Doubly Linked List Null Pointer Exception

Question

I'm trying to work on a remove function for a doubly linked list, but I keep getting a Null Pointer exception on the current.prev.next = current.next; part. I don't think I really understand what a null pointer exception is, because I have no idea what to do to fix this. The log function is just one I wrote to write to an output file, and retval[1] is the element I'm searching to delete.

Node current = head;

    while(current != null)
    {
            if((current.data).compareTo(retval[1]) == 0)
            {
                    if(current.prev == null)
                        head = current.next;

                    if(current.next == null)
                        tail = current.prev;

                    current.prev.next = current.next;
                    current.next.prev = current.prev;

                    current = null;

                    valid++;
                    log(line + "\n" + "Sucsessfully Removed \n");

            }
            else
            {
                log(line + "\n" + InvalidTransaction + " - Element does not exist \n");     
            }

            current = current.next;
        }

I'm sure it's something stupid, but I don't know what it is. Any help would be greatly appreciated.

Vinod Kumar Y S · Accepted Answer · 2013-10-07 01:20:21Z

1

Just replace

current.prev.next = current.next;
current.next.prev = current.prev;

with

if(null != current.prev) current.prev.next = current.next;
if(null != current.next) current.next.prev = current.prev;

And also you need to break the loop once the element is found.

answered Oct 7, 2013 at 1:20

Vinod Kumar Y S

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Comments

tbodt · Accepted Answer · 2013-10-07 01:19:12Z

0

A NullPointerException means that you tried to do something with an object that was null. Since current definitely wasn't null, then current.prev must have been null. Go from there.

answered Oct 7, 2013 at 1:19

tbodt

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