Class vs. Interface

Question

I have a quite basic question:

When should we decide to use Interface or Class for a specific class?

For example: says we have 2 classes, Customer and Doctor

Answer 1

• Interface - describe the behavior
• Class - do the behavior

A class extending another class inherits the behavior. On the other hand, implementing an interface just says it need to behave that way but the class still has to know the how-to.

Besides single inheritance limitations, code using interfaces are easier to refactor and test, e.g. provide a mock implementation for a database access object in unit tests.

So, the real answer is, it depends on your design.

It may be that you use interfaces to describe behavior, and abstract parent classes to implement the behavior that can be inherited by subclasses. Or maybe the subclasses are so different that each implements the interface in their own way.