Java Question - Adding User Input To Countdown Timer

Hey,

I was just wondering how hard it would be to take a program that already has the time hardcoded into it (it is a countdown program) and make it so that the user can input the amount of time they want? This is the code that I have now, and when the program starts, I want it to ask how long the program should run.


package countdowntimer;
import java.awt.event.*;
import javax.swing.*;

public class CountdownTimer extends JFrame implements ActionListener, Runnable
{
    private long startTime;
    private final static java.text.SimpleDateFormat timerFormat = new java.text.SimpleDateFormat("mm : ss : SSS");
    private final JButton startStopButton = new JButton("Start/Stop");
    private Thread updater;
    private boolean isRunning = false;
    private final Runnable displayUpdater =  new Runnable()
    {
        public void run()
        {
            displayElapsedTime(CountdownTimer.this.startTime - System.currentTimeMillis());
        }
    };
    public void actionPerformed(ActionEvent ae)
    {
       if(isRunning)
       {
           long elapsed = startTime - System.currentTimeMillis();

           isRunning = false;
           try
           {
               updater.join();
               // Wait for updater to finish
           }
           catch(InterruptedException ie) {}
           displayElapsedTime(elapsed);
           // Display End Result
       }
       else
       {
           startTime = 2*60*1000+System.currentTimeMillis();
           isRunning = true;
           updater = new Thread(this);
           updater.start();
       }
    }
    private void displayElapsedTime(long elapsedTime)
    {
        startStopButton.setText(timerFormat.format(new java.util.Date(elapsedTime)));
    }
    public void run()
    {
        try
        {
            while(isRunning)
            {
                SwingUtilities.invokeAndWait(displayUpdater);
                Thread.sleep(50);
            }
        }
        catch(java.lang.reflect.InvocationTargetException ite)
        {
            ite.printStackTrace(System.err);
            // Prints Error - shouldn't ever happen
        }
        catch(InterruptedException ie) {}
        // Ignore and return!
    }
    public CountdownTimer()
    {
        startStopButton.addActionListener(this);
        getContentPane().add(startStopButton);
        setSize(100,50);
        setVisible(true);
    }
    public static void main(String[] arg)
    {
        new CountdownTimer().addWindowListener(new WindowAdapter()
                {
                    public void windowClosing(WindowEvent e)
                    {
                        System.exit(0);
                    }
                });
    }
}

Thanks for all the help!
Bobby