Re: Problem with program exception in thread error

Knute Johnson <>
Thu, 07 Feb 2008 14:47:15 -0800
<47ab8a73$0$27861$> wrote:

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(
    at remoteshutdown.WakeOnLan.readFile(
    at remoteshutdown.mainScreen
    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
    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
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at Source)

This is the code that I am using:

package remoteshutdown;

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

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

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

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

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

    public void WOL() {
     Object[] choices =
     for (Object aChoice : choices) {

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

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

        String ipStr = ""; //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,

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

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


    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
        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

Since this isn't compilable it is very hard to tell what your problem is
or where it is. NullPointerExceptions are "Thrown when an application
attempts to use null in a case where an object is required." Just check
to see what object is null in line 74 and that should tell you what your
problem is.


Knute Johnson
email s/nospam/knute/

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