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.

In Inheritance (class): we could set these 2 classes to inherit from parent class Person.

Couldn't we do the same with Interface? Says we have InterfacePerson and have both Customer and Doctor implement the interface?

Thus, this lead to: when do we decide to use one over the other and vice versa?

Answer 1

• Read the wikipedia article

• Read a book, then read the chapters about OOP again

• In your example, Person should be a class, because it contains implementation details that are common to both a Doctor and a Customer.

• interfaces don't have (and don't need) implementation details - they only denote what objects which implement them are doing. Not how. Why is this useful? Because when you are using the object you don't care how it's going to do its job.

Let's take a look at a simple example - there is an interfaces Comparable (in Java at least). It denotes that its implementors can be compared with each other. So you can have two classes:

class Doctor implements Comparable {..}

class Customer implements Comparable {..}

Now you can have a common method which takes any set of objects which implement Comparable and call comparable1.compareTo(comparable2), because you know they can perform comparison - it's denoted by their interface.

Answer 2

• 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.