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ArrayDeque Example in Java

beginner· Collections · Queue & Stack

Problem

ArrayDeque is a resizable double-ended queue — elements can be added or removed from either the front or the back, not just one end like a plain queue.

Add elements to both ends of an ArrayDeque, then remove one from each end.

Input

addFirst(10), addLast(20), addFirst(5), addLast(25)

Output

Deque: [5, 10, 20, 25]
Removed from front: 5
Removed from back: 25
Deque now: [10, 20]

Java Program

Java

import java.util.ArrayDeque;
import java.util.Deque;

public class ArrayDequeExample {
    public static void main(String[] args) {
        Deque<Integer> deque = new ArrayDeque<>();
        deque.addFirst(10);
        deque.addLast(20);
        deque.addFirst(5);
        deque.addLast(25);
        System.out.println("Deque: " + deque);

        System.out.println("Removed from front: " + deque.removeFirst());
        System.out.println("Removed from back: " + deque.removeLast());
        System.out.println("Deque now: " + deque);
    }
}

▶ Run Code

Output

Deque: [5, 10, 20, 25]
Removed from front: 5
Removed from back: 25
Deque now: [10, 20]

Core Logic

Calling addFirst() and addLast() in whatever order builds up the deque from both directions at once, and the same first/last symmetry applies when removing elements again.

How It Works

1addFirst(10) starts the deque with a single element.
2addLast(20) appends to the opposite end, then addFirst(5) and addLast(25) keep extending both ends, ending with [5, 10, 20, 25].
3removeFirst() removes and returns the element currently at the front — 5.
4removeLast() removes and returns the element currently at the back — 25 — leaving [10, 20].

Building the deque from both ends produces [5, 10, 20, 25], and removing one element from each end brings it back down to [10, 20].

💡

Key Point: Every one of these four methods works on a specific end — there's no ambiguity about which side is affected, unlike a plain List where add() always appends to the end.

Complexity

Time Complexity: O(1)Space Complexity: O(n)

Why: ArrayDeque's resizable circular array lets it add or remove from either end in amortized constant time, while holding up to n elements at once.

Key Concepts

ArrayDequeaddFirst()addLast()removeFirst()removeLast()

ArrayList Basic Operations

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