Which should inherit which?

Question

This is one of the boring academic OOP questions, but it is not a homework. I got the question from a newbie programmer about one of those stupid textbooks examples about OOP.

Imagine that you are designing a Square class and a Cube class, which should inherit which?

I see a relationship, but what it is, I can not really see!

Could you give me a logical argument with OOP in mind.

Answer 1

Neither! Since a square is not a cube, and a cube is not a square, neither should inherit from the other. Square can inherit from polygon and cube can inherit from polyhedron, but the two are, themselves, mutually exclusive.

Answer 2

There's no inheritance. Inheritance is a "is-a" relationship (Well, sometimes not even a "is-a" relationship, as mentioned in the link below). A cube is not a square, and a square is not a cube.

How would you construct, it depends on how you model. You could go with something like a cube has 6 squares (a cube is not, it has 6 squares; a composition), or a cube has a side size, just like square. But once there is no "is-a", an inheritance would be dangerous zone...

Also, in a inheritance, everything that is valid for the base class must be valid for the Derived one. It's the Square extends Rectangle problem. For instance:

Supposing Cube inherits Square: If your Square has a method changeArea(double area) and getSide(), the same should be possible for the Cube. But it is not, since the area of a cube is 6 times the area of a square (it has 6 squares).

Supposing Square inherits Cube: If your Cube has a setVolume(double volume) method, your square will be broken, once it has no volume

Finally, if you want to use inheritance, you could create a GeometryObjectWithEqualSides object, then both could inherit from it. :)