public class ImprovedClock{
 
	private volatile long first = 0, second = 0;
	private volatile boolean startPressed = false, stopPressed = false;
 
	public void start(){
		if(!startPressed){
			synchronized(this){
				first = System.currentTimeMillis();
				startPressed = true;
				stopPressed = false;
			}
		}
	}
 
	public boolean hasStarted(){
		return startPressed;
	}
 
	public void stop(){
		if(!stopPressed){
			synchronized(this){
				second = System.currentTimeMillis();
				startPressed = false;
				stopPressed = true;
			}
		}
	}
 
	public boolean hasStopped(){
		return stopPressed;
	}
 
	public long getElapsedTime(){
		long time = 0;
		synchronized(this){
			time = second - first;
			first = 0;
			second = 0;
		}
		return time;
	}
 
	public double getSeconds(){
		return ((double)getElapsedTime()/1000);
	}
}

import javax.swing.JFrame;
import java.awt.event.KeyEvent;
import java.awt.event.KeyAdapter;
 
public class TimeMonitorTest extends JFrame{
 
 
	public TimeMonitorTest(){
		final ImprovedClock ic = new ImprovedClock();
		addKeyListener(
			new KeyAdapter(){
				@Override public void keyPressed(KeyEvent ke){
					if(!ic.hasStarted()){
						ic.start();
						System.out.println("Clock started!");
					}
				}
 
				@Override public void keyReleased(KeyEvent ke){
					ic.stop();
					System.out.println("Time elapsed: " + ic.getSeconds() + " seconds!");
				}
			}
		);
		setSize(300, 300);
                setLocation(500, 500);
		setVisible(true);
	}
 
	public static void main(String... args){
		TimeMonitorTest tmt = new TimeMonitorTest();
	}
}