Doubly linked list removing first element

Question

I'm pretty new to java, and my doubly linked list addFront method and addRear method works fine, but only dequeue method doesn't works. In the main method, I making a test to remove front element using dequeueFront ()method, when I remove front element it works, but If I continued removing front element,

Exception in thread "main" java.lang.NullPointerException
    at DoublyLinkedDeque.dequeueFront(DoublyLinkedDeque.java:97)

comes out,

Line97 is the frontNode.previous=null

I just wondering how to remove front element form the double linked list properly.

public E dequeueFront() throws NoSuchElementException 
{
    if(frontNode!=null)
    {
        E frontElement=frontNode.element;
        frontNode=frontNode.next;
        frontNode.previous=null;
        frontNode.next=null;
        numElement--;
        if(numElement==0)
            rearNode=null;
        return frontElement;
    }
    else
        throw new NoSuchElementException();
}

Answer 1

 frontNode=frontNode.next;

If there is no next after frontNode, then frontNode.next returns null.

If frontNode is null then trying to do:

 frontNode.previous=null;

Will throw a null pointer exception because how can you access the previous node from a node that doesn't exist? Without thinking about how to make it work with your implementation, just doing a null check will stop this from happening.

if(frontNode != null){
    frontNode.previous = null;
}

You will have to do this check with the ".next" also

Or, maybe you can check if numElements = 1. I'll let you think of how to solve that problem.

Answer 2

It should look more like

public E dequeueFront() throws NoSuchElementException {
if(frontNode!=null)
{
    E frontElement=frontNode.element;
    N oldFront = frontNode ;
    frontNode=oldFront.next; // frontNode may now be null
    if( frontNode != null ) frontNode.prev=null; // remove link to oldFront
    oldFront.previous=null; // should be unessary
    oldFront.next = null ;

    numElement--;
    if(numElement==0)
        rearNode=null;
    return frontElement;
}
else
    throw new NoSuchElementException();
}