Deque

Two ends of one structure

A Deque (double-ended queue) lets us add, inspect, and remove elements at both the front and the back. Unlike a regular queue, it supports operations on both ends. That means one deque can act as either a stack or a queue, and some algorithms genuinely need both ends at the same time.

Deque<Integer> deque = new ArrayDeque<>();

deque.addFirst(20);
deque.addFirst(10);
deque.addLast(30);

int left = deque.removeFirst(); // 10
int right = deque.removeLast(); // 30

After these operations, only 20 remains in the structure. With ArrayDeque, operations on both ends are amortized O(1).

Choose methods according to the role

The Deque interface contains many similar methods. If you use it as a stack, it is convenient to stick to push, pop, and peek. If you use it as a queue, work explicitly with addLast, removeFirst, and peekFirst. This makes the direction of data movement visible in the code itself.