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();

    }

}