Problem with Event Handling in Swing while developing a GraphGUI in Java

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Java

Swing

Eclipse

Hi,
I am creating a GraphGUI. I have attached 2 panels a GraphPanel(for graph sheet) and a COntrolPanel(for button controls). I placed two buttons a 'sine' and 'cosine' which should display it's corresponding wave on GraphPanel upon clicking them. Right now I am doing event handling for my GUI and I am having problem there.
Here is the code:
1) Main Class:

 public class GraphGUI extends JFrame {
	public static void main(String[] args) {
		SwingUtilities.invokeLater(new Runnable() {
			public void run() {
				JFrame f = new MainFrame("GraphGUI");
				f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
				f.setVisible(true);
				f.setSize(900, 725);
				f.setResizable(false);
			}
		});
	}
} 

2)Frame Class:

 public class MainFrame extends JFrame {
	private GraphPanel gPanel;
	private ControlPanel pPanel;	
	public MainFrame(String title) {
		super(title);		
		pPanel = new ControlPanel();
		gPanel = new GraphPanel();		
		Container con = getContentPane();		
		con.add(pPanel, BorderLayout.EAST);
		con.add(gPanel, BorderLayout.WEST);		
		pPanel.addGraphListener(new GraphListener() {
			@Override
			public void graphEventOccured(GraphEvent event) {
				System.out.println("hi");				
			}			
		}, gPanel);
	}
} 

3) GraphPanel Class:

public class GraphPanel extends JPanel {
	public GraphPanel() {
		super();
		setBorder(BorderFactory.createLineBorder(Color.black));		
		Dimension size = getPreferredSize();
		size.width = 666;
		setPreferredSize(size);
		setBackground(Color.black);
	}	
	protected void paintComponent(Graphics g) {
		super.paintComponent(g);
		Graphics2D g2 = (Graphics2D)g;
		g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
		final int h = getHeight();
		final int w = getWidth();
		System.out.println("h = " + h + "\tw = " + w);
		g2.setColor(Color.RED);
		g2.drawLine(300, 0, 300, h);
		g2.drawLine(0, 340, w, 340);
		g2.setColor(Color.GREEN);
		int i = 1;
		int z  = 1;
		while(z > 0) {
			z = 300 - (i*20);
			g2.drawLine(z, 0, z, h);
			i++;
		}
		i = 1;
		while(z < w) {
			z = 300 + (i*20);
			g2.drawLine(z, 0, z, h);
			i++;
		}
		i = 1;
		while(z > 0) {
			z = 340 - (i*20);
			g2.drawLine(0, z, w, z);
			i++;
		}
		i = 1;
		while(z < h) {
			z = 340 + (i*20);
			g2.drawLine(0, z, w, z);
			i++;
		}
	}
}

4) ControlPanel Class:

 public class ControlPanel extends JPanel {
	private JButton sineBtn;
	private JButton cosineBtn;	
	private EventListenerList listenerList = new EventListenerList();
	public ControlPanel() {
		super();
		setBorder(BorderFactory.createLineBorder(Color.green));
		setLayout(new GridBagLayout());
		Dimension size = getPreferredSize();
		size.width = 300;
		setPreferredSize(size);
		setLayout(new GridBagLayout());
		
		sineBtn = new JButton("Sine");
		cosineBtn = new JButton("Cosine");
		
		//action listener for sineBtn
		sineBtn.addActionListener(new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent arg0) {
				fireGraphEvent(new GraphEvent(this));//exception here when Sine Button is clicked
			}			
		});
		//action listener for cosineBtn
		cosineBtn.addActionListener(new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent arg0) {
				fireGraphEvent(new GraphEvent(this));//exception here when Cosine Button is clicked				
			}
		});		
		setLayout(new GridBagLayout());
		GridBagConstraints gc = new GridBagConstraints();		
		//adding buttons
		gc.anchor = GridBagConstraints.CENTER;
		gc.weightx = 0.5;
		gc.weighty = 0.5;
		//changing the default size of the buttons
		gc.ipadx = 40;
		gc.ipady = 40;
		//adding sineBtn
		gc.gridx = 0;
		gc.gridy = 0;
		add(sineBtn, gc);
		//adding cosineBtn
		gc.gridx = 0;
		gc.gridy = 1;
		add(cosineBtn, gc);
	}	
	public void fireGraphEvent(GraphEvent event) {
		Object[] listeners = listenerList.getListenerList();
		for(int i = 0; i < listeners.length; i++) {
			if(listeners[i] == GraphListener.class) {
				((GraphListener)listeners[i]).graphEventOccured(event);//exception here for both events
			}
		}
	}
	public void addGraphListener(GraphListener listener, GraphPanel gPanel) {
		listenerList.add(GraphListener.class, listener);
	}	
	public void removeGraphListener(GraphListener listener) {
		listenerList.remove(GraphListener.class, listener);
	}
} 

5)GraphEvent Class:

public class GraphEvent extends EventObject {
    public GraphEvent(Object source) {
        super(source);
        System.out.println("Source : " + source);
    }
}

6) GraphListener Interface:

public interface GraphListener extends EventListener {
    public void graphEventOccured(GraphEvent event);
}

Right now I am not at the stage of plotting waves, I am just checking whether my events are occurring as desired and expected. Exceptions are occurring in ControlPanel class as mentioned in the code. Please show me where am I erring and point me in the right direction for plotting sine waves and cosine waves in my GUI.

Thank you for your valuable time.