Key Points about ArrayDeque and LinkedList:

Underlying Data Structure:

• ArrayDeque: Uses a resizable array to store elements.
• LinkedList: Uses a doubly-linked list data structure.

Performance:

• ArrayDeque: Generally faster for add/remove operations at both ends (beginning and end) because of its array-based structure.
• LinkedList: Also provides constant time for add/remove operations at both ends but might be slightly slower for these operations due to the overhead of managing node references.

Memory Overhead:

• ArrayDeque: More memory-efficient due to using a contiguous block of memory.
• LinkedList: Uses more memory per element since it needs to store two references (next and previous) for each node in addition to the actual data.

Capacity:

• ArrayDeque: Has an initial capacity but grows as needed.
• LinkedList: This does not have a capacity constraint as it dynamically allocates memory for each new node.

Implements Interfaces:

• ArrayDeque: Implements the Deque interface.
• LinkedList: Implements both the List and Deque interfaces, making it more versatile in terms of API methods.

Null Elements:

• ArrayDeque: Does not support storing null elements.
• LinkedList: Allows null elements.

Use Cases:

• ArrayDeque: Better suited for stack and queue implementations where rapid operations at both ends are crucial and there’s no need for indexed access.
• LinkedList: More versatile due to list capabilities. Suitable for situations where list functionalities and deque operations are both needed.

Access Time:

• ArrayDeque: Fast for operations at the ends but slower for random access as it might require O(n) time in the worst case.
• LinkedList: Provides O(n) time for random access since it needs to traverse from the start or end to reach a particular index.

Difference Between ArrayDeque and LinkedList in Java

Criteria	ArrayDeque	LinkedList
Underlying Data Structure	Resizable-array	Doubly-linked list
Interfaces Implemented	Deque	List, Deque
Insertion & Removal at Ends	Amortized constant time (Fast)	Constant time (Fast)
Element Access	No direct access by index	Access by index can take O(n) in worst case
Memory Overhead	Less overhead as it uses a continuous block of memory	More overhead due to storing two references (for previous and next node) alongside data
Null Elements	Does not allow null elements	Allows null elements
Capacity	Has an initial capacity that grows when needed	No capacity restrictions as elements are added dynamically
Use-case	Preferred when used primarily as a stack or queue (double-ended queue)	Preferred when you need a list with listIterator capabilities, or if you’ll be inserting/removing elements from the middle frequently

Example: Difference Between ArrayDeque and LinkedList in Java

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

public class ArrayDequeVsLinkedListDemo {

    public static void main(String[] args) {

        // Create an ArrayDeque and perform operations
        Deque<String> arrayDeque = new ArrayDeque<>();
        arrayDeque.offer("apple");
        arrayDeque.offerFirst("banana");
        arrayDeque.offerLast("cherry");
        arrayDeque.pollFirst();

        System.out.println("ArrayDeque:");
        arrayDeque.forEach(System.out::println);

        // Create a LinkedList and perform operations
        LinkedList<String> linkedList = new LinkedList<>();
        linkedList.offer("diamond");
        linkedList.offerFirst("emerald");
        linkedList.offerLast("fossil");
        linkedList.pollFirst();

        System.out.println("\nLinkedList:");
        linkedList.forEach(System.out::println);
    }
}

Output:

ArrayDeque:
apple
cherry

LinkedList:
diamond
fossil

Explanation:

1. ArrayDeque: An ArrayDeque is a resizable-array implementation of the Deque interface. It is usually more efficient than LinkedList for queue operations (offer, poll, etc.) because it uses a circular array internally. This helps in achieving O(1) time complexity for insertion and deletion. However, ArrayDeque does not implement the List interface, so it cannot be used as a general-purpose List.

2. LinkedList: A LinkedList is a doubly-linked list implementation of both the List and Deque interfaces. It supports all list operations, including indexing, and can also be used as a stack or queue. While LinkedList offers O(1) time complexity for insertions and deletions at the beginning and end, access times for elements in the middle can be slower than with an ArrayList because of the need to traverse the list.

3. In the given example, both the ArrayDeque and LinkedList are used as Deque to perform similar operations. While both classes provide similar functionalities as a double-ended queue, the underlying data structures are different, leading to different performance characteristics.

4. When choosing between the two, consider the primary use case: if you need fast queue operations without random access, ArrayDeque might be more efficient. If you need a general-purpose list with deque operations, LinkedList is the way to go.