Re: A question regarding singly-linked lists.

 Daniel Pitts <>
Tue, 18 Sep 2007 04:37:51 -0000
On Sep 17, 8:58 pm, mmoski <> wrote:

Here is some code. It reads in a line, then puts each token in that
line into a list node. The problem is that it seems that the next
token simply gets put into the data field of the head node. This is
causing the .next to NEVER equal null and crashes JCreator. This list
is implemented myself because this is my first time using a linked
list and I don't want to use any pre-defined list classes. Any
suggestions as to why the head node won't stay as the head node?

import java.util.Scanner; //import everything

public class Parse {

        public static void main (String[] args) throws IOException {

                BufferedReader br = new BufferedReader( new

                Node head = null; // set up head node
                Node prev = null; // set up a node to keep track of position
                Node a = new Node(); //set up node that will hold data
                String line = null;
                String word = null;

                while(((line = br.readLine()) != null) && (!line.equals(""))) { //
reads in a line
                Scanner sc = new Scanner(line);

                        while(sc.hasNext()){ //scans line for tokens
                                word =;
                       = word; //sets data to the next word
                        if(head == null){ //if there is no head create one and make
it's data field equal a
                                head = a;
                        }else{ //if there is a just a head and 0 or more other nodes
make the current node's next field point to a
                       = a;
                        prev = a; // set the 'keeping track of' node to equal a.

                Node pointer = head;

        while(pointer != null){ //test to see if the pointer node exists
                System.out.println("Working " +; //print out the
data of the pointer node
                pointer =;//set the pointer to the next node



class Node{
        public Node next = null;
        public String data;


Any help would be freakin' awesome. Thanks.

I have two pieces of help for you:

How to fix your code:
You need put your "Node a = new Node()" inside your for loop.

You're reusing the same node every time (so you end up with head = = prev (oops)

How to fix your post:
When posting code to usenet, try to make sure that your lines fit in
80 columns. You're // comments have wrapped to the next line, and if
someone tries to cut/paste that into a compiler, they'll get an
error. You can usually avoid this by using /* */ style comments.

Also, its more common in Java to write a comment to explain the
following line, not to comment at the end of the line. Although
that's not as steadfast of a rule. For example:

/* While we have something to look at
while (pointer != null) {
  /* Print out the data */
  /* Set the pointer to the next node */
  pointer =;

One more thing... Although, many teachers expect you to comment
excessively, its generally better to comment on WHY you chose to do a
set of statements, than to explain what each statement does. If
you're repeating yourself, the comment is just line noise.

/* While something() returns true */
while (something()) {

I know how to read code, so the "While something() returns true"
comment is utterly unhelpful. More advanced programmers will document
their code by using meaningful names for everything, and breaking down
the process into multiple methods.

while (moreToProcess()) {
   Processable processable = getNextThingToProcess();

Isn't that nicer than:

/* While there is more to process */
while (node != null) {
    /* Get the next thing to process */
   Processable processable =;
   /* Do the processing */

Some people might disagree with me, but I think you'll find many

Hope this helps

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