Linked lists help... how exactly is this a NullPointerException?

Question

So I have a linked list class with variables head and tail both initialized to "null", with variables for size, declare, and traveler (not really relevant to this though). We have to create methods to manipulate said list.
For a test I declared an instance of this linked list:
static SinglyLinkedList list = new SinglyLinkedList();One of the methods we have to create is an append method, here's the code for that:
public void append(Object dataToAdd)
          if(head == null & tail == null)
               head = new Node(dataToAdd, null);
               tail = head;
          else
               tail.next = new Node(dataToAdd, null);
               tail = tail.next;
     }Testing it with...
list.append("A");Gives null pointer exceptions at the if statement in the append method as well as at the list.append line. But why? What is exactly pointing to null?

Answer 1

Ah... I didn't think of head not being null but tail being null.
I initiated the traveler node in the linked list class to head.
then I added this in between the if and else statements I had:
else if(head != null & tail == null)
               while(traveler.next != null)
                    traveler = traveler.next;
               tail = traveler;
               tail.next = new Node(dataToAdd, null);
               tail = tail.next;
                        }But it's still giving me the same errors.
Edited by: klawson88 on Feb 12, 2009 12:32 AM