What is Java 21s new Sequence Interface

Question

What is the new Sequence interface in Java 21? What does it add and what may be common issues to consider?

Answer 1

The Sequence interface is Java’s standardization of sequential Collections that have a logical ordering, or as JEP 431: Sequenced Collections puts it, “a defined encounter order”. A SortedSet would be able to keep its natural ordering, a List keeps its insertion order etc.

The methods that it adds are

SequencedCollection<E> reversed();
void addFirst(E);
void addLast(E);
E getFirst();
E getLast();
E removeFirst();
E removeLast();

The getFirst() method can replace list.get(0), deque.getFirst(), sortedSet.first(), and linkedHashSet.iterator().next(). The same for getLast() except LinkedHashSet had no get last and you would have to iterate the entire Set.

There is some strangeness IMO to this regarding SortedSet. SortedSet is forced to implement the addFirst() and addLast() methods which don’t make sense in context and the Jep says “these methods can throw UnsupportedOperationException”.

Reversed() also standardizes how to iterate backwards. Previously, there was navSet.descendingSet(), deque.descendingIterator(), and for (var it = list.listIterator(list.size()); it.hasPrevious();) {} and again LinkedHashSet has no method. Now reversed() allows all the standard iteration techniques of fors, streams etc. and includes LinkedHashSet. One final note is that if a codebase used these method names they will run into issues. See the Jep for a full description of the item.