Question

how abstraction and encapsulation are interrelated?

Answer 1

Narue's answer is good, of course. But on a less formal basis, I see encapsulation and abstraction as sort of two sides of the same coin. In both cases, you usually hide away the details that the user of your library or class doesn't need to know about (hiding data and/or implementation details (like helper functions)). In a sense, encapsulation is asking yourself: What are the things the user shouldn't see? and abstraction is asking yourself: What are the only things the user should see? Most software components have things the user really shouldn't see (e.g. see the PImpl idiom) and things that are part of the interface of this software component (see "programming by contract in OOP"). In an ideal world, the split is clear, in the real world, the line is blurred, so it makes sense to think of both sides of the question (should this thing be hidden? is this thing part of the (abstract) interface?).

Answer 2

encapsulation is a way of implementing abstraction...hope it helps