Question

I am working on a project to create a simple auction server that multiple clients connect to. The server class implements Runnable and so creates a new thread for each client that connects.
I am trying to have the current highest bid stored in a variable that can be seen by each client. I found answers saying to use AtomicInteger, but when I used it with methods such as atomicVariable.intValue() I got null pointer exception errors.

What ways can I manipulate the AtomicInteger without getting this error or is there an other way to have a shared variable that is relatively simple?

Any help would be appreciated, thanks.

Update

I have the AtomicInteger working. The problem is now that only the most recent client to connect to the server seems to be able to interact with it. The other client just sort of freeze.

Would I be correct in saying this is a problem with locking?

Answer 1

Well, most likely you forgot to initialize it:

private final AtomicInteger highestBid = new AtomicInteger();

However working with highestBid requires a great deal of knowledge to get it right without any locking. For example if you want to update it with new highest bid:

public boolean saveIfHighest(int bid) {
    int currentBid = highestBid.get();
    while (currentBid < bid) {
        if (highestBid.compareAndSet(currentBid, bid)) {
            return true;
        }
        currentBid = highestBid.get();
    }
    return false;
}

or in a more compact way:

for(int currentBid = highestBid.get(); currentBid < bid; currentBid = highestBid.get()) {
    if (highestBid.compareAndSet(currentBid, bid)) {
        return true;
    }
}
return false;

You might wonder, why is it so hard? Image two threads (requests) biding at the same time. Current highest bid is 10. One is biding 11, another 12. Both threads compare current highestBid and realize they are bigger. Now the second thread happens to be first and update it to 12. Unfortunately the first request now steps in and revert it to 11 (because it already checked the condition).

This is a typical race condition that you can avoid either by explicit synchronization or by using atomic variables with implicit compare-and-set low-level support.

Seeing the complexity introduced by much more performant lock-free atomic integer you might want to restore to classic synchronization:

public synchronized boolean saveIfHighest(int bid) {
    if (highestBid < bid) {
        highestBid = bid;
        return true;
    } else {
        return false;
    }
}