linked list in java insert at end

Question

Guys I do have problem while inserting a new node at the end of a list. insert at begening works fine but in insert at end it shows null pointer exception.

Please look in method inserAtEnd and suggest me how to deal with it:

package linkedlist;

class Node{
    Node next;
    Object data;
    Node(Object x)
    {
        data=x;
    }
}
class LL
{
    Node first;

    LL(Node f)
    {
       first =f;
    }
    void insertAtBeg(Object x)
    {

        Node n= new Node(x);
        n.next=first;
        first=n;

    }
    void insertAtEnd(Object x)
    {
        Node n= new Node(x);
        Node k;
        k=first;
        while(k!=null)
        {
            k=k.next;
        }

      k.next=n;


    }
    void print()
    {

        Node p=first;
        while(p!=null)
        {

            System.out.print(p.data+"-->");
            p=p.next;
        }
    }

}
public class LinkedList {


    public static void main(String[] args) {
        LL l = new LL(null);
        l.insertAtBeg("Raj");
        l.insertAtBeg("Am ");
        l.insertAtBeg("I ");
        l.insertAtBeg("100 ");
        l.insertAtEnd("Hello");
        l.print();
        System.out.println("returned in main");

    }

}

Please provide the entire stack trace so we know where the error occurs. — NitrogenReaction
– NitrogenReaction, Commented Aug 2, 2015 at 16:37
Also, please add a language tag (It looks like Java, but I can't be sure). Additionally, please follow Java conventions (If this is even Java). — NitrogenReaction
– NitrogenReaction, Commented Aug 2, 2015 at 16:49

NitrogenReaction · Accepted Answer · 2015-08-02 16:46:17Z

1

The problem is your loop is going till k is null, you want to go till k.next is equal to null. You could also solve it by having a variable for the last node(This is what is almost always done). The code I've provided fixes the problem without adding any new variables however:

void insertAtEnd(Object x) {
    Node n= new Node(x);
    Node k;
    k=first;

    while(k.next != null) {
        k=k.next;
    }

    k.next=n;
}

answered Aug 2, 2015 at 16:46

NitrogenReaction

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3 Comments

NitrogenReaction Over a year ago

My answer solves your problem, so would you mind accepting it or telling me what else I can add?

Raj Gawade Over a year ago

thank you Sir, your answer solved my problem. It works now :)

NitrogenReaction Over a year ago

Glad I could help. I would appreciate it if you would accept my answer however.

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