Sequenced Collections

Before Java 21, asking "give me the first element" was different for every ordered collection. List used get(0), Deque used peekFirst(), and LinkedHashSet had no way to do it at all without walking the iterator. Sequenced Collections (JEP 431) fix this by adding three new interfaces, SequencedCollection, SequencedSet, and SequencedMap, that give every ordered collection a consistent API for first, last, and reverse access.