Java: Đồng hồ số với thread

package thread;

import javax.swing.*;
import java.text.*;
import java.util.*;

public class DigitalWatch implements Runnable {

  JFrame f;
  Thread t = null;
  //    int hours = 0, minutes = 0, seconds = 0;
  String timeString = "";
  JButton b;

  DigitalWatch() {
    f = new JFrame();

    t = new Thread(this);
    t.start();

    b = new JButton();
    b.setBounds(100, 100, 100, 50);

    f.add(b);
    f.setSize(300, 400);
    f.setLayout(null);
    f.setVisible(true);
  }

  public void run() {
    try {
      while (true) {

//                Calendar cal = Calendar.getInstance();
//                hours = cal.get(Calendar.HOUR_OF_DAY);
//                if (hours > 12) {
//                    hours -= 12;
//                }
//                minutes = cal.get(Calendar.MINUTE);
//                seconds = cal.get(Calendar.SECOND);

        SimpleDateFormat formatter = new SimpleDateFormat("hh:mm:ss");
//                Date date = cal.getTime();
        timeString = formatter.format(new Date());

        printTime();

        t.sleep(1000);  // interval given in milliseconds  
      }
    } catch (Exception e) {
    }
  }

  public void printTime() {
    b.setText(timeString);
  }

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