Problem with program exception in thread error

Hi all,

I am currently developing a java application that will allow the user
to perform wake on lan. The program gets the host name and mac address
from a CSV file. When the user presses the button the application will
read the file in and then perform the wake on lan on the computer.
However I keep on getting an error and the application is not working.
This is the error that i am getting.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at remoteshutdown.WakeOnLan.WOL(WakeOnLan.java:74)
    at remoteshutdown.WakeOnLan.readFile(WakeOnLan.java:60)
    at
remoteshutdown.mainScreen.btnAddDetails_actionPerformed(mainScreen.java:
563)
    at remoteshutdown.mainScreen
$mainScreen_btnAddDetails_actionAdapter.actionPerformed(mainScreen.java:
980)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown
Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown
Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown
Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

This is the code that I am using:

package remoteshutdown;

import java.awt.Dimension;
import java.awt.Point;
import java.io.*;
import java.net.*;
import java.util.*;

public class WakeOnLan {
    String macStr;
// String mac;
// String Computer;
    Hashtable<String, String> macTable = new Hashtable<String,
String>();

    public static final int PORT = 9;

    public void readFile() {
        try {
            // Open the file that is the first
            // command line parameter
            FileInputStream fstream = new FileInputStream("C:\
\Documents and Settings\\All Users\\Application Data\\Remote Shutdown\
\DHCP Export.csv");
            // Get the object of DataInputStream
            DataInputStream in = new DataInputStream(fstream);
            BufferedReader br = new BufferedReader(new
InputStreamReader(fstream));
            String strLine;
            //Read File Line By Line
            while ((strLine = br.readLine()) != null) {
             System.out.println("strLine is: " + strLine );
             StringTokenizer st = new StringTokenizer(strLine, ",.");

             String Computer = st.nextToken();

             if(strLine.contains("."))
             {
             int firstIndex = strLine.indexOf(".");
             int lastIndex = strLine.lastIndexOf(".");
             if (firstIndex == lastIndex) {
             st.nextToken();
             }
             else {
         st.nextToken();
             st.nextToken();
             }
             }
             String mac = st.nextToken();
             System.out.println("\t\tComputer : " + Computer+" for
"+mac);

             macTable.put((String)Computer, (String)mac);
                // Print the content on the console
            //Close the input stream
            }
           System.out.println("CLOSING IN");
            in.close();

        } catch (Exception e) { //Catch exception if any
            System.err.println("Error: " + e.toString());
        }
        WOL();
    }

    public void WOL() {
     Object[] choices =
mainScreen.lstComputerNames.getSelectedValues();
     for (Object aChoice : choices) {

     String wakeComputer = (String)macTable.get(aChoice);

     System.out.println("wakeComputer is:" + wakeComputer);

        String ipStr = "255.255.255.255"; //broadcast address

     String originalMac = wakeComputer.substring(0,2) + ":" +
wakeComputer.substring(2,4) + ":" + wakeComputer.substring(4,6) + ":"
+ wakeComputer.substring(6,8) + ":" + wakeComputer.substring(8, 10) +
":" + wakeComputer.substring(10,12);
     System.out.println("original mac: " + originalMac);

        String macStr = originalMac; //"00:30:4F:1C:95:dF";

        try {
// wait(5000);
            byte[] macBytes = getMacBytes(macStr);
            byte[] bytes = new byte[6 + 16 * macBytes.length];
            for (int i = 0; i < 6; i++) {
                bytes[i] = (byte) 0xff;
            }
            for (int i = 6; i < bytes.length; i += macBytes.length) {
                System.arraycopy(macBytes, 0, bytes, i,
macBytes.length);
            }

            InetAddress address = InetAddress.getByName(ipStr);
            DatagramPacket packet = new DatagramPacket(bytes,
bytes.length, address, PORT);
            DatagramSocket socket = new DatagramSocket();
            socket.send(packet);
            socket.close();

            System.out.println("Wake-on-LAN packet sent.");
        }
        catch (Exception e) {
            System.out.println("Failed to send Wake-on-LAN packet: +
e");
            System.exit(1);
        }
     }

    }

    private static byte[] getMacBytes(String macStr) throws
IllegalArgumentException {
        byte[] bytes = new byte[6];
        String[] hex = macStr.split("(\\:|\\-)");
        if (hex.length != 6) {
            throw new IllegalArgumentException("Invalid MAC
address.");
        }
        try {
            for (int i = 0; i < 6; i++) {
                bytes[i] = (byte) Integer.parseInt(hex[i], 16);
            }
        }
        catch (NumberFormatException e) {
            throw new IllegalArgumentException("Invalid hex digit in
MAC address.");
        }
        return bytes;
    }

}

Any help in this matter would be highly appreciated. Thank you