Accessing derived class properties when implementing an interface method with abstract base class as parameter?

This is a clue that a design change might help. Your current interface says "I work on Foo s". If the implementation only works on Fum s, then you've got a mismatch in what you're saying and what you're doing.

If you're really only expecting Fum s, then declare the parameter in the interface as a Fum .

I think what you're doing is fine because Fum extends Foo you should be able to cast it in the way that you are.

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