Why was the Sequenced Collections API introduced in Java 21?

Before Java 21, working with the "ends" of a collection meant learning a different API for every type: list.get(0) for lists, deque.peekFirst() for deques, and no first-class method at all for grabbing the first entry of a LinkedHashMap or LinkedHashSet.

This inconsistency made simple, common operations - "give me the first element," "give me this reversed" - harder to express uniformly, and pushed developers toward verbose or error-prone workarounds like index arithmetic or wrapping a map in an ArrayList just to reverse it.

JEP 431 fixes this by defining what "having an encounter order" means as a first-class concept in the type system: SequencedCollection, SequencedSet, and SequencedMap. Any type that already has a defined order gets a uniform way to expose it, and any type that doesn't (like HashSet) simply doesn't implement these interfaces.

The result is fewer surprises and less boilerplate: code that needs "the first element of this ordered thing" can be written once against SequencedCollection and work correctly whether the underlying object is a List, a Deque, or a LinkedHashSet.