Abstract vs Interface

Difference between abstract class and interface

Abstract class and interface both are used to achieve abstraction where we can declare the abstract methods. Abstract class and interface both can't be instantiated.

But there are many differences between abstract class and interface that are given below.

Abstract class	Interface
1) Abstract class can have abstract and non-abstract methods.	Interface can have only abstract methods.
2) Abstract class doesn't support multiple inheritance.	Interface supports multiple inheritance.
3) Abstract class can have final, non-final, static and non-static variables.	Interface has only static and final variables.
4) Abstract class can have static methods, main method and constructor.	Interface can't have static methods, main method or constructor.
5) Abstract class can provide the implementation of interface.	Interface can't provide the implementation of abstract class.
6) The abstract keyword is used to declare abstract class.	The interface keyword is used to declare interface.
7) Example: public class Shape{ public abstract void draw(); }	Example: public interface Drawable{ void draw(); }

Simply, abstract class achieves partial abstraction (0 to 100%) whereas interface achieves fully abstraction (100%).

Example of abstract class and interface in Java

Let's see a simple example where we are using interface and abstract class both.

1.    //Creating interface that has 4 methods  

2.    interface A{  

3.    void a();//bydefault, public and abstract  

4.    void b();  

5.    void c();  

6.    void d();  

7.    }  

8.      

9.    //Creating abstract class that provides the implementation of one method of A interface  

10. abstract class B implements A{  

11. public void c(){System.out.println("I am C");}  

12. }  

13.   

14. //Creating subclass of abstract class, now we need to provide the implementation of rest of the methods  

15. class M extends B{  

16. public void a(){System.out.println("I am a");}  

17. public void b(){System.out.println("I am b");}  

18. public void d(){System.out.println("I am d");}  

19. }  

20.   

21. //Creating a test class that calls the methods of A interface  

22. class Test5{  

23. public static void main(String args[]){  

24. A a=new M();  

25. a.a();  

26. a.b();  

27. a.c();  

28. a.d();  

29. }}  

Test it Now

Output:

       I am a

       I am b

       I am c

       I am d