Dynamic Clock in java

Question

I want to implement a clock within my program to diusplay the date and time while the program is running. I have looked into the getCurrentTime() method and Timers but none of them seem to do what I would like.

The problem is I can get the current time when the program loads but it never updates. Any suggestions on something to look into would be greatly appreciated!

Answer 1

This sounds like you might have a conceptual problem. When you create a new java.util.Date object, it will be initialised to the current time. If you want to implement a clock, you could create a GUI component which constantly creates a new Date object and updates the display with the latest value.

One question you might have is how to repeatedly do something on a schedule? You could have an infinite loop that creates a new Date object then calls Thread.sleep(1000) so that it gets the latest time every second. A more elegant way to do this is to use a TimerTask. Typically, you do something like:

private class MyTimedTask extends TimerTask {

   @Override
   public void run() {
      Date currentDate = new Date();
      // Do something with currentDate such as write to a label
   }
}

Then, to invoke it, you would do something like:

Timer myTimer = new Timer();
myTimer.schedule(new MyTimedTask (), 0, 1000);  // Start immediately, repeat every 1000ms

Answer 2

What you need to do is use Swing's Timer class.

Just have it run every second and update the clock with the current time.

Timer t = new Timer(1000, updateClockAction);
t.start();

This will cause the updateClockAction to fire once a second. It will run on the EDT.

You can make the updateClockAction similar to the following:

ActionListener updateClockAction = new ActionListener() {
  public void actionPerformed(ActionEvent e) {
      // Assumes clock is a custom component
      yourClock.setTime(System.currentTimeMillis()); 
      // OR
      // Assumes clock is a JLabel
      yourClock.setText(new Date().toString()); 
    }
}

Because this updates the clock every second, the clock will be off by 999ms in a worse case scenario. To increase this to a worse case error margin of 99ms, you can increase the update frequency:

Timer t = new Timer(100, updateClockAction);