Collections Framework

Queues, Deques & Priority Queues

Queue offers FIFO with polite failure modes; Deque adds both ends (and replaces the legacy Stack); PriorityQueue always surrenders the smallest element. ArrayDeque is the right default for both stack and queue.

Two method families: throwing (add/remove/element) vs value-returning (offer/poll/peek)
ArrayDeque: circular array, faster than LinkedList as a queue and than Stack as a stack
PriorityQueue: binary heap; poll is O(log n); iteration is not sorted
Blocking variants (ArrayBlockingQueue, LinkedBlockingQueue) live in Concurrent Collections

Stack and queue via Deque

Deque<Task> queue = new ArrayDeque<>();
queue.offer(task);            // enqueue at tail
Task next = queue.poll();     // dequeue from head (null if empty)

Deque<Frame> stack = new ArrayDeque<>();
stack.push(frame);            // addFirst
Frame top = stack.pop();      // removeFirst (throws if empty)

PriorityQueue

Queue<Job> jobs = new PriorityQueue<>(Comparator.comparing(Job::deadline));
jobs.addAll(pending);
while (!jobs.isEmpty()) {
    process(jobs.poll());     // always the earliest deadline
}