LinkedList Iterator

For an assignment, I had to create my own LinkedList class -- called BasicLinkedList -- that has many methods similar to LinkedList. I have to create a BasicListIterator that can iterate through the list, which contains the next(), previous(), hasNext(), and hasPrevious() methods. I'm having trouble with this, and keep getting NullPointerExceptions whenever I try to test it. Please let me know what I'm doing wrong!!

Code:

---

private class Node
  {
      public E data;
      public Node next;
      public Node prev;

  }
  private Node head;


private class MyListIterator implements BasicListIterator<E>
  {
      private Node position = head;
     
      public E next()
      {
        if(!(hasNext()))
        {
            throw new NoSuchElementException();
        }
        else
        {
            E data = position.data;
            position = position.next;
            return data;
        }
      }
     
      public E previous()
      {
        if(!(hasPrevious()))
        {
            throw new NoSuchElementException();
        }
        else
        {
            System.out.println(position);
            E data = position.prev.data;
            position = position.prev;
            return data;
        }
      }
     
      public boolean hasNext()
      {
        if(position == null)
        {
            return false;
        }
        else
        {
            return true;
        }
      }
     
      public boolean hasPrevious()
      {
        if(position == head)
        {
            return false;
        }
        else
        {
            return true;
        }
      }
     
     
     
      public void remove()
      {
        throw new UnsupportedOperationException();
      }
  }

---

We don't read minds. Post your errors and indicate on which lines they occur.

Sorry. Here are my tests. When I call iter.previous(), I get this error:
Exception in thread "main" java.lang.NullPointerException
at BasicLinkedList$MyListIterator.previous(BasicLinke dList.java:38)

Code:

---

BasicLinkedList<Integer> list = new BasicLinkedList<Integer>();
int[] ints = new int[] {99, 88, 77, 66, 55};
list.add(ints[0]);
BasicListIterator<Integer> iter = list.basicListIterator();

iter.next() == ints[0];
iter.previous() == ints[0];

---

Code:

---

E data = position.prev.data;

---

Is that line 38?
When position is head then prev is null. So prev.data will throw a NPE.

I changed it to just "position.data" but am still getting the same error...

Think about it!

When you are at the head does it have a previous?

No -- but we aren't at head. After calling iter.next(), we're at the next node; so prev would be the node we just went over.

I see you have a print statement. Change it to print previous.data to see exactly which node is throwing the NPE. Then trace back as to what and why it is null.

Tried that... Still can't figure it out. :(