Comparison between ArrayList and LinkedList in Java

When working with the Java Collection Framework, two commonly used classes for storing dynamic data are ArrayList and LinkedList. Both implement the List interface but differ in internal implementation, performance, and use cases.

What is ArrayList?

• ArrayList in Java is backed by a dynamic array.
• It provides fast random access (O(1)) but slower insertions and deletions in the middle or beginning (O(n)).
• Best choice when read-heavy operations are frequent.

What is LinkedList?

• LinkedList is implemented as a doubly linked list of nodes.
• Each node stores data along with references to the previous and next nodes.
• It provides faster insertions and deletions (O(1) at head/tail) but slower random access (O(n)).
• Best choice when insert/delete-heavy operations are frequent.

Key Differences between ArrayList and LinkedList in Java

1. Data Structure

Feature	ArrayList	LinkedList
Internal Structure	Resizable dynamic array	Doubly-linked list of nodes
Storage	Contiguous memory locations	Nodes connected via next and prev pointers
Memory Overhead	Less, only array elements	More, each node stores prev and next references

2. Performance / Time Complexity

Operation	ArrayList	LinkedList
Access by index (get)	O(1)	O(n)
Add at end (add(E e))	O(1) amortized	O(1)
Add at beginning (addFirst)	O(n)	O(1)
Add at specific index	O(n)	O(n)
Remove first/last element	O(n)/O(1)	O(1)
Remove by index/value	O(n)	O(n)
Search (contains, indexOf)	O(n)	O(n)

3. Iteration Performance

• ArrayList: Iteration is faster due to contiguous memory (cache-friendly).
• LinkedList: Iteration is slower since it traverses nodes one by one.

4. Use Cases

ArrayList	LinkedList
Frequent access by index	Frequent insertion/deletion at start/middle
Less memory overhead	More memory usage due to node pointers
Implementing random-access lists	Implementing Queue, Deque, Stack

5. Summary

1. ArrayList is backed by an array → good for read-heavy operations.
2. LinkedList is a doubly-linked list → good for insert/delete-heavy operations.
3. Choose based on operation frequency:
   • Frequent random access → ArrayList
   • Frequent add/remove from head/middle → LinkedList

Conclusion

Both ArrayList and LinkedList are powerful implementations of the List interface in Java. The choice depends on your use case:

• ArrayList → Best for fast access and iteration.
• LinkedList → Best for fast insertions/deletions.