List: ArrayList vs LinkedList
Ordered, indexed, duplicates allowed — and when each concrete class makes sense.
The List contract
A List<E> is an ordered sequence where elements are accessed by zero-based index. Duplicate elements are allowed. The core operations:
| Method | Meaning |
|---|---|
add(e) | Append at end |
add(i, e) | Insert at index |
get(i) | Element at index |
set(i, e) | Replace at index |
remove(i) | Remove by index |
remove(e) | Remove first occurrence by value |
size() | Number of elements |
contains(e) | Linear search |
List<String> fruits = new ArrayList<>(List.of("apple", "banana"));
fruits.add("cherry"); // [apple, banana, cherry]
fruits.add(1, "apricot"); // [apple, apricot, banana, cherry]
String s = fruits.get(2); // "banana"
fruits.remove("apple"); // [apricot, banana, cherry]
System.out.println(fruits.size()); // 3ArrayList backs elements in a resizable array. Random access (get(i)) is O(1). Insertion/deletion in the middle shifts elements — O(n).
LinkedList backs elements in a doubly linked node chain. Head/tail insertion/deletion is O(1). Random access traverses the chain — O(n).
In practice ArrayList is the right default for almost every list. Use LinkedList only when you are inserting/deleting heavily at both ends and rarely doing indexed access.
List.of(...) (Java 9+) creates an immutable list. No add, set, or remove — any mutation throws UnsupportedOperationException.