Adding nodes to end of linked list

Question

Having trouble adding nodes to the end of a linked The code is pretty self explanatory, the addToEnd method adds a single node to the end of a linkedlist.

public class ll5 {
    // Private inner class Node

    private class Node{
        int data;
        Node link;

        public Node(int x, Node p){
            data = x;
            link = p;
        }
    }
    // End of Node class

    public Node head;

    public ll5(){
        head = null;
    }

    public void addToEnd(int data) {
        Node p = head;
        while (p.link != null)
            p=p.link;
        p.link=new Node(data, null);
    }


    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        ll5 list = new ll5();

        list.printList();
        System.out.println("How many values do you want to add to the list");
        int toAdd = input.nextInt();

        for(int i = 0; i < toAdd; i++) {
            System.out.println("Enter value " + (i + 1));
            list.addToEnd(input.nextInt());
        }

        System.out.println("The list is:");
        list.printList();

        input.close();
    }

}

Why is it giving me an NullPointerException error?? The error is somewhere in the while loop in the addToEnd method.

I guess that Node p = head makes p = null since head is null when you call addToEnd the first time. — StepTNT
– StepTNT, Commented Apr 19, 2016 at 20:20

11thdimension · Accepted Answer · 2016-04-19 20:21:16Z

2

You haven't handled the initial condition when list has nothing and head is null. Because of that you're getting NPE.

Following method should work.

public void addToEnd(int data) {
    Node p = head;
    if( p == null) {
        head = new Node(data, null);
    } else {
        while (p.link != null)
            p=p.link;
        p.link=new Node(data, null);
    }
}

answered Apr 19, 2016 at 20:21

11thdimension

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Comments

Ethan F. · Accepted Answer · 2016-04-19 20:22:57Z

0

That's because the head is null at the beginning

public ll5(){
    head = null; // <-- head is null
}

public void addToEnd(int data) {
    Node p = head;  //<-- you assigned head, which is null, to p
    while (p.link != null) //<-- p is null, p.link causes NullException
        p=p.link;
    p.link=new Node(data, null);
}

answered Apr 19, 2016 at 20:22

Ethan F.

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