A member or type annotated with VisibleForTesting claims that its visibility is higher than necessary, only for testing purposes. In particular:

• If public or protected member/type is annotated with VisibleForTesting, it's assumed that package-private access is enough for production code.
• If package-private member/type is annotated with VisibleForTesting, it's assumed that private access is enough for production code.
• It's illegal to annotate private member/type as VisibleForTesting.

This annotation means that the annotated element exposes internal data and breaks encapsulation of the containing class; the annotation won't prevent its use from production code, developers even won't see warnings if their IDE doesn't support the annotation. It's better to provide proper API which can be used in production as well as in tests.