JDK 21 comes with new SequencedCollection/SequencedSet/SequencedMap types, with SequencedCollection as super-interface of List and Deque, and the other implemented by LinkedHashSet/LinkedHashMap. While we won't be able to benefit from the new hierarchy and its new operations in the framework codebase yet, we should provide everything we can for application usage: e.g. detection in CollectionFactory and support for the new operations in our org.springframework.util List implementations.
Comment From: jhoeller
CollectionFactory supports user declarations of type SequencedCollection/SequencedSet/SequencedMap now, e.g. for injection points.
AutoPopulatingList inherits all SequencedCollection default methods as declared in the List interface, automatically supporting them on JDK 21 in a good enough fashion.
For the time being, LinkedCaseInsensitiveMap and LinkedMultiValueMap do not implement any SequencedMap operations since they would have to explicitly implement that interface rather than inherit its declaration. None of those operations seem to matter for the common usage of those classes. If necessary, we can revisit this with extended JDK 21 implementations of LinkedCaseInsensitiveMap/LinkedMultiValueMap declaring implements SequencedMap in our new spring-core multi-release jar, with the primary feature of a reversed Map view having to be provided (and maintained) then.