Question

In Java, you can do the following :

public interface IEngine{}
public interface ICoolEngine extends IEngine{}

public interface Car
{
   IEngine getEngine();
}
public interface ICoolCar extends ICar
{
    @Override
    ICoolEngine getEngine();
}

While this nicely solves a problem I’ve been grappling with, something about it “feels” wrong.

Am I committing some nasty design faux pas here?

Answer 1

No, you are doing the right thing. Covariant returns just specify that the class, and classes below it, must return a specific subclass of the original general class argument that the parent class returned. It also means that your subclasses are still compatible with the original interface that requires that it return an Engine, but if you know that it is an ICoolCar, that it has an ICoolEngine – because the more specific interface knows of more specific functionality. This applies to interfaces as well as classes – this is correct, proper and useful to boot.