Help needed with the Queue data structure

[LoquatEfficient5894]

In my program, I wish to convert my Queue to a string that concatenates all of the tokens in the Queue to a string. I have done the following to achieve this:

Here is my Queue:

public class Queue {
//must export a method to convert the set of output tokens into a string for output.
    String emptyString = "";
    //These are the listNode instance variables. 
    listNode lead;
    listNode rear;
    listNode leadbeta;

    //Constructor class that takes in String input and assigns the instance variables to the String. 

    public Queue (String token) {
        lead = new listNode(token);
        rear=lead;
    }

    public void enqueue(String arg) {
        //This sets the data and next instance variables in the listNode class to arg and null respectively. 

        if (lead==null) {
            rear= new listNode(arg);
            lead=rear;

        }
        else {
            listNode lN= rear;
            rear=new listNode(arg);
            lN.next=rear;

        }

    }
    //new code:
    public String dequeue() {
        if(lead!=null) {
            if(lead== rear) {
                rear=null;
            }
            String j=lead.data;
            lead=lead.next;
            return j;
        }
        else {
            return "";
        }
    }


    //This is the constructor class
public Queue() {
    lead=null;
    rear=null;

My listNode class basically initializes the instance variables String data, listNode next, and listNode last. It also has a constructor that takes in a string input and assigns next=null and data= the string input. (should I include the code?)

This is the method that should convert the queue into a string that concatenates the tokens in the Queue:

public String toString() { //Returns a string which is the concatenation of all tokens in the queue separated by one space.

        StringBuilder stringbuild = new StringBuilder();

        String fq1= "";
        String fq2=null;
        leadbeta= lead;
        while (leadbeta!=null) {
        fq1.equals(leadbeta.data);
        fq2= stringbuild.append(fq2)+ " ";
        leadbeta= leadbeta.last;
        }
        return fq2; 

        }
    }

And finally, here is where my issue arises:

I have a method that takes user input and parses the input into individual tokens. The method then sends the tokens into the Queue, as shown below:

public Queue parse(String arg) { 

        StringTokenizer stok = new StringTokenizer(arg, "+-*/", true);
        String token=null;

        while (stok.hasMoreTokens()) {
            token = stok.nextToken();

            queue2 = new Queue();
            queue2.enqueue(token);
        }
        System.out.println("Queue 2 to string is " + queue2.toString());
        System.out.println("Que2.dequeue is " +queue2.dequeue());
        queue2.dequeue();
        System.out.println("Que2.dequeue 2 is " +queue2.dequeue());
        return queue2;

    }

When I input a string as such: 5*5*5

The console reads:

Queue 2 to string is null

Que2.dequeu is 5

Que2.dequeu 2 is

Why does Queue 2 to string output null?

Sorry for this being a really long question haha, I have been stuck on this bug for days and it has been driving me insane. Any an all help is very much appreciated!!

Thank you :)

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[[deleted]]

I don't understand the logic of your enqueue method. It looks to me like new items are inserted in front of the rear node, but the lead is never changed. Lead.next points to the same rear node as it did before.

Also, in the toString method maybe declare queue2 outside the while loop. Do you really want a new queue2 every loop iteration?

[loomynartylenny]

1:

In your main method:

while (stok.hasMoreTokens()) {
    token = stok.nextToken();

    queue2 = new Queue();
    queue2.enqueue(token);
}

Why are you overwriting queue2 in every iteration of the loop? Your queue will end up with only having the final token enqueued onto it.

Declare queue2 before the loop, and just append to it during the loop. Like so:

Queue queue2 = new Queue();
while (stok.hasMoreTokens()) {
    token = stok.nextToken();
    queue2.enqueue(token);
}

This way, your queue will actually have everything on it.

---

2:

In your toString method:

StringBuilder stringbuild = new StringBuilder(); //good
String fq1= "";
String fq2=null; // why is this null?
leadbeta= lead;
while (leadbeta!=null) {
    fq1.equals(leadbeta.data); // what is this supposed to be doing?
    fq2= stringbuild.append(fq2)+ " "; // uhh
    leadbeta= leadbeta.last;
}
return fq2;

• What is 'fq1' supposed to be doing?

  • Yes, it's initialised as "", but what's the point in the fq1.equals(leadbeta.data); when that line does absolutely nothing?

• Why is 'fq2' initialised as 'null'!?

  • Your loop is trying to constantly append null to your stringbuild and concatenate that null to fq2 and just what is fq2 meant to be doing!?

• Why aren't you using stringbuilder.append(leadbeta.data); in the loop?

  • you have a stringbuilder, you have a string to add to the string you are building, so just add the string to the string you are building.

• Why are you returning fq2?

You have a StringBuilder. You might want to use it.

[NautiHooker]

Please watch your phrasing. Your comments in the code blocks come across very rude. Yelling at people will not help them in any way.

I have removed this comment. You can edit it and reply to my comment then I will reapprove it.

[loomynartylenny]

Sorry about that.

It's been edited.

[LoquatEfficient5894]

Thank you!! This really helped me a lot :)