There are lost of discussion on the internet about the Interface vs Abstract class. Also, as base class whether we have to use interface, abstract class or normal class.
I am trying to point out few considerations on which we can take decision about Interface vs Abstract class vs Class.
Abstract Class vs Interface
I am assuming you are having all the basic knowledge of abstract and interface keyword. I am just briefing the basics.
We can not make instance of Abstract Class as well as Interface.
Here are few differences in Abstract class and Interface as per the definition.
Abstract class can contain abstract methods, abstract property as well as other members (just like normal class).
Interface can only contain abstract methods, properties but we don’t need to put abstract and public keyword. All the methods and properties defined in Interface are by default public and abstract.
public abstract class Vehicles
private int noOfWheel;
private string color;
public abstract string Engine
public abstract void Accelerator();
public interface Vehicles
We can see abstract class contains private members also we can put some methods with implementation also. But in case of interface only methods and properties allowed.
We use abstract class and Interface for the base class in our application.
This is all about the language defination. Now million doller question:
How can we take decision about when we have to use Interface and when Abstract Class.
Basicly abstact class is a abstract view of any realword entity and interface is more abstract one. When we thinking about the entity there are two things one is intention and one is implemntation. Intention means I know about the entity and also may have idea about its state as well as behaviour but don’t know about how its looks or works or may know partially. Implementation means actual state and behaviour of entity.
Enough theory lets take an example.
I am trying to make a Content Management System where content is a genralize form of article, reviews, blogs etc.
So content is our base class now how we make a decision whether content class should be Abstract class, Interface or normal class.
First normal class vs other type (abstract and interface). If content is not a core entity of my application means as per the business logic if content is nothing in my application only Article, Blogs, Review are the core part of business logic then content class should not be a normal class because I’ll never make instance of that class. So if you will never make instance of base class then Abstract class and Interface are the more appropriate choice.
Second between Interface and Abstract Class.
As you can see content having behavior named “Publish”. If according to my business logic Publish having some default behavior which apply to all I’ll prefer content class as an Abstract class. If there is no default behavior for the “Publish” and every drive class makes their own implementation then there is no need to implement “Publish” behavior in the base case I’ll prefer Interface.
These are the in general idea of taking decision between abstract class, interface and normal class. But there is one catch. As we all know there is one constant in software that is “CHANGE”. If I made content class as Interface then it is difficult to make changes in base class because if I add new method or property in content interface then I have to implement new method in every drive class. These problems will over come if you are using abstract class for content class and new method is not an abstract type. So we can replace interface with abstract class except multiple inheritance.
CAN-DO and IS-A relationship is also define the deference between Interface and abstract class. As we already discuss Interface can be use for multiple inheritance for example we have another interface named “ICopy” which having behavior copy and every drive class have to implements its own implementation of Copy. If “Article” class drive from abstract class Content as well as ICopy then article “CAN-DO” copy also.
IS-A is for “generalization” and “specialization” means content is a generalize form of Article, Blogs, Review and Article, Blogs, Review are a specialize form of Content.
So, abstract class defines core identity. If we are thinking in term of speed then abstract is fast then interface because interface requires extra in-direction.