Google API libraries mark every class as "final" so it's not trivial to mock-extend it for tests. But third-party IO is exactly the thing you'd want to mock.
Probably because they zealously followed "Effective Java" book.
Probably because they zealously followed "Effective Java" book.
You write an adapter.
Not mentioning of course needless copy-pasting dosens of members in the adapter. And it must be in prod code, not tests, even though it's documentation would say "Adapter for X, exists only for tests, to be able to mock X".
Moreover, that wrapper library is now a pretty large piece of code, and we'd want to maintain and test as well. But cannot without hacks.