2.5.2 ArrayDeque Explained

The ArrayDeque class in Java is a part of the Java Collections Framework and implements the Deque interface. It is a resizable-array implementation of a double-ended queue, allowing elements to be added or removed from both ends efficiently. Understanding ArrayDeque is crucial for managing data in scenarios where elements need to be accessed from both ends.

Key Concepts

1. Double-Ended Queue (Deque)

A Deque (pronounced "deck") is a linear collection that supports element insertion and removal at both ends. The ArrayDeque class provides this functionality using a resizable array, making it more efficient than a linked list for most operations.

Example

        ArrayDeque<String> deque = new ArrayDeque<>();
        deque.addFirst("First");
        deque.addLast("Last");
        System.out.println(deque); // Output: [First, Last]
    

2. Resizable Array

The ArrayDeque uses a resizable array to store its elements. This array automatically grows as elements are added, ensuring that the deque can handle a large number of elements without running out of space.

Example

        ArrayDeque<Integer> deque = new ArrayDeque<>();
        for (int i = 0; i < 1000; i++) {
            deque.add(i);
        }
        System.out.println(deque.size()); // Output: 1000
    

3. Efficient Operations

The ArrayDeque provides constant-time performance for adding and removing elements at both ends. This makes it an efficient choice for scenarios where elements need to be accessed from both the front and the back.

Example

        ArrayDeque<String> deque = new ArrayDeque<>();
        deque.addFirst("Front");
        deque.addLast("Back");
        System.out.println(deque.removeFirst()); // Output: Front
        System.out.println(deque.removeLast()); // Output: Back
    

4. No Null Elements

The ArrayDeque does not permit the inclusion of null elements. Attempting to add a null element will result in a NullPointerException.

Example

        ArrayDeque<String> deque = new ArrayDeque<>();
        try {
            deque.add(null);
        } catch (NullPointerException e) {
            System.out.println("Null elements are not allowed.");
        }
    

Examples and Analogies

Think of an ArrayDeque as a double-ended train where passengers can board and alight from both the front and the back. The train (deque) has a flexible number of carriages (resizable array) that can expand as more passengers (elements) join. Passengers cannot board with a ticket that says "null" (no null elements).

By mastering the ArrayDeque class, you can efficiently manage and manipulate data in scenarios where elements need to be accessed from both ends, making your Java SE 11 applications more flexible and performant.