Previewed in JDK 15 and again in JDK 16, sealed classes and interfaces restrict which other classes and interfaces may extend or implement them.Candidates for migration are informally designated as value-based classes in API specifications. The design and implementation of primitive classes in Java is now sufficiently mature that the migration of certain classes of the Java platform to primitive classes can be anticipated in a future release. Primitive classes declare instances to be identity-free and capable of inline or flattened representations, where instances can be copied freely between memory locations and encoded using values of instances’ fields.

Driving this effort is the Valhalla Project, which is pursuing a significant enhancement to the Java programming model in the form of primitive classes. Warnings are provided about improper attempts to synchronize on instances of any value-based classes in the Java platform. The warnings for value-based classes proposal designates the primitive wrapper classes as value-based and deprecates their constructors for removal, prompting new deprecation warnings.

Java 11 se development kit