Java Iterator on doubly linked list

Question

Hi I'm very new to Java and have this problem with building a nested Iterator class for a Doubly Linked List. I'm getting this error on E next method when running the test program. The goal of the next method in the Iterator is to return the next item in the Doubly Linked List.

Can anyone advice a fix on my code? Any help is greatly appreciated!

Error message:

Exception in thread "main" java.lang.NullPointerException at dlinkedlist.Deque$DoubleListIterator.next(Deque.java:51)

    public E next() {
        if (!hasNext()) throw new NoSuchElementException();
        last = current;
        E value = current.item;
        current = current.next; 
        index++;
        return value;
    }
    public void remove() { throw new UnsupportedOperationException(); }
  }// end class ListIterator

"next element" in what sence? Forward iteration or backward iteration? I believe you need a forward iteration here. — Aakash
– Aakash, Commented May 4, 2015 at 6:13
Yes, forward iteration, how do I accomplish that? And do I also need a backward method for the iterator? I'm still bit confused. Please help, thanks! — Robert
– Robert, Commented May 4, 2015 at 6:15
What is 'head' - your code snippet does not show such variable. Is the NPE thrown for the first call to 'next'? What is the dataset you insert to your list - give us more to analyze. — Rafal G.
– Rafal G., Commented May 4, 2015 at 6:16

Aakash · Accepted Answer · 2015-05-04 06:15:02Z

2

It seems your current object is null. Can you check it?

answered May 4, 2015 at 6:15

Aakash

2,12915 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Aakash Over a year ago

Well, the class Deque is in-complete. It does not include most of the required methods to work as a proper dequeue. Maybe OP can check the algorithm here basicdatastructures.blogspot.in/2007/12/…

Robert Over a year ago

public Deque() { head = new Node(); tail = new Node(); head.next = tail; tail.prev = head; N = 0; }

Aakash Over a year ago

Now it seems better. You should not get this exception now. :)

Robert Over a year ago

Sorry unfortunately I still get them, I suspect there must be something wrong with the E next method.

Aakash Over a year ago

Where is your add() method which adds a new node in dequeue? Also, give complete stacktrace.

Collectives™ on Stack Overflow

Java Iterator on doubly linked list

1 Answer 1

5 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related