NullPointerException in the implementation of Doubly Linked List

Question

I am getting a NullPointerException when I attempt to add an object with InsertFront() method. The DList code is:

public class DList {
 protected DListNode head;
  protected int size;

protected DListNode newNode(Object item, DListNode prev, DListNode next) {
    return new DListNode(item, prev, next);
  }

public DList() {
    head=newNode(null,head,head);
    size=0;
  }


public void insertFront(Object item) {
     head.next.prev=newNode(item, head, head.next);
     head.next=head.next.prev;
     size++;
}

However, this error no longer shows up as soon as I change the DList constructor to this:

 public DList() {
        head=newNode(null,head,head);
        head.prev=head;
        head.next=head;
        size=0;
      }

Now, I do understand that assigning the head.next & head.prev values solved the problem; But I don't understand what was the need for stating this seperately when I already assigned the 'head' variable as the prev and next nodes in the 1st line of the constructor:

head=newNode(null,head,head);

Please explain.

Answer 1

In the initial constructor, this may not be doing what you think it is:

head=newNode(null,head,head);

Note that head is null initially, so the call is really something like this:

head=newNode(null,null /*prev*/, null /*next*/);

In insertFront you try to reference head.next.prev , but since head.next was null you get an exception.

Another way to think of your old constructor is to decompose it into 2 lines:

DListNode temp=newNode(null,head,head); // remember that head is null here
head=temp;

The method arguments are evaluated before the variable assignment.