窗体焦点监听事件WindowFocusListener

 

public class Demo extends JFrame {
    JLabel label;//定义变量，以便在自定义方法类中调用

    public Demo() {
        setBounds(100, 100, 300, 300);
        setResizable(false);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        Container c = getContentPane();
//面板，黑色背景，以适应两张图片的黑色背景
        JPanel panel = new JPanel();
        panel.setBackground(Color.BLACK);
        c.add(panel);
//Label，用来展示图片
        label = new JLabel();
        panel.add(label);
//调用自定义方法类
        addWindowFocusListener(new MyWindowFocusListener());

        setVisible(true);
    }

    class MyWindowFocusListener implements WindowFocusListener {

        public void windowGainedFocus(WindowEvent e) {
            label.setIcon(new ImageIcon(Demo.class.getResource("light.png")));
        }

        public void windowLostFocus(WindowEvent e) {
            label.setIcon(new ImageIcon(Demo.class.getResource("dark.png")));
        }
    }

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