Doubly Linked List - null pointer exception again

Question

I'm getting this exception which seemed to be that I was doing things with a node that was null. Can someone explain how I am doing that? What is the constructor supposed to look like? I've seen it empty or with header and trailer dummy nodes..

    //default constructor
public AddressList() {

}

    //get size
public int size() {
    return counter;
}

public void addEntryNode(){
    //create the new node
    EntryNode n = new EntryNode();
    //set the data
    System.out.println("Enter first name: ");
    n.myFirstName = keyboard.next();
    n.setFirstName(n.myFirstName);

    System.out.println("Enter Last Name: ");
    n.myLastName = keyboard.next();
    n.setLastName(n.myLastName);

    System.out.println("Enter Email Address: ");
    n.myEmail = keyboard.next();
    n.setEmail(n.myEmail);

    System.out.println("Enter Phone Number: ");
    n.myPhoneNum = keyboard.next();
    n.setPhoneNum(n.myPhoneNum);

    n.setIndex(index);

    //add nodes to head of list
    head.setPrev(n);
    n.setNext(head);
    head = n;

    //up the count and index 
    counter++;

Seems like it was the head.setPrev(n). Should that be set to null instead? — jackie
– jackie, Commented Mar 6, 2012 at 1:09

kwarrick · Accepted Answer · 2012-03-06 01:17:15Z

1

Well, this seems like the most likely fix. Initialize the head to null, and add a condition for when the list is empty.

public AddressList() {
    head = null
}

And your condition would look like this:

if (head == null) {  // empty list
    head = n
}
else {
    head.setPrev(n);
    n.setNext(head);
    head = n;
}

answered Mar 6, 2012 at 1:17

kwarrick

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