NullPointerException while implementing Queue using Linked List

Question

I am following Coursera Algorithm 1 course, and right now implementing Queues using linked list, but getting a NullPointerException. Please help me out.

package algo_packages;

public class QueueLinkedList {
    private Node first, last;
    public class Node{
        String item;
        Node next;
    }

    public QueueLinkedList(){
        first = null;
        last = null;
    }
    public boolean isEmpty(){
        return first == last;
    }
    public void enqueue(String item){
        Node oldLast = last;
        last = new Node();
        last.item = item;
        last.next = null;
        if(isEmpty()){
            first = last;
        }
        else {
            oldLast.next = last;
        }
    }
    public String dequeue(){
        String item = first.item;
        first = first.next;
        if (isEmpty()) last = null;
        return item;
    }
}

I am getting the exception at:

oldLast.next = last;

oldLast.next caused the NullPointerException when I tried to debug the program.

Eran · Accepted Answer · 2014-10-22 12:44:00Z

1

The first time you enqueue an item isEmpty() returns false, since it checks if first==last, but first is still null and last is no longer null (since you already assigned the new Node to it). This brings you to access oldLast.next when oldLast is null, hence the NullPointerException.

A possible fix :

public void enqueue(String item)
{
    Node oldLast = last;
    Node newNode = new Node();
    newNode.item = item;
    newNode.next = null;
    if(isEmpty()) { // last is not assigned yet, so isEmpty returns correct result
        last = newNode;
        first = last;
    } else {
        last = newNode;
        oldLast.next = last;
    }
}

edited Oct 22, 2014 at 12:44

answered Oct 22, 2014 at 12:38

Eran

395k57 gold badges726 silver badges793 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

brso05 · Accepted Answer · 2014-10-22 12:50:19Z

0

When you check isEmpty() it always returns false for enqueue because you are setting last to a new Node() which will never equal first. You don't need to check if list isEmpty() because if list is empty then first == last so you don't need to assign first = last because they are already equal. Try this:

public void enqueue(String item){
    Node oldLast = last;
    last = new Node();
    last.item = item;
    last.next = null;
    if(oldLast != null)
    {
        oldLast.next = last;
    }
    else
    {
        first = last;
    }
}

answered Oct 22, 2014 at 12:50

brso05

13.3k2 gold badges25 silver badges41 bronze badges

Collectives™ on Stack Overflow

NullPointerException while implementing Queue using Linked List

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related