Queues, Deques & Priority Queues

Queue, Deque, PriorityQueue

ArrayDeque is Java's best stack AND queue — faster than the legacy Stack class and LinkedList:

Deque<String> stack = new ArrayDeque<>();
stack.push("a");          // addFirst
stack.pop();              // removeFirst — LIFO

Deque<String> queue = new ArrayDeque<>();
queue.offer("a");         // addLast
queue.poll();             // removeFirst — FIFO

Two method flavors exist for a reason:

• add/push/offer throw or lie differently on capacity failure
• element/peek vs remove/poll: the first pair throws when empty, the second returns null — choose explicitly, don't mix casually.

PriorityQueue is a heap: poll() always yields the smallest element according to its comparator, not insertion order:

PriorityQueue<Task> q = new PriorityQueue<>(Comparator.comparingInt(Task::priority));

O(log n) insert/poll — the tool for "always process the most urgent next".