Re: Reading from hashtables

Mon, 4 Feb 2008 06:58:58 -0800 (PST)
On 4 Feb, 14:43, Lew <> wrote:

Some side notes for you that will help code readability (as when you're as=


for help) and your runtime performance and engineering. wrote:

i [sic] am currently developing a java [sic] application that will read =

in a CSV

file which includes computer names and mac addresses. I have two
seperate functions within a class. One function is called readFile();
and the other function is called WOL();.

Method names in Java should begin with a lower-case letter and be spelled =


camel case, by well-established convention.

In the readFile() function it reads the CSV file in and then adds the
data into a hashtable. I then want to retrieve a computer name from
the hashtable being ran from the WOL function. However when I output
the hash table to the console it says that the table is null. When I
use the exact same code and place that within the readFile function it
works fine. The code is below:

package remoteshutdown;

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

public class WakeOnLan {
    String macStr;

Why are your instance variables package-private instead of private?

// String mac;
// String Computer;
    Hashtable<String, String> macTable = new Hashtable<String,

Hashtable isn't your best choice. You don't seem to be making any use o=

f its

synchronization, so HashMap is likely the better choice. Actually, Hash=


hasn't been the right choice since 1998. You probably should declare th=


variable 'macTable' as a Map <String, String> rather than locking it down =


the implementation.

    public static final int PORT = 9;

    public void readFile() {
           try {

PLEASE stop embedding TABs in Usenet posts. It screws up the indentatio=


something fierce.

            // Open the file that is the first
            // command line parameter
            FileInputStream fstream = new FileInputStream(=


\Documents and Settings\\All Users\\Application Data\\Remote Shutdown\
\DHCP Export.csv");
            // Get the object of DataInputStream
            DataInputStream in = new DataInputStream(fstre=


            BufferedReader br = new BufferedReader(new

DataInputStream is the wrong choice here. Just build your InputStreamRe=


run on the FileInputStream. From the Javadocs:> A data input stream let=

s an application read primitive Java data types

from an underlying input stream in a machine-independent way.


This is not at all what you are doing. So once again, do not use
DataInputStream to read characters like this.

            String strLine;

Once again, avoid embedding implementation ("str") in variable names.

            //Read File Line By Line
            while ((strLine = br.readLine()) != null) {

Here's an interesting alternative idiom that confines the scope of your 'l=


variable to the loop and no wider, thus making safer code:

     for ( String line; (line = br.readLine()) != null; )

                   System.out.println("strLine is: "=

 + strLine );

If you must use such logging statements without using logging calls, at le=


use System.err. Either way, logging statements that you have to remove =


code later are a bad idea.

                   StringTokenizer st = new Str=

ingTokenizer(strLine, ",.");

                   String Computer = st.nextToken(=


                           int firstIndex =

= strLine.indexOf(".");

                           int lastIndex ==


                           if (firstIndex =

== lastIndex) {



                           else {
                   String mac = st.nextToken();
                   System.out.println("\t\tComputer =

: " + Computer+" for


                   macTable.put((String)Computer, (S=


Why are you casting Strings to Strings in a method that only takes Strings=



                // Print the content on the console
            //Close the input stream
           System.out.println("CLOSING IN");

Interesting choice to close the middle output path, not the lowest or high=


Note that in this location the close() call is not guaranteed to run. Y=


risk running around with unclosed streams.

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


    public void WOL() {

Should be named 'wol'.

           String Computer = (String)

Others have pointed out that you show no definition for 'mainScreen'.

        System.out.println("the mac table has inside: " + macTab=


           String wakeComputer = (String)macTable.get("cbo=


Why are you casting the get() result to String? get() already returns a=


           System.out.println("wakeComputer is:" + wakeCompu=


           System.out.println("computer variable is: " + Com=


        String ipStr = ""; //broadcast address
        String macStr = "00:30:4F:1C:95:DF"; //CBO_TEST2

        try {
            byte[] macBytes = getMacBytes(macStr);
            byte[] bytes = new byte[6 + 16 * macBytes.leng=


            for (int i = 0; i < 6; i++) {
                bytes[i] = (byte) 0xff;
            for (int i = 6; i < bytes.length; i += macBy=

tes.length) {

                System.arraycopy(macBytes, 0, bytes, i,

            InetAddress address = InetAddress.getByName(ip=


            DatagramPacket packet = new DatagramPacket(byt=


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 p=

acket: +


That is too strong a reaction to the failure. In fact, calling System.e=


from any non-main() is a mistake. What if the invoking code expects to =


Do not kill the program from a low-level subroutine.



    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;


You stated that 'readFile()' is called before 'WOL()', but provide no evid=


of that. How are you certain that that order pertains?

Also, since you do nothing in readFile() to notify a caller if it fails, a=


you sure that readFile() actually built the Map that you think it did?

Are you sure that wol() is called on the same instance of 'WakeOnLan' on w=


'readFile()' was called?

Lew- Hide quoted text -

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Thank you for your help. I have now resolved this issue

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