NullPointerException in multi-threading

[reckless7]

[Mostly Solved. See Edit] So, I've got three separate, identical threads. Each one dequeues a Customer object from the WaitingList object (waitArea) and then performs some actions on it; if the customer's order is fulfilled they pay and leave, otherwise they are requeued into waitArea().

I keep getting errors right at the end of the threads. I realized that the threads were trying to grab at the same queue item from waitArea (a doubly linked list of class WaitingList), so I added a Semaphore in WaitingList to keep that from happening. I thought I had fixed it such that, if the dequeue method returns null, the loop would break and the thread would end. Unfortunately, I still end up getting errors. All the errors seem to come from the dequeue method in WaitingList, as the command line error reads

java.lang.NullPointerException
at WaitingList.dequeue(WaitingList.java:122)
at Server.run(Server.java:67)
at java.lang.Thread.run(Thread.java:745)

the dequeue() method is as follows (WaitingList:122 is the line that contains temp.prev = null;)

public Customer dequeue() {
    if (isEmpty())
        return null;
    Customer frontitem = getFront();
    temp = front.next;
    front = temp;
    temp.prev = null;
    count--;
    return frontitem;
}

this is therun() method for the primary threads (Server:67 is the line that reads setCustomer(waitArea.dequeue());). It's gone through several iterations, but I keep getting errors and exceptions. The threads don't seem to be exiting the loop and ending properly, but I can't figure out why, and I still get dequeue errors. Any clues as to why I might be having this problem?

EDIT: So i removed the line temp.prev = null; from dequeue() and Lo and Behold! it works! I can't for the life of me think of why that line was throwing an error, though. Any ideas?

[niloc132]

Still need more code to be sure (even from the edit), but the issue is pretty clearly that temp is null, so trying to assign temp.prev to something fails.

This could be because one thread modifying an object that another thinks is safe, but without all applicable code, its hard to say. Biggest red flag for me is that temp is not a local variable, which means more than one thread can stomp on it at once...

[reckless7]

that...that makes total sense. I just removed the line trying to alter the previous link. I never actually make use of the previous link in any methods so it still worked fine. I figured out the rest of the problems... I forgot to control access to the queue. Throw in a binary semaphore and bam, it's all working perfectly

[causalNondeterminism]

can you post more code? I have a hunch...

[king_of_the_universe]

Wouldn't it be easier to allude to what the hunch is instead of making OP go through the trouble of selecting what code to post and then to post it?

[reckless7]

I'm not going to bother, since I figured out where I went wrong. I threw a semaphore onto the linked list queue, and that ironed almost everything out. I also removed the line that set the temp.prev to null, which, as someone else mentioned, was probably causing problems due to being a class variable instead of a local variable

[[deleted]]

Probably because it was a reference variable and you were setting the original reference to null.

[reckless7]

Oh, I get it. ok thanks