Painting Clock, Overlaps the current String display

Code :

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import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.util.Calendar;
import javax.swing.*;
 
public class MyClock extends JPanel implements Runnable {
 
    Thread runner;
 
    public MyClock() {
 
        setDoubleBuffered(true);
        setFocusable(true);
        setLayout(null);
    }
 
    public void paintComponent(Graphics g) {
 
        Calendar now = Calendar.getInstance();
        int hrs = now.get(Calendar.HOUR_OF_DAY);
        int min = now.get(Calendar.MINUTE);
        int sec = now.get(Calendar.SECOND);
 
        String time = zero(hrs) + ":" + zero(min) + ":" + zero(sec);
 
        g.drawString(time, 60, 40);
 
        repaint();
    }
 
    public String zero(int num) {
 
        String number = (num < 10) ? ("0" + num) : ("" + num);
        return number;                                   
    }
 
    public void start() {
 
        if (runner == null) {
 
            runner = new Thread(this);
        }
 
        runner.start();
    }
 
    public void run() {
 
        while (runner == Thread.currentThread()) {
 
            try {
 
                Thread.sleep(1000);
            }
            catch (InterruptedException e) {
 
                System.out.println("Thread failed");
            }
        }
    }
 
    public static void main(String[] args) {
 
        JFrame f = new JFrame();
        f.setSize(400, 300);
        f.add(new MyClock());
        f.setLocationRelativeTo(null);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
    }
}

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i just gode this code from the web, with a little bit of modification, i need to implement a digital clock for another project.. i just cant figure out why does the next painting overlaps the current one.. any help please.

by the way.. my goal is.. to create a separate class as a Clock, but extends as a JPanel not a JFrame, i just want to add/call it to any JFrame window i have or in any JPanel/Container

1- Don't just throw code together you get from the internet. Take the time to really understand what's going on. You'll save yourself a ton of headaches. Recommended reading: Lesson: Performing Custom Painting (The Java™ Tutorials > Creating a GUI With JFC/Swing)

2- Don't call repaint() from a paint method, including paintComponent(). Use a Swing Timer instead, otherwise you're going to be constantly drawing, which is really going to eat up a lot of resources.

3- Your zero() method is doing the job of a DecimalFormat instance.

4- JPanel's paintComponent() method redraws the background, which in effect clears out any previously drawn frames. Start your paintComponent() method with a call to super.paintComponent() in order to maintain that functionality.