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

First thing to remember, Classes can be instantiated, Interfaces cannot.

Secondly, a Class can only extend ONE Class. Interfaces are not limited by this and so we can have multiple inheritance like so

public class foo extends Person implements Man, Mammal

foo is a Person. It is also a Man and a Mammal;

The only issue is Interfaces cannot have variables or method implementations where as a class(or abstract class for that matter) can.

Generally I would say stick with interfaces and avoid abstract classes if you can.