Question

I’ve written a TimerTask to display current date and time in a JLabel. Following is the TimerTask code and that works well in normal scenario. When I change the system date and time when the GUI is running, the timer stops running. There was no exception when I changed system date and time and the Timer just stops running.
Can anyone tell me what is happening?

private void startTimer()
{
    // Start the clock
    timer = new Timer();
    timer.schedule(new TimeTask(), 0, 1000);
}

class TimeTask extends TimerTask
{
    public void run()
    {
        try {
            clockLabel.setText(new SimpleDateFormat("EEE , dd MMM , HH:mm:ss").format(Calendar.getInstance().getTime()));
            System.out.println(clockLabel.getText());
        } catch(Exception ex) {
            ex.printStackTrace();
            System.out.println("Exception : " + ex.getMessage());
        }
    }
}

Answer 1

Don’t use TimerTask with Swing as you can easily run into concurrency issues as TimerTask will be calling code off of the EDT. Instead use a Swing Timer; this is specifically what it is built for — calling code periodically on the Swing event thread.

i.e.,

private void startTimer() {
  timer = new Timer(TIMER_DELAY, new TimerListener());
  timer.start();
}

private class TimerListener implements ActionListener {
  private final String PATTERN = "EEE , dd MMM , HH:mm:ss";
  private final DateFormat S_DATE_FORMAT = new SimpleDateFormat(PATTERN);

  @Override
  public void actionPerformed(ActionEvent e) {
     Date date = Calendar.getInstance().getTime();

     String dateString = S_DATE_FORMAT.format(date);
     clockLabel.setText(dateString);         
  }
}