Re: retrieve data from hashtable

Lew <>
Mon, 21 Jan 2008 17:41:42 -0500
<> wrote:

I have sorted out the mistake where I am creating a new Hashtable each
time I add a record. Where you see CBO_TEST is just a test to see if
it worked if I hard coded the computer name, which unfortunately it
doesn't. This is the code that I am using now:

package remoteshutdown;

import java.util.*;


public class WakeOnLan {
    Hashtable macTable = new Hashtable();

Please, please, please, please, please do not use TAB characters to indent.
It makes the listings too wide for comfortable or accurate Usenet reading, as
mentioned earlier.

Why are you using Hashtable and not, say, HashMap?

     String ipStr;
    String macStr;

Given how this variable is used, it should be a method-local variable, not an
instance variable. This is part of why you don't get the expected results.

Hint: macStr is initialized to null. When is it changed?

     String mac;

Hint: mac is initialized to null. When is it changed?

// String Computer;

Variable names begin with a lower-case letter by convention.

     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);

Do not use DataInputStream to read character data.

This was mentioned before also.

            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();

Variable names should start with a lower-case letter by convention.

             mac = st.nextToken();
             System.out.println("Computer TOKENIZED: " + Computer);
             System.out.println("Mac Token: " + mac);
             macTable.put(Computer, mac);
             Computer = (String)
             System.out.println("computer is showing: " + Computer);
             macStr = mac;

Upon loop exit, this sets the 'macStr' variable to whatever the last value was
through the loop. It also leaves 'mac' set to that value. Could it be that
that last value is null?


Why do you do a get, then throw away the returned value?

             System.out.println("Inside the HashMap: " + macTable);

In what sense is this code "inside" the "HashMap [sic]"?

Why does your message refer to a HashMap when you don't even use HashMap?

             // WOL();
                // Print the content on the console
            //Close the input stream

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

Your exception handling is not going to guarantee that the InputStream is
closed, nor that further logic is suppressed if there is a problem.


Your indentation makes it hard to distinguish where methods begin and end.

     public void WOL() {

Method names should begin with a lower-case letter by convention.

         final int PORT = 9;

Variable names should begin with a lower-case letter and be spelled with camel
case, by convention ('port'). (Static compile-time constants are an exception.)

         String ipStr = "";
        String choice = (String)
        System.out.println( choice );
        // will be -1 if there aare none or muliple selections.
        int which = mainScreen.lstComputerNames.getSelectedIndex();
        System.out.println( which );

        // detecting multiple selections
        System.out.println( "--multiples--" );

        Object[] choices = mainScreen.lstComputerNames.getSelectedValues();

Wouldn't this be better as a String []?

// macStr = "00:07:E9:93:18:EB";
        for ( Object aChoice : choices )

You make no use of the 'aChoice' variable. Shouldn't you?

         System.out.println("The selection for wake on lan is: " +
        System.out.println("The mac address is: " + mac);

Which MAC address will 'mac' hold at this point? (See hint above.)

         try {
            byte[] macBytes = getMacBytes(macStr);

'mac', 'macStr' - choose one. Set it from the Map, not by the coincidence of
your last input.

Here is where some of your problem lies. You are not retrieving anything from
that Map you built so laboriously.

And don't embed implementation types in variable names - what if you later
decide 'macStr' should be some type other than String? This was also
mentioned upthread.

             byte[] bytes = new byte[6 + 16 * macBytes.length];

Similarly, 'bytes' is a bad name for a variable. Plus, it's dangerously close
to a keyword; a small typo would create a big error.

             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);

I suggest that you read the Javadocs for arraycopy():

The number of components copied is equal to the length argument

A "component" in this case is a byte.

Your call as written will repeatedly write the same bytes to successive
regions of the destination array.

You should also consider using
which adds type safety.


            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: " +

Or you could continue the loop.


    private static byte[] getMacBytes(String macStr) throws
IllegalArgumentException {

You do not need to declare a RuntimeException in the method signature, in
fact, it goes against their purpose to do so.

Since the method is private, you do not need to throw any exception. You
could, for example, return null and test for that in the caller.

This method likely should not be static.

         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
        return bytes;

However the same problem persists where I am searching the hashtable [sic]
and the result is null. I am fairly new to Java and would appreicate
any help that you can provide.

One does not usually "search" a Map but indexes into it using the key value.
However, I see nowhere in your code where you do either.

How do you think you are retrieving anything from the Map?


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