Java Iterator Doubly Linked list

Question

Hi I'm very new to Java and trying to create a Deque class by implementing a doubly linked-list format. When I run the code(DequeApp), I get a NullPointerException refer back to my Iterator.next(Deque.java:44).

Error messages:  **Exception in thread "main" java.lang.NullPointerException
    at dlist.Deque$DoubleListIterator.next(Deque.java:44)



        public E next() {
                if (!hasNext()) {throw new NoSuchElementException();}
                else{
                E temp = current.item;
                current = current.next;
                return temp;}
            }

Very similar code (there are not that many ways to write linked lists...), @Aakash, although the index is incremented correctly there. — tucuxi
– tucuxi, Commented May 5, 2015 at 8:32
I understand that the algorithm would be same and most implementations would also be same, but the OP has asked same question in 2 different places, with same problem. He is getting NPE for which the solution was already provided. Though I have marked the question as duplicate, I also gave the solution to the problem myself as well. — Aakash
– Aakash, Commented May 5, 2015 at 8:40
Yes sorry that i asked the question twice, it's just that I didn't fully understand the other post, sorry I'm only starting to learn java, will hopefully get better at it, thanks Aakash! — Robert
– Robert, Commented May 5, 2015 at 8:44

Aakash · Accepted Answer · 2015-05-05 08:37:53Z

3

I have made two changes.

As tucuxi already told, increment index.

Start current from head, not head.next.

private class DoubleListIterator implements Iterator<E> {
// instance variable
private Node current = head;
private int index = 0;

public boolean hasNext() {
    return index < N;
}

public E next() {
    if (!hasNext()) {
        throw new NoSuchElementException();
    } else {
        index++;
        E temp = current.item;
        current = current.next;
        return temp;
    }
}

public void remove() {
    throw new UnsupportedOperationException();
}
}

answered May 5, 2015 at 8:37

Aakash

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1 Comment

Aakash Over a year ago

Most welcome. But @tucuxi said right. Build debugging habit and make sure everything is in place where they should be.

tucuxi · Accepted Answer · 2015-05-05 08:40:13Z

2

You forgot to increment your index counter in the DoubleListIterator. You write:

public E next() {
    if (!hasNext()) {
        throw new NoSuchElementException();
    } else {
        E temp = current.item;
        current = current.next;
        return temp;
    }
}

And you should have written:

public E next() {
    if (!hasNext()) {
        throw new NoSuchElementException();
    } else {
        index ++; // <---- without this, hasNext() always returns true
        E temp = current.item;
        current = current.next;
        return temp;
    }
}

Note also that I have changed the indenting format to that of Oracle's guidelines.

A second error is that you initialize your Iterator as follows:

    private Node current=head.next;

However, this makes it impossible to retrieve head (as you are already pointing to its next node). And it makes you index counter off-by-one. Corrected code:

    private Node current=head;

edited May 5, 2015 at 8:40

answered May 5, 2015 at 8:29

tucuxi

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2 Comments

Robert Over a year ago

Hi tucuxi, sorry I'm still getting the same exception. Exception in thread "main" java.lang.NullPointerException at dlist.Deque$DoubleListIterator.next(Deque.java:46) Which is this line "E temp = current.item;"

tucuxi Over a year ago

Found another error further up. You should really do a full pass with a debugger making sure everything looks good; untested code is always full of errors on the first run.

acearch · Accepted Answer · 2015-05-05 09:50:54Z

0

Another option to using index variable is

Maybe you could try "current.next != null" within hasNext.

But if it is already working with index no issues then.

answered May 5, 2015 at 9:50

acearch

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