Obviously modelling your program like Car extends Vehicle is bad, but thinking in terms of real-life relations makes thinking about complex subtyping relations

Obviously modelling your program like Car extends Vehicle is bad, but thinking in terms of real-life relations makes thinking about complex subtyping relations like covariance much easier to grasp. Compare: > If B and C are subtypes of A, then List<T> must be invariant over T since List<B> should not be a subtype of List<A>, which can contain both B and C. with: > List<Cat> can't become List<Animal>, because you might add Dogs to that list!