Private inner types(Classes, Interfaces, enums) should NOT be placed as return type for public method. We need to ignore such types if they are extends or implements smth (usually public interface) so clients of such class could keep them by public interface.
incomplete code: https://github.com/BinnaryWolf/sevntu.checkstyle/tree/forbid_return_private_branch
mai-list thread: https://groups.google.com/forum/#!topic/sevntu-checkstyle/Bsa1-Dv__xM
All info for this issue would be here: