What is Deque & ArrayDeque in Java?

Java provides several data structures to store and manipulate data, and two of them, Deque and ArrayDeque, are specifically designed for managing elements in a flexible way. They are part of the java.util package and are commonly used when working with stacks, queues, or double-ended queues. Let’s dive deeper into understanding these structures.

What is a Deque?

The term Deque stands for Double-Ended Queue. It is an interface in Java that extends the Queue interface and allows elements to be added or removed from both ends of the queue. This flexibility makes it suitable for implementing stacks, queues, and deque operations.

Key Characteristics of Deque:

1. Double-Ended: You can add or remove elements from both the front and the rear.
2. No Capacity Restrictions: Deque implementations can dynamically grow in size (e.g., ArrayDeque).
3. Supports Both Stack and Queue Operations: It can act as:
   • Queue: First-In-First-Out (FIFO) operations.
   • Stack: Last-In-First-Out (LIFO) operations.

Deque Operations:

The Deque interface provides methods for operations on both ends:

• Insertions: addFirst(), addLast(), offerFirst(), offerLast()
• Deletions: removeFirst(), removeLast(), pollFirst(), pollLast()
• Access Elements: getFirst(), getLast(), peekFirst(), peekLast()

What is ArrayDeque?

ArrayDeque is one of the most commonly used implementations of the Deque interface. It is backed by a resizable array and provides an efficient and flexible alternative to other queue or stack implementations like LinkedList and Stack.

Key Characteristics of ArrayDeque:

1. Resizable Array: Unlike a fixed-size array, it dynamically resizes to accommodate more elements.
2. No Null Elements: It does not allow null values.
3. Fast Operations: Provides faster operations than LinkedList for both ends, as it avoids overhead from node pointers.
4. Not Thread-Safe: It is not synchronized, so external synchronization is required for concurrent access.

How to Use Deque and ArrayDeque in Java

1. Using Deque as a Queue (FIFO):

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

public class DequeAsQueue {
    public static void main(String[] args) {
        Deque<Integer> deque = new ArrayDeque<>();

        // Adding elements (to the rear)
        deque.addLast(10);
        deque.addLast(20);
        deque.addLast(30);

        // Removing elements (from the front)
        System.out.println(deque.removeFirst()); // Output: 10
        System.out.println(deque.removeFirst()); // Output: 20

        // Peeking at the front
        System.out.println(deque.peekFirst()); // Output: 30
    }
}

2. Using Deque as a Stack (LIFO):

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

public class DequeAsStack {
    public static void main(String[] args) {
        Deque<String> stack = new ArrayDeque<>();

        // Pushing elements (to the top)
        stack.push("Java");
        stack.push("Python");
        stack.push("C++");

        // Popping elements (from the top)
        System.out.println(stack.pop()); // Output: C++
        System.out.println(stack.pop()); // Output: Python

        // Peeking at the top
        System.out.println(stack.peek()); // Output: Java
    }
}

Advantages of ArrayDeque

1. Efficient Performance: Compared to LinkedList and Stack, ArrayDeque is faster for stack and queue operations due to its array-based implementation.
2. Dynamic Resizing: Automatically resizes when the capacity is exceeded, unlike traditional arrays.
3. Versatility: Can be used as both a stack and a queue.
4. No Overhead: Unlike LinkedList, it doesn’t need to maintain references for nodes, saving memory.

Methods in Deque (Summary)

When to Use Deque and ArrayDeque?

1. Deque:
   • Use when you need operations at both ends (front and rear).
   • Suitable for applications like sliding windows, task scheduling, and BFS/DFS.
2. ArrayDeque:
   • Use when you want a faster alternative to Stack or LinkedList for queue operations.
   • Ideal for LIFO (stack) or FIFO (queue) implementations in non-threaded environments.

The Deque interface and its ArrayDeque implementation are powerful tools for managing collections in Java. With support for double-ended operations, Deque provides versatility for a wide range of use cases, while ArrayDeque delivers performance and memory efficiency. By understanding their functionality, you can implement efficient data structures tailored to your application’s needs.