Is PriorityQueue a FIFO Queue?

Question

PriorityQueue implements Queue, but is PriorityQueue a FIFO data structure like Queue?

Answer 1

From the Queue interface:

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering

So PriorityQueue is an exception and it becomes a FIFO queue only if the comparator sorts in that order.

Answer 2

No, it is not. As per Javadoc

The elements of the priority queue are ordered according to their natural ordering, or by a Comparator provided at queue construction time, depending on which constructor is used

AND

The head of this queue is the least element with respect to the specified ordering