Question

I have seen many answers on stackoverflow, but I didn’t find an answer that is matching mine.
Apart from all those difference, Does it make sense if we say an abstract class abstracts the implementation of behaviour while an interface abstracts the type which implements the behaviour.

Answer 1

The main differences from design point of view are that:

• you can declare a contract on constructor of the implementing classes, by creating a protected constructor in the base abstract class.
• you can provide implementations of methods usable by base classes
• you can make a wrapper around the contract (e.g. validate method arguments)
• you can provide a “calling scheme” when you create non-abstract methods that call abstract methods of the type, implemented by derived classes. This can be useful for implementing abstraction of an algorithm in derived classes, while the base class implements all the handling logic – prepares and validates data, and lets the actual processing algorithm to be implemented by derived classes.

So I would say you are correct in the statement that “an abstract class abstracts the implementation of behaviour while an interface abstracts the type which implements the behaviour”

Abstract class: provides requirement to implement some methods (you override methods of the abstract class)

Interface: defines only a contract. Indicates that a class that implements the interface has methods of the interface (you implement an interface)

For example:

• by implementing an interface on an existing class, you just declare adding the interface methods to the contract of the class. The class may already implement all the methods of the interface and you do not need to change anything in the existing class.

• by changing the base type to an abstract class, you are required to override all the methods, even if methods with the same names as abstract methods of the base class already exist on the type.