Stuck with a nullpointerexception on linked list implementation

Question

When trying to add things at a specific index to my linkedlist, I am receiving a nullpointerexception. I know why, but I don't know how to fix it.

Here is my code for the nodes of the linked list

public class LinkedList<Key> implements LinkedListInterface<Key>{
        private Node head;
    private Node tail;
    private int size = 0;

    private class Node{
        Key key;
        Node next;
    }

And here is my code where I'm trying to add to the linked list at a specific index.

 public void add(int index, Key key) {
        if(index < size){
        Node left = null, newNode = null;
        Node right = head;
        for(int i = 0; i < index; i++){
            left = right;
            right = right.next;
        }
        left.next = newNode;
        newNode.next = right;
        newNode.key = key;
        size++;
        }
        else{
            addToEnd(key);
        }
    }
private void addToEnd(Key key) {
        Node lastNode = new Node();
        if(size == 0){
            tail = head = lastNode;
        }
        else{
            lastNode.key = key;
            tail.next = lastNode;
            tail = lastNode;
        }
        size++;     
    }

I know that by default my nodes will be null, but I am unable to do anything with them until they have a value. However, it seems that in order to be able to assign a value to them, they can't be null. I just can't wrap my head around it.

Thanks for the help

http://stackoverflow.com/questions/21632621/add-nodes-in-linked-list/21633256#21633256 — Joop Eggen, Feb 11 '14 at 23:09

score 1 · Answer 1 · answered Feb 11 '14 at 23:08

1

It looks to me like you just need to initialize newNode up there on the second line of add(...), rather than setting the variable to null.

answered Feb 11 '14 at 23:08

MattPutnam

2,927
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score 0 · Accepted Answer · answered Feb 11 '14 at 23:07

0

When adding a node, you need memory allocated for the new node.

So, when you create newNode, allocate memory for it:

Node left = null, newNode = new Node();

answered Feb 11 '14 at 23:07

Steven Hansen

3,189
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Are you free to skype for a few minutes? I'm experiencing a really weird bug when debugging and I don't even know how to describe it without sharing my screen. – user2738003 Feb 11 '14 at 23:55

Stuck with a nullpointerexception on linked list implementation

2 Answers2