用java在窗口上编写一个数字时钟

好的，以下是用Java编写数字时钟的示例代码：

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class DigitalClock extends JPanel implements Runnable {
    private Thread thread = null;
    private SimpleDateFormat dateFormat = new SimpleDateFormat("HH:mm:ss");
    private Font clockFont = new Font("Arial", Font.BOLD, 36);

    public DigitalClock() {
        setBackground(Color.BLACK);
    }

    public void start() {
        if (thread == null) {
            thread = new Thread(this);
            thread.start();
        }
    }

    public void stop() {
        if (thread != null) {
            thread = null;
        }
    }

    public void run() {
        while (thread != null) {
            repaint();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void paint(Graphics g) {
        super.paint(g);
        Calendar calendar = Calendar.getInstance();
        String time = dateFormat.format(calendar.getTime());
        g.setFont(clockFont);
        g.setColor(Color.GREEN);
        g.drawString(time, 20, 50);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Digital Clock");
        DigitalClock digitalClock = new DigitalClock();
        frame.add(digitalClock);
        frame.setSize(200, 120);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        digitalClock.start();
    }
}

这段代码创建了一个继承自`JPanel`的类`DigitalClock`，并实现了`Runnable`接口。在`run()`方法中，程序不断地重绘数字时钟。在`paint()`方法中，获取当前时间并在窗口上绘制数字时钟。`main()`方法中创建了一个`JFrame`窗口，并将`DigitalClock`添加到窗口中。最后启动`DigitalClock`线程，数字时钟开始运行。