java.lang.NullPointerException when trying to add elements to a circular linked list

Question

I'm trying to add an element to the circular linked list but it throws NullPointerException whenever I try to access the node. Here's my code: EDIT: I already read the post explaining NullPointerExceptions but nothing worked.

public class CircularLinkedList<T> {
static private class Node<T> {
    public T data;
    public Node<T> next;

    Node(T data, Node<T> next) {
        this.data = data;
        this.next = next;
    }
}

private Node<T> first;

public void addFirst(T x){
    if (first.data.equals(null)){
        first.data = x;
        first.next = first;
    }else{
        Node<T> second = first;
        first.data = x;
        first.next = second;
    }
}

EDIT: I already initialized the Node with private Node<T> first = new Node(null, null);, keeps throwing the error. Also compared with first.data == null, now it terminates the process.

EDIT:Here's the full code:

public class CircularLinkedList<T> {
    static private class Node<T> {
        public T data;
        public Node<T> next;

        Node(T data, Node<T> next) {
            this.data = data;
            this.next = next;
        }
    }

    private Node<T> first = new Node(null, null);

    public void addFirst(T x){
        if (first.data == null){
            first.data = x;
            first.next = first;
        }else{
            Node<T> second = first;
            first = new Node(x, second);
        }
    }

    public void print() {
        while (first.data == null) {
            System.out.println(first.data);
            first = first.next;
        }
    }

    public static void main(String[] args) {
        CircularLinkedList<String> lst = new CircularLinkedList<String>();
        lst.addFirst("Z");
        lst.addFirst("A");
        lst.addFirst("B");
        lst.print();
    }
}

Well, I read through that but i can't figure out how to initialize the Node, I mean, i do first = null, but keeps throwing the same NullPointerExeption when i do first.data.equals(null) — Ruben, Oct 08 '16 at 15:59
probably change `addFirst(T x)` to a `addFirst(Node root)` method — Mad Matts, Oct 08 '16 at 16:03
don't use equals to check for null, you probably want `if (first.data == null){` and some init of `first` (like in provided answer) — , Oct 08 '16 at 16:06
You should check `first == null`, then. Obviously `first.data` => `null.data` is the problem — OneCricketeer, Oct 08 '16 at 16:06

Andriy Kryvtsun · Accepted Answer · 2016-10-08T16:04:48.510

2

You get NPE in first.data.equals(null) statement as initially first variable itself is null. You have to initialize it at start like this

private Node<T> first = new Node(null, null);

Also you have to compare with null differently

if (first.data == null) {

edited Oct 08 '16 at 16:04

answered Oct 08 '16 at 16:03

Andriy Kryvtsun

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Did it, i keep getting the same error – Ruben Oct 08 '16 at 16:04
@Ruben see my later note about `null` comparison – Andriy Kryvtsun Oct 08 '16 at 16:06
That took the error away but now when i run it, it terminates it, doesn't even let me debugg – Ruben Oct 08 '16 at 16:08
@Ruben you have a problem in your `else` statement. You have to put `first = new Node(x, second)` after `Node second = first;` and remove other lines. – Andriy Kryvtsun Oct 08 '16 at 16:13
It terminates the process anyways. – Ruben Oct 08 '16 at 16:15
@Ruben share full you code with `main` method – Andriy Kryvtsun Oct 08 '16 at 16:16
Edited the thread to add it – Ruben Oct 08 '16 at 16:19
@Ruben you have 2 problems here: 1. add correct `class CircularLinkedList` definition; 2. you print method should look like this `public void print() { while (first.data != null) { System.out.println(first.data); if (first.next == null) return; first = first.next; } }` – Andriy Kryvtsun Oct 08 '16 at 16:27
Oh my lord, that did it, thank you kind man. – Ruben Oct 08 '16 at 16:33

java.lang.NullPointerException when trying to add elements to a circular linked list

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