How to debug my Java linked-list queue?

Question

I had basically no issue with linked lists in C++, but this is getting to me for some reason. I had a single node being printed out using the other classes in the package that were provided, but as I go on I just keep running into walls.

The code below is in shambles due to me tinkering around. I just have no idea where to go from here. As of now that is getting a null pointer exception.

Just for information: poll() is just removing the current head and returning it, offer() is adding to the rear. As of now the exception is at oldLast.next = last in the offer method.

I am not asking for anyone to completely solve this. I just need some tips to progress.

public class FIFOQueue implements Queue {

//put your name as the value of the signature.
String signature = "name";

Node head = new Node(null);
Node pointer = head;
Node first;
Node last;
Node prev;
Node curr;

class Node {
    Process process;
    Node next;


    Node(Process p) {
        this.process = p;
        this.next = null;
    }

}

@Override
public void offer(Process p) {


    if(head == null)
    {
        head = new Node(p);
        first = head;
        last = head;

    }

    else
    {

        Node oldLast = last;
        Node newNode = new Node(p);

        last = newNode;
        oldLast.next = last;


    }



}


@Override
public Process poll() {


    if(isEmpty())
        throw new NoSuchElementException();

    Node oldPointer = first;

    first = first.next;
    head = first;


        return oldPointer.process;
}

@Override
public boolean isEmpty() {

return head == null;

}

@Override
public String getSignature() {
    return signature;
}

}

Answer 1

I think your core issue is here:

Node prev;
Node curr;

These are confusing you. Remove them.

1. Node prev; - This should be in the Node class.
2. Node curr; - This should be a local variable, not an instance variable.

Also

Node head = new Node(null);

does not gel with

if(head == null)
{
    head = new Node(p);

Either make head == null mean the list is empty or something else - but be consistent.

Answer 2

(Posted on behalf of the OP) .

public void offer(Process p) {


    if(head.process == null)
    {
        head = new Node(p);
        first = head;
        last = head;
    }


        last.next = new Node(p);
        last = last.next;

}

This solved my issue. Can't believe I let this confuse me.