java.util · java.base · since Java 1.6

ArrayDeque

declaration

public class ArrayDeque<E> extends AbstractCollection<E>
        implements Deque<E>, Cloneable, Serializable

Circular-array Deque — the right default for both stacks and queues: faster than java.util.Stack (unsynchronized) and than LinkedList (no nodes).

Null elements are forbidden (null is the "empty" sentinel for poll)
Not thread-safe — synchronize externally or use a concurrent Deque implementation across threads.

Key methods

`ArrayDeque()`	Default capacity, backed by a resizable circular array.
`ArrayDeque(int numElements)`	Presize to skip regrowth copies.

Example

Java

Deque<Integer> stack = new ArrayDeque<>();
stack.push(1);
stack.push(2);
stack.push(3);
stack.pop(); // 3 — LIFO

Deque<Integer> queue = new ArrayDeque<>();
queue.offer(1);
queue.offer(2);
queue.offer(3);
queue.poll(); // 1 — FIFO

Same class, two disciplines: push/pop for LIFO, offer/poll for FIFO — pick the pair that matches intent.