How implementation of java.util.queue uses LIFO?

Question

In Java doc:

[...] Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out)

How implementation of java.util.queue uses LIFO instead of FIFO?

Answer 1

Implementation of the Queue can base on FIFO, priorities and LIFO - that is what official documentation says.

When a programmer first sees "Queue" he automatically thinks "it must be FIFO order" (or eventually prioritized order). But as documentation says there must be possibility to use Queue interface for LIFO ordering. Let me explain you how it can be done.

// FIFO queue usage
Queue<Integer> queue = new LinkedList<>();
queue.add(1);
queue.add(2);

queue.remove(); // returns 1
queue.remove(); // returns 2

// LIFO queue usage
Queue<Integer> queue = Collections.asLifoQueue(new ArrayDeque<>());
queue.add(1);
queue.add(2);

queue.remove(); // returns 2
queue.remove(); // returns 1

As you can see depending on the implementation, Queue interface can be used also as a LIFO.

Answer 2

You can use a java.util.LinkedList and use the pop() and push() methods and use it like a stack, which is a LIFO queue.