Question

What is meant by proper inheritance?

Answer 1

I would like to add to what Justin and Baxter said.

The term proper inheritance is not really well-defined. Properly using inheritance is quite a subjective issue…

Consider the following example:

• An interface: Bird
• A concrete class: Ostrich

Should Ostrich inherits from Bird ? From a zoological point of view it makes sense, but from a Computer Science point of view… not so much. If Bird has a fly method, then how am I supposed to handle this in Ostrich::fly 😡 ?

There is somewhat of a war in the CS community. Indeed you’ll regularly see books where Circle inherits from Ellipse (or the other way around) when it doesn’t really makes sense from a CS point of view.

So my own little definition:

Considering that the interface defines precise semantics for each of its methods, a concrete class should only inherit from the interface if the implementation of each of the method matches the semantics specified.