ArrayList vs LinkedList in Java: Which One Should You Use?

If you're working with lists in Java, you've probably come across ArrayList and LinkedList. Both implement the List interface, both store ordered collections of objects, but under the hood, they work very differently. Choosing the right one can make a big difference in performance and memory usage.

Let’s break it down in a simple and practical way so you can decide which one fits your needs best.

How They Work Internally

ArrayList

  • Implementation: Uses a dynamically resizable array.
  • Access Speed: Super fast! Retrieving an element by index is O(1) because it directly accesses memory.
  • Insertion/Deletion: Can be slow (O(n)) if done in the middle or beginning because shifting elements is required.
  • Memory Usage: More memory-efficient compared to LinkedList since it only stores data.

📌 Best Use Case: When you need quick access to elements by index, like in a UI list or search feature.

Example: An e-commerce platform where products are mostly read, sorted, and displayed but rarely modified.

LinkedList

  • Implementation: Built as a doubly linked list (each node points to the previous and next node).
  • Access Speed: Slow for random access (O(n)) since you have to traverse the list from the start.
  • Insertion/Deletion: Super fast O(1) when adding/removing elements anywhere in the list.
  • Additional Capabilities: Implements the Deque interface, making it useful for stacks and queues.

📌 Best Use Case: When frequent insertions and deletions are needed, especially at the start or middle of the list.

Example: A messaging system where new messages arrive frequently, and old ones are removed dynamically.

When to Use Each One

Use ArrayList When:

✅ You need fast random access (e.g., displaying items in a UI list).
✅ The list is mostly read-heavy, with few insertions or deletions.
✅ Memory efficiency is a concern.

Use LinkedList When:

✅ Frequent insertions and deletions happen, especially at different positions.
✅ You need queue-like behavior (FIFO, LIFO operations).
✅ Sequential access is the primary use case.

When to Avoid Them

🚫 Avoid ArrayList if: Your use case involves frequent insertions/deletions in the middle of the list. The shifting of elements can become a performance bottleneck.

🚫 Avoid LinkedList if: You need fast random access to elements. Since it requires traversing from the start, performance can degrade significantly for large lists.

Conclusion

Choosing between ArrayList and LinkedList boils down to your application’s access and modification patterns:

✔ Use ArrayList for fast lookups and when you mostly read from the list.
✔ Use LinkedList for frequent insertions/deletions, especially in the middle or beginning of the list.

Understanding these differences will help you optimize your Java applications for speed and efficiency.

Happy coding! 🚀