Abstract classes vs. interfaces vs. mixins

Question

Could someone please explain to me the differences between abstract classes, interfaces, and mixins? I\'ve used each before in

Answer 1

Since many of guys have explained about the definitions and usage, I would like to highlight only important points

Interface:

1. To define a contract ( preferably stateless - I mean no variables )
2. To link unrelated classes with "has a" capabilities.
3. To declare public constant variables (immutable state)

Abstract class:

1. Share code among several closely related classes. It establishes "is a" relation.

2. Share common state among related classes ( state can be modified in concrete classes)

I am closing the difference with a small example.

Animal can be an abstract class. Cat and Dog, extending this abstract class establishes "is a" relation.

Cat is a Animal

Dog is a Animal.

Dog can implement Bark interface. Then Dog has a capability of Barking.

Cat can implement Hunt interface. Then Cat has a capability of Hunting.

Man, who is not Animal, can implement Hunt interface. Then Man has a capability of Hunting.

Man and Animal (Cat/Dog) are unrelated. But Hunt interface can provide same capability to unrelated entities.

Mixin:

1. If you want a mixture of both abstract class and interface. Especially useful when you want to force a new contract on many unrelated classes where some of them have to re-define new behaviour and some of them should stick to common implementation. Add common implementation in Mixin and allow other classes to re-define the contract methods if needed

If I want to declare an abstract class, I will follow one of these two approaches.

1. Move all abstract methods to interface and my abstract class implements that interface.

   interface IHunt{
       public void doHunting();
   }
   abstract class Animal implements IHunt{
   
   }
   class Cat extends Animal{
       public void doHunting(){}
   }
   

Related SE question :

What is the difference between an interface and abstract class?