The code I type in title doesn't compile. It says: Cannot infer arguments.
After that I did BlockingQueue<Integer> a = new PriorityBlockingQueue<>(2); which compiles just fine. What was my mistake, so I can avoid it next time?
Yes, I know that constructor in Queue interface has parameter(2) for initial capacity, while in BlockingQueue parameter(2) represents max elements. Does this have anything to do with the error?
Well for starters, PriorityQueue is not a BlockingQueue. It wont be able to infer any generic arguments for PriorityQueue<T> because there is no T that will be valid. That being said, Cannot infer arguments sounds like it may be due to a side effect of this change somewhere else in your code.
Looking at the documentation for BlockingQueue, you should instead use one of the classes under "All Known Implementing Classes:", find a library with BlockingQueues, or implement your own.
Your other option is to use another type of queue instead, like AbstractQueue<Integer> in place of BlockingQueue<Integer>.