What are the Key Differences Between LinkedList and ArrayDeque as Queue Implementations in Java?

Introduction

In Java, the Queue interface represents a collection designed for holding elements prior to processing. Implementing the Queue interface allows us to manage data in a First-In-First-Out (FIFO) manner. Two common classes that implement the Queue interface are LinkedList and ArrayDeque. While both classes provide similar functionality, they differ in their internal structure, performance characteristics, and ideal use cases. Understanding these differences is essential when choosing the right data structure for your specific needs.

In this article, we will compare LinkedList and ArrayDeque as Queue implementations in Java. We will discuss the internal working of each, highlight key differences, explore their performance, and provide relevant code examples.

1. Overview of LinkedList and ArrayDeque

LinkedList

LinkedList is a doubly-linked list implementation of the List and Deque interfaces. It supports operations like insertion, removal, and access to elements at both ends of the list. Being part of the Java Collections Framework, it implements the Queue interface, allowing it to function as a FIFO queue.

Internal Structure:
LinkedList consists of a series of nodes, each containing a reference to the next and previous node in the list. This linked structure allows for efficient insertion and deletion operations, especially when adding or removing elements from the head or tail of the list.
Queue Operations:
When used as a queue, LinkedList supports operations like offer(), poll(), peek(), and remove() for inserting and removing elements in FIFO order.

ArrayDeque

ArrayDeque is a resizable array-based implementation of the Deque interface. It also implements the Queue interface, allowing for FIFO-based queue operations. Unlike LinkedList, which uses pointers to connect nodes, ArrayDeque relies on an underlying dynamic array that expands as needed.

Internal Structure:
ArrayDeque uses a circular array to store elements. When the array is full, it dynamically resizes itself to accommodate more elements. This makes it more space-efficient than a linked list when handling small to medium-sized queues.
Queue Operations:
Similar to LinkedList, ArrayDeque provides queue operations like offer(), poll(), peek(), and remove(). However, these operations can perform better in some cases due to the underlying array structure.

2. Key Differences Between LinkedList and ArrayDeque

While both LinkedList and ArrayDeque implement the Queue interface, their internal data structures lead to significant differences in performance, memory usage, and behavior.

Memory Efficiency

LinkedList:
A LinkedList stores each element as a separate node, with each node containing references to both the previous and next nodes. This leads to higher memory consumption per element because each node needs additional memory for the pointers. The memory overhead can be significant, especially for smaller elements.
ArrayDeque:
ArrayDeque, on the other hand, uses a dynamic array to store its elements. This structure tends to use less memory per element because it does not require storing extra pointers for each element. However, resizing the array can sometimes incur additional memory costs, particularly when the array needs to expand or shrink.

Performance Considerations

Insertion and Deletion Operations:
- LinkedList:
  LinkedList offers constant-time O(1) insertion and removal at both ends (head and tail), which makes it efficient for queues. However, accessing elements by index (i.e., random access) is costly, with a time complexity of O(n) due to the need to traverse the list.
- ArrayDeque:
  ArrayDeque provides O(1) time complexity for inserting and removing elements at both ends, thanks to its underlying array structure. However, the array resizing operations can be costly in some scenarios, leading to an amortized time complexity of O(1) for insertions and deletions, but it can sometimes involve O(n) time if resizing occurs.
Access Time:
- LinkedList:
  For LinkedList, accessing an element by index is slow (linear time, O(n)), since it must traverse the list. However, if you only need to interact with the elements at the front or rear, performance is optimal.
- ArrayDeque:
  ArrayDeque also does not support random access well (no direct access by index). Although accessing elements at the head or tail is efficient, accessing a random element is inefficient (O(n) time), like LinkedList.

Thread Safety

Neither LinkedList nor ArrayDeque is thread-safe. However, you can use wrapper classes like Collections.synchronizedQueue() to make them thread-safe if required.

3. Code Examples

Let’s take a look at some code examples that demonstrate how to use both LinkedList and ArrayDeque as queue implementations in Java.

Using LinkedList as Queue:

import java.util.LinkedList;
import java.util.Queue;

public class LinkedListQueueExample {
    public static void main(String[] args) {
        Queue<Integer> queue = new LinkedList<>();
        
        // Adding elements to the queue
        queue.offer(10);
        queue.offer(20);
        queue.offer(30);
        
        // Peeking at the front element
        System.out.println("Front element: " + queue.peek());
        
        // Polling elements from the queue (removing and returning)
        System.out.println("Polled element: " + queue.poll());
        System.out.println("Polled element: " + queue.poll());
        
        // Remaining elements in the queue
        System.out.println("Remaining queue: " + queue);
    }
}

Output:

Front element: 10
Polled element: 10
Polled element: 20
Remaining queue: [30]

Using ArrayDeque as Queue:

import java.util.ArrayDeque;
import java.util.Queue;

public class ArrayDequeQueueExample {
    public static void main(String[] args) {
        Queue<Integer> queue = new ArrayDeque<>();
        
        // Adding elements to the queue
        queue.offer(100);
        queue.offer(200);
        queue.offer(300);
        
        // Peeking at the front element
        System.out.println("Front element: " + queue.peek());
        
        // Polling elements from the queue (removing and returning)
        System.out.println("Polled element: " + queue.poll());
        System.out.println("Polled element: " + queue.poll());
        
        // Remaining elements in the queue
        System.out.println("Remaining queue: " + queue);
    }
}

Output:

Front element: 100
Polled element: 100
Polled element: 200
Remaining queue: [300]

4. When to Use LinkedList vs. ArrayDeque

Both LinkedList and ArrayDeque are useful queue implementations, but their suitability depends on specific scenarios.

Use LinkedList When:
- You need a double-ended queue (deque) and plan to perform frequent insertions or deletions at both ends.
- You need a queue with dynamic growth or shrinkage and can tolerate the higher memory overhead per element.
- You need to frequently remove elements from the front or back but don’t care about the cost of random access.
Use ArrayDeque When:
- You need a simple queue with fast insertion and removal operations.
- You have a predictable queue size that doesn’t require frequent resizing.
- You need to minimize memory overhead and benefit from the compactness of an array-based structure.

5. Conclusion

In summary, both LinkedList and ArrayDeque are viable options for implementing queues in Java, but they each have distinct advantages and limitations. LinkedList excels in scenarios where frequent insertions and deletions from both ends are needed, while ArrayDeque provides better memory efficiency and is more suitable for smaller or medium-sized queues with a low need for resizing.

Understanding these differences allows you to make an informed decision about which implementation is best suited for your application. Whether you choose LinkedList or ArrayDeque depends on your performance requirements, memory constraints, and specific use case.

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