How to solve the following problem? Thanks.

Question

Create an interface with at least one method, in its own package. Create a class in a separate package. Add a protected inner class that implements the interface. In a third package, inherit from your class and, inside a method, return an object of the protected inner class, upcasting to the interface during the return.

Answer 1

Life is essentially simple, we just love to make it complicated. I am quite sure, the effect you are trying to achieve by what you are doing can be done in a much simpler way.
In theory it is fine - on production floor if you do it, being the architect I most definitely am going to fire you.
It is easy to obfuscate code by pulling these kind of stunts - nobody (even you yourself) will understands this later.
Keep your thinking as well as your code and logic simple, systems become easier to build and maintain.
I shall intentionally not answer your question. There are certain knowledge which is better not given - if the potential lies that it shall be used otherwise :D
No offence intended though
Ironluca