What is the difference between ArrayList and LinkedList in Java?

Both implement the `List` interface but with different internal structures:

**ArrayList:**
- Backed by a dynamic array
- O(1) random access by index
- O(n) insertion/deletion in middle (shifting elements)
- Better for read-heavy operations

**LinkedList:**
- Doubly-linked list
- O(n) access by index (must traverse from head)
- O(1) insertion/deletion at head/tail
- Better for frequent add/remove at ends

```java
List<String> al = new ArrayList<>(); // Use for most cases
List<String> ll = new LinkedList<>(); // Use when adding/removing from ends frequently
```

**Rule of thumb:** Use ArrayList 90% of the time. Use LinkedList only when you have a specific reason (frequent queue/deque operations).