public void addListeners() {
		
		// Start Button Pressed
		start.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				
				String input = tf.getText();
				
				try{
					int newGoal = Integer.parseInt(input);
					if (worker == null) {	
						worker = new WorkerThread(0, newGoal);
						worker.start();
					}else {
						worker.interrupt();
						worker = null;
						worker = new WorkerThread(0, newGoal);
						worker.start();
					}					
				}catch (NumberFormatException ex) {
					tf.setText("Enter an int value.");
				}
								
			}
		});
		
		// Stop Button Pressed
		stop.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				if (worker != null) {
					worker.interrupt();
					worker = null;
				}
				display.setText("0");
			}
		});
	}