LinkedList Iterator

[cpguy]

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!!

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();
      }
   }

[Junky]

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

[cpguy]

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)

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];

[Junky]

E data = position.prev.data;

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

[cpguy]

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

[Junky]

Think about it!

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

[cpguy]

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.

[Junky]

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.

[cpguy]

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