Java's backward compatibility applies only to the spec, i.e. the API. If any change to any method in the ~6M MLOC standard library is a "breaking change," then nothing all changes are "breaking changes," and it doesn't matter if there's good reason for them or not; we want things to not break, so internal code needs to be encapsulated. As to the last remaining popular "non-API-APIs", which is mostly just Unsafe, it is, indeed not encapsulated, and over time it will be shrunk only as replacement APIs are available.