Concurrent Set Queue

Question

Maybe this is a silly question, but I cannot seem to find an obvious answer.

I need a concurrent FIFO queue that contains only unique values. Attempting to add a value that already exists in the queue simply ignores that value. Which, if not for the thread safety would be trivial. Is there a data structure in Java or maybe a code snipit on the interwebs that exhibits this behavior?

Answer 1

There's not a built-in collection that does this. There are some concurrent Set implementations that could be used together with a concurrent Queue.

For example, an item is added to the queue only after it was successfully added to the set, and each item removed from the queue is removed from the set. In this case, the contents of the queue, logically, are really whatever is in the set, and the queue is just used to track the order and provide efficient take() and poll() operations found only on a BlockingQueue.