Can't get rid of NullPointerException for Comparator

Question

I've been struggling to get rid of a NullPointerException that occurs when calling the line of code:

 if (priorityComparator.compare(temp.next.value, newNode.value) >= 0 )

The full code is:

 public class HeaderLinkedPriorityQueue<E> extends 
       AbstractPriorityQueue<E> implements PriorityQueue<E> {

  //Some other methods, constructors etc.

 public boolean add (E e) {

  ListNode<E> temp = highest;


  ListNode<E> newNode = new ListNode<E>(e, null);

  if (temp.next == null){
      //first node in a list.
      temp.next = newNode;
      objectCount++;
      return true;
  }

  //if the value of the first element following the header node is greater than the newNode add to back.
  if (priorityComparator.compare(temp.next.value, newNode.value) >= 0 ) {
      temp.next.next = newNode;
      objectCount++;
  }
  else {
      //add before the first node in the list. have temp.next point to newNode and have newNode point to the old temp.next.
      newNode.next = temp.next;
      temp.next = newNode;
      objectCount++; 
  }
  return true;
 }

 //class variables.
 private ListNode<E>           highest     = new ListNode(null, null); 
 private int                   objectCount = 0;
 private Comparator<? super E> priorityComparator;

I don't see anything wrong with the parameters so I'm really stumped. How can I fix this?

Answer 1

It seems like you didn't initialize your PriorityComparator.

private Comparator<? super E> priorityComparator;

should be something like

private Comparator<? super E> priorityComparator = new PriorityComparator();