What's the difference between abstraction and encapsulation?

Question

In interviews I have been asked to explain the difference between abstraction and encapsulation. My answer has been along the lines of

• Abstraction allows us to represent complex real world in simplest manner. It is the process of identifying the relevant qualities and behaviors an object should possess; in other words, to represent the necessary feature without representing the background details.

• Encapsulation is a process of hiding all the internal details of an object from the outside real world. The word "encapsulation", is like "enclosing" into a "capsule". It restricts clients from seeing its internal view where the behavior of the abstraction is implemented.

I think with above answer the interviewer was convinced, but then I was asked, if the purpose of both is hiding, then why there is a need to use encapsulation. At that time I didn't have a good answer for this.

What should I have added to make my answer more complete?

Answer 1

Abstraction has to do with separating interface from implementation. (We don't care what it is, we care that it works a certain way.)

Encapsulation has to do with disallowing access to or knowledge of internal structures of an implementation. (We don't care or need to see how it works, only that it does.)

Some people do use encapsulation as a synonym for abstraction, which is (IMO) incorrect. It's possible that your interviewer thought this. If that is the case then you were each talking about two different things when you referred to "encapsulation."

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It's worth noting that these concepts are represented differently in different programming languages. A few examples:

• In Java and C#, interfaces (and, to some degree, abstract classes) provide abstraction, while access modifiers provide encapsulation.
• It's mostly the same deal in C++, except that we don't have interfaces, we only have abstract classes.
• In JavaScript, duck typing provides abstraction, and closure provides encapsulation. (Naming convention can also provide encapsulation, but this only works if all parties agree to follow it.)