ArrayList vs LinkedList
Compare the internal structure, performance characteristics, and use cases of ArrayList and LinkedList to make informed data structure choices.
Structural Comparison
ArrayList
Storage:Contiguous array in memory
Memory:Elements only (+ capacity overhead)
Access:Direct via index calculation
Growth:Doubles capacity when full
10
20
30
40
LinkedList
Storage:Scattered nodes in memory
Memory:Elements + pointers per node
Access:Sequential traversal required
Growth:Add nodes as needed
head→
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→nullTime Complexity Comparison
| Operation | ArrayList | LinkedList | Winner |
|---|---|---|---|
| get(index) | O(1) | O(n) | |
| set(index, value) | O(1) | O(n) | |
| addFirst(value) | O(n) | O(1) | |
| addLast(value) | O(1)* | O(1) | Tie |
| add(middle, value) | O(n) | O(n) | Tie** |
| removeFirst() | O(n) | O(1) | |
| removeLast() | O(1) | O(1)*** | Tie |
| Memory per element | ~4-8 bytes | ~24-40 bytes |
* O(1) amortized - occasionally O(n) when resizing
** LinkedList is faster for the actual insertion, but both require O(n) to find the position
*** O(1) for doubly-linked list, O(n) for singly-linked list
Interactive Performance Demo
Click an operation to see a simulated comparison of how many steps each data structure needs (assuming 1000 elements).
When to Use Which?
Choose ArrayList when...
- ✓You need frequent random access (get/set by index)
- ✓Most additions are at the end (append operations)
- ✓Memory efficiency is important
- ✓You iterate through elements frequently
- ✓You know the approximate size upfront
Common use cases: Storing data for display, caching, implementing dynamic arrays, sorting algorithms
Choose LinkedList when...
- ✓You need frequent insertions/deletions at the beginning
- ✓You need a queue or deque (double-ended queue)
- ✓Insertions/deletions at known positions (with iterator)
- ✓You don't need random access
- ✓Size changes dramatically and unpredictably
Common use cases: Implementing stacks, queues, undo/redo functionality, LRU caches, task scheduling
Memory Usage Comparison
For storing 1000 integers (assuming 64-bit JVM):
ArrayList
- Object header: ~16 bytes
- Array reference: ~8 bytes
- Size field: ~4 bytes
- Array: 1000 × 4 = 4,000 bytes
- Total: ~4,028 bytes
LinkedList (Doubly)
- List header: ~24 bytes
- Per node: ~40 bytes each
- 1000 nodes: 1000 × 40 = 40,000 bytes
- Total: ~40,024 bytes
Key insight: LinkedList uses approximately 10x more memory than ArrayList for the same data. Each node carries overhead for object headers and two pointers.
Quick Reference Summary
| Criteria | Best Choice | Reason |
|---|---|---|
| Random access by index | ArrayList | O(1) vs O(n) |
| Insert at beginning | LinkedList | O(1) vs O(n) |
| Insert at end | Either | Both O(1) |
| Delete from beginning | LinkedList | O(1) vs O(n) |
| Memory efficiency | ArrayList | ~10x less memory |
| Iteration speed | ArrayList | Better cache locality |
| Queue/Deque operations | LinkedList | O(1) at both ends |