ThreadPool's uses BlockingQueue to store tasks in queue.
I want executor that takes random task from queue. So first task and last task in queue have equal chances to be taken from.
Is that possible to do?
Yes it is technically possible to implement a thread pool which chooses the next task randomly. You can instantiate ThreadPool with a caller-supplied queue.
While it seems strange (even dangerous or subversive!) to some people1, a Queue is not necessarily FIFO. Indeed the javadoc for Queue states:
Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner.
So all you need to do to implement random thread pool behavior is to implement your own BlockingQueue class with a take() that selects the element randomly.
Alternatively, @Ben Manes' idea is use a PriorityBlockingQueue and assign random priorities. (This is simpler, but there is an overhead in keeping the queue heapified: O(1) on average, but O(logN) in the worst case.)
1 - Actually, in the real world queues are largely a social convention. Some cultures apparently don't follow this convention; e.g. https://www.thelocal.it/20150410/my-italian-habits-that-foreigners-just-dont-get. By contrast: https://www.standard.co.uk/lifestyle/london-life/british-people-display-amazing-queuing-etiquette-without-being-told-a3528366.html