This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Because you use a wildcard in the declaration of house, the compiler no longer knows it is a AnimalHouse<Cat>. For all it knows it could be an AnimalHouse<Dog>, AnimalHouse<Bird> or even AnimalHouse<String>*. You need to replace the "?" with either "Cat" or "? super Cat".
* Change the declaration of AnimalHouse to this:
This way the generic type is bound to be Animal or any subtype.