I'm in a bit of a quandary. I have class animal, let's say it stores basic things like position, etc.. So then I have a class smartAnimal that extends animal. Let's say smartAnimal adds some functionality of being able to make decisions without the user's input.
Now, I have a class dog that extends animal. It doesn't extend smartAnimal because it doesn't need that functionality. So far I'm good: a dog is an animal and a smartAnimal is an animal. But now I want smartDog that would extend smartAnimal but smartDog should also be a dog. For example, a lot of the functionality in regular dog, let's say .sniff(), .poop(), and .bark() should also be in my smartDog but it won't be unless I literally copy+paste the code, because Java doesn't allow multiple inheritance.
Composition seems inelegant because if I had:
class smartDog{
smartAnimal thisSmartAnimal;
dog thisDog; }
...I'd be duplicating the "stuff" in animal, like, for example, the position. I would have to deal with two sets of position variables! And duplicates of whatever properties there are in animal.
Inheriting multiple interfaces doesn't sound like the proper solution either, because let's say dog was an interface, .sniff(), .poop(), and .bark()'s implementation shouldn't have to vary and be re-implemented for smartDog and whatever other classes.
What's the best practice here? That is, to have both classes dog and smartDog?
EDIT:
From some of the comments/answers, let me clarify. In my specific project, a smartAnimal actually has a whole neural network that controls it. It's a lot of code, and doesn't make sense to abstract it to an interface. The core of the NN functionality is the same no matter what animal has it, but each smartAnimal subclass would implement the outputs of the AI "brain" differently. Conversely, each animal has it's own functionality that operates the same whether a human controls it or its own AI agent controls it.
The answer is that this isn't a realistic inheritance hierarchy, which you can tell from the fact that you used the words "adds some functionality" to describe adding methods in a subclass. This isn't proper inheritance. In proper inheritance (in Java, at least) every method should exist at the top level of the inheritance hierarchy. Inheritance is for code reuse, not for adding functionality as you go deeper into the tree. Therefore, there's not really an answer to your question as given.