In many interviews I am facing these questions

Anand SA

4y1.3k.NET

1. Difference between abstract class and interface

We can declare methods virtually in a class and override them in child classes. Then why use of interface.

3. Why are you implementing all the interface methods even though you wont use them.

I know interface concept and abstract class concept but unable to understand their exact usage. I hope someone will explain me.

4 Replies

Know the answer? Post it — somebody with the same question will find it here.

Abstract Class	Interface
Abstract class can have implementations for some of its members.	interface cannot have implementation for any of its members
An abstract class can contain fields.	Interfaces cannot contain fields
An abstract class can inherit from another class or another interface.	An interface can inherit from another interface only and cannot inherit from any class.
A class cannot inherit from multiple classes at the same time.	A class and Struct can inherit from more than one interface at the same time
Abstract class members can have access modifiers	Interface members cannot have access modifiers. It is by default public.
A class that inherit from abstract class must provide implementations for all inherited abstract members	A class that inherit an interface must provide implementation of all interface otherwise we will get compiler error.
We cannot create an instance of abstract class, but abstract class reference variable can point to derived class object.	We cannot create an instance of interface, but an interface reference variable can point to a derived class object

Vishal Yelve4y ago
Please refer Shivprasad Koirala sir's video, which will help you to clear your doubt.

https://youtu.be/0EnSPBVrbG0

(1) Difference between abstract class and interface.

Abstract class	Interface
1) Abstract classes can have abstract and non-abstract methods.	The interface can have only abstract methods. Since Java 8, it can have default and static methods also.
2) Abstract class doesn't support multiple inheritances.	The interfacesupports multiple inheritances.
3) Abstract class can have final, non-final, static and non-static variables.	The interface has only static and final variables.
4) Abstract class can provide the implementation of the interface.	Interface can't provide the implementation of an abstract class.
5) The abstract keyword is used to declare an abstract class.	The interface keyword is used to declare the interface.
6) An abstract class can extend another Java class and implement multiple Java interfaces.	An interface can extend another Java interface only.
7) An abstract class can be extended using the keyword "extends".	An interface can be implemented using the keyword "implements".
8) A Java abstract class can have class members like private, protected, etc.	Members of a Java interface are public by default.
9)Example: public abstract class Shape{ public abstract void draw(); }	Example: public interface Drawable{ void draw(); }