[SpartanGuy07]

My assignment is to create a Queue class from a singly linked list and an array list. I am getting an error from the compiler and I am not really sure how I can fix it.

I'll just post some snippets in hopes that someone will be able to help me figure out what the error means exactly.

Queue class

Spoiler

//package name removed
public class Queue_SL<E> implements IQueue<E> {
	private SLinkedList<E> theSList;
	public Queue_SL() {
		this.theSList = new SLinkedList<E>();
	}
	@Override
	public void enqueue(E e) {
		this.theSList.addLast(e);
	}

Singly Linked List Class

Spoiler

//package name removed
public class SLinkedList<E> {

	private Node<E> head;
	private Node<E> tail;
	private long size;

	public SLinkedList() {
		this.head = null;
		this.tail = null;
		this.size = 0;
	}

	public void addLast(Node<E> n) {
		if (n == null) {
			throw new NullPointerException();
		}
		if (head == null) {
			head = n;
		}
		if (tail != null) {
			tail.setNext(n);
		} else {
			n.setNext(null);
		}
		tail = n;
		this.size++;
	}

Node Class

Spoiler

//package name removed
public class Node<E> {

	private E element;
	private Node<E> next;

	Node(E e, Node<E> n) {
		setElement(e);
		setNext(n);
	}

	public Node(E e) {
		this(e, null);
	}

	public E getElement() {
		return this.element;
	}

	public Node<E> getNext() {
		return this.next;
	}

	public void setElement(E e) {
		this.element = e;
	}

	public void setNext(Node<E> n) {
		this.next = n;
	}

	@Override
	public String toString() {
		return getElement().toString();
	}

}

The error I receive is:


I am not sure why the issue is occuring, but if it is a problem with the my method or code otherwise, I would appreciate someone explaining the problem to me.

Thank you.

[ianian112]

Your addlast is asking for a Node<E>, when it should be asking for E data. Your enqueue is trying to pass it E data, which is what is causing the error.

This post has been edited by ianian112: 07 February 2012 - 02:34 PM

[SpartanGuy07]

You say it should be asking for E data? Does that mean I need to redesign the addLast method?

We actually had to design the Singly Linked List last week, and write tests for it. So I know it works the way it was intended to with just simply adding the values into the list. Is there something I am doing wrong with the Queue on top of the Singly Linked Lists? Or like I said do I need to redesign things?

This post has been edited by SpartanGuy07: 07 February 2012 - 02:55 PM

[ianian112]

All you need to do in modify your addLast a littlebit

 public void addLast(Node<E> n) {

to

 public void addLast(E n) {
Node<E> a = new Node<E>(n);
}

[SpartanGuy07]

ianian112, on 07 February 2012 - 05:02 PM, said:

All you need to do in modify your addLast a littlebit

 public void addLast(Node<E> n) {

to

 public void addLast(E n) {
Node<E> a = new Node<E>(n);
}

OH RIGHT! I think I get it! Cause the new item will always be added to the end of the list, addLast should just always create a new node every time it is invoked right?

Wow I feel stupid that I didn't realize that! Thank you!

[blackcompe]

Like ian said, addLast expects a node not an instance of E. Create a node in enqueue with the value e and pass it to addLast. Ian was saying that your linked list should accept E and not a node, which is true, if you want to write an exportable list. As a client of your linked list, I shouldn't know anything about nodes. Instead I have to do this:

main() {
SLinkedList<Integer> llist = new SLinkedList<Integer>();
Node<Integer> elem = new Node<Integer>(2343);
llist.addLast(elem);
}

when the class should have been designed so that I can do this:

SLinkedList<Integer> llist = new SLinkedList<Integer>();
llist.addLast(2343);

What happens when I write code that depends on your Node implementation? Perhaps I want to keep track of the head of the list for odd reason.

Node<Integer> elem = new Node<Integer>(2343);
int first  = elem.getElement();

Later, you *learn why encapsulation is so important* and you decide to make your node class a private inner class of the linked list or you decide to remove the getter/setters. All of your client's codes break.

[SpartanGuy07]

Alright I understand what you mean. Thank you guys.