MessageSetImpl

Clients of @PublicApi can expect that programs compiled against a given version will remain binary compatible with later versions of the @PublicApi as per each product's API policy as long as the client does not implement/extend @PublicApi interfaces or classes (refer to each product's API policy for the exact guarantee---usually binary compatibility is guaranteed at least across minor versions).

Note: since @PublicApi interfaces and classes are not designed to be implemented or extended by clients, we may perform certain types of binary-incompatible changes to these classes and interfaces, but these will not affect well-behaved clients that do not extend/implement these types (in general, only classes and interfaces annotated with @PublicSpi are safe to extend/implement).