Lesson 04 · Sequenced Collections

Objectives

After this lesson you will be able to:

• Use the Java 21 SequencedCollection, SequencedSet, and SequencedMap interfaces.
• Access first/last elements and obtain a reversed view.
• Know which existing types gained these methods.

What Java 21 added (JEP 431)

Before Java 21 there was no common type for "a collection with a defined encounter order and both ends reachable." Java 21 adds three interfaces:

• SequencedCollection<E> — addFirst/addLast, getFirst/getLast, removeFirst/ removeLast, and reversed().
• SequencedSet<E> — a SequencedCollection that is also a Set.
• SequencedMap<K,V> — firstEntry/lastEntry, putFirst/putLast, pollFirstEntry/pollLastEntry, reversed(), plus sequenced keySet/values/entrySet.

SequencedCollection<Integer> sc = new ArrayList<>(List.of(1, 2, 3));
sc.getFirst();          // 1   (was list.get(0))
sc.getLast();           // 3   (was list.get(list.size()-1))
sc.addFirst(0);         // [0, 1, 2, 3]
sc.reversed();          // a reversed VIEW: [3, 2, 1, 0]

Which types implement them

Type	New super-interface
List (e.g. ArrayList, LinkedList)	SequencedCollection
Deque (e.g. ArrayDeque)	SequencedCollection
LinkedHashSet	SequencedSet
SortedSet/TreeSet	SequencedSet
LinkedHashMap	SequencedMap
SortedMap/TreeMap	SequencedMap

Exam trap

reversed() returns a view backed by the original — changes write through, and iterating it costs nothing extra. A plain HashSet/HashMap is NOT sequenced (no defined order), so it has none of these methods. getFirst()/getLast() throw NoSuchElementException on an empty collection.

Beyond the exam

reversed() on a SequencedMap returns a SequencedMap whose encounter order is reversed; on a TreeMap that's the descending order — a cleaner alternative to descendingMap() for simple needs.

Key Takeaways

• Java 21's SequencedCollection/SequencedSet/SequencedMap unify ordered access: getFirst/getLast, addFirst/addLast, and reversed().
• List, Deque, LinkedHashSet, TreeSet, LinkedHashMap, and TreeMap implement them; HashSet/HashMap do not (unordered).
• reversed() is a live view, not a copy; getFirst/getLast throw NoSuchElementException when empty.

Lesson Quiz

Lesson Quiz · Sequenced Collections0 / 4

1. Which method gives the first element of a List in Java 21?

   • Alist.first()
   • Blist.getFirst()
   • Clist.head()
   • Dlist.peek()

2. Does HashSet implement SequencedSet?

   • AYes
   • BNo — it has no defined order
   • COnly when sorted
   • DOnly in Java 21

3. What does reversed() return?

   • AA sorted copy
   • BA live reversed view of the original
   • CA new ArrayList
   • Dnull

4. What does getFirst() do on an empty List?

   • AReturns null
   • BReturns 0
   • CNoSuchElementException
   • DIndexOutOfBoundsException

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Next: Comparison & Sorting. Run the matching code in labs/src/main/java/com/jse21/m05_collections/.