JEP 431: Sequenced Collections

Status: Delivered in Java 21
Author: Stuart Marks
Reviewed by: Brian Goetz
Component: java.util:collections

🧭 Summary

JEP 431 introduces SequencedCollection, SequencedSet, and SequencedMap — new interfaces that define a consistent, reversible encounter order for Java collections. These interfaces unify how elements are accessed from the front or back and allow bidirectional iteration across all collection types that maintain order.

💡 Motivation

Java lacked a unified abstraction for collections with a defined order. Although classes like List, Deque, LinkedHashSet, and SortedSet implied order, there was:

• No shared interface that declared operations like getFirst(), getLast(), or reversed()
• Inconsistent APIs across types for accessing first/last elements
• No reverse stream support without copying or complex logic

🧱 New Interfaces

SequencedCollection<E>

A base interface for collections with defined order.

interface SequencedCollection<E> extends Collection<E> {
  SequencedCollection<E> reversed();
  void addFirst(E);
  void addLast(E);
  E getFirst();
  E getLast();
  E removeFirst();
  E removeLast();
}