List: ArrayList vs LinkedList

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()); // 3

ArrayList 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.