState = e.getNewState();// 获得窗体现在的状态

String from = "";// 标识窗体以前状态的中文字符串

String to = "";// 标识窗体现在状态的中文字符串

switch (oldState) {// 判断窗台以前的状态

case JFrame.NORMAL:// 窗体处于正常化

from = "正常化";

break;

case JFrame.MAXIMIZED_BOTH:// 窗体处于最大化

from = "最大化";

break;

default:// 窗体处于最小化

from = "最小化";

}

switch (newState) {// 判断窗台现在的状态

case JFrame.NORMAL:// 窗体处于正常化

to = "正常化";

break;

case JFrame.MAXIMIZED_BOTH:// 窗体处于最大化

to = "最大化";

break;

default:// 窗体处于最小化

to = "最小化";

}

System.out.println(from + "——>" + to);

}

public static void main(String[] args) {

new Demo();

}

这里 addWindowFocusListener报错，怎么办？