java.util · java.base · since Java 21

SequencedCollection

declaration

public interface SequencedCollection<E> extends Collection<E>

Java 21 unification of collections with a well-defined encounter order: first/last access and a reversed view, inherited by List, Deque, and SortedSet.

Retrofitted onto List, Deque, LinkedHashSet, and SortedSet in Java 21 — one consistent end-access API.

Key methods

`void addFirst(E) / addLast(E)`	Insert at the front / back (may be unsupported on immutable views).
`E getFirst() / getLast()`	Peek the ends (throws NoSuchElementException if empty).
`E removeFirst() / removeLast()`	Remove and return an end element.
`SequencedCollection<E> reversed()`	A reverse-order VIEW — writes flow through.