What are Sequenced Collections in Java 21?

JEP 431 introduces three new interfaces — SequencedCollection, SequencedSet, and SequencedMap — to the collections hierarchy. Before Java 21 there was no unified API to access the first or last element of a collection, or to iterate in reverse order; each concrete class had its own ad-hoc approach.

// SequencedCollection adds:
interface SequencedCollection extends Collection {
    void addFirst(E e);
    void addLast(E e);
    E getFirst();      // replaces get(0) / peekFirst()
    E getLast();       // replaces get(size-1) / peekLast()
    E removeFirst();
    E removeLast();
    SequencedCollection reversed(); // reversed VIEW
}

// Now works uniformly on List, Deque, LinkedHashSet...
List list = new ArrayList<>(List.of("a", "b", "c"));
System.out.println(list.getFirst());  // "a"
System.out.println(list.getLast());   // "c"
list.addFirst("z");                   // ["z", "a", "b", "c"]

for (String s : list.reversed()) {
    System.out.print(s + " "); // c b a z
}

// SequencedMap adds firstEntry(), lastEntry(), reversed()
SequencedMap map = new LinkedHashMap<>();
map.put("one", 1); map.put("two", 2); map.put("three", 3);
System.out.println(map.firstEntry()); // one=1
System.out.println(map.lastEntry());  // three=3

The reversed() method returns a view — it does not copy the collection. Modifications through the reversed view affect the backing collection. All List, Deque, SortedSet, LinkedHashSet, SortedMap, and LinkedHashMap implementations now implement the appropriate sequenced interface.