Need Help Getting A Simple Stop-Watch Working?

So I am experimenting with a little stop-watch object and I can't figure out why it isn't working. I have a gui with the following;
timeinfield is a jtextfield which the user enters the start time of the clock.
timerfield is a jtextfield where the clock is displayed timing.
startbutton is a jbutton where the user clicks to start the clock.
stopbutton is a jbutton where the user clicks to stop the clock.

I want the clock to look like 00.00.00 and keep updating until the user clicks stop. Then I am going to do something with the stop value later on.

Right now, the timerfield just displays Running: 0 seconds.
Nothing else happens, maybe I am doing this wrong for it is my first time trying something like this. I am about to start learning how to use threads so I thought this might help get me started.

Could someone help me write something like this that corresponds to the buttons and areas I have above? This is what I have so far;

Code :

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    private void startbuttonActionPerformed(java.awt.event.ActionEvent evt) {
 
            String starttim = timeinfield.getText();
            long startTime = Long.valueOf(starttim);
            boolean running = false;
            Timer timer = null;
            timer.stop();
            long time = (System.currentTimeMillis() - startTime)/1000;
 
            timerfield.setText("Running:" + time + "seconds");
            if(running==false){
            running = true;
            timerfield.setText("Running: 0 seconds");
 
                                }
            if(stopbutton.isSelected()) {
            timer.stop();
            running = false;
            long endTime = evt.getWhen();
            double seconds = (endTime - startTime)/1000.0;
            timerfield.setText("Time: " + seconds + "sec.");
            }
// going to need timer.stop() while timer is null or null pointer exception occurr
 
    }

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You declare timer but initialize it to null. What do you expect to happen? This is going to cause a NullPointerException every time.