The Sequenced Collection API introduces several new interfaces into the collections framework, providing enhancements to many existing collections classes. The new API facilitates access to elements at each end of a sequenced collection, and provides the ability to view and iterate such collections in reverse order. See [JEP 431](https://openjdk.org/jeps/431) for additional information.

The introduction of new collections interfaces, along with default methods, introduces some compatibility risk, including the possibility of both source and binary incompatibilities. The introduction of default methods in an interface hierarchy may cause conflicts with methods declared on existing classes or interfaces that extend collections interfaces - this could result in either source or binary incompatibilities. The introduction of new interfaces also introduces new types into the system, which can change the results of type inference, leading in turn to source incompatibilities.

For a discussion of potential incompatibilities and possible ways to mitigate them, please see the document [JDK 21: Sequenced Collections Incompatibilities](https://inside.java/2023/05/12/quality-heads-up/).