What does "abstraction" mean in the context of "class cohesion" or "class cohesiveness" code metrics?

Question

I encountered this phrase on this site in a discussion of code metrics in Eclipse, specifically discussing the concept of "lack of cohesion":

Cohesion is an important concept in OO programming. It indicates whether a class represents a single abstraction or multiple abstractions. The idea is that if a class represents more than one abstraction, it should be refactored into more than one class, each of which represents a single abstraction.

What is a "single abstraction" in this context?

From Difference between Encapsulation and Abstraction, I got that abstraction generally is just showing necessary details to the user (through the use of interfaces and abstract classes). And here: What is abstraction?, I got again that abstraction is to hide implementation.

How would you apply these ideas to the single abstraction term used in that article?

Im just asking what the author means by a single abstraction — committedandroider, Dec 07 '14 at 02:06
Single abstraction corresponds (as per my understanding) to having the class work towards single task. Providing a layer over single task. — nitishagar, Dec 07 '14 at 02:14
A layer as in providing a way to access certain functionality implemented. — nitishagar, Dec 07 '14 at 02:19
so if i had something like a car interface with a method drive, would the layer in this case be the the interface because that I can use that to access the drive method? — committedandroider, Dec 07 '14 at 02:31
Well yes. Abstraction generally provides just interface (not particularly referring to java interface). — nitishagar, Dec 07 '14 at 02:34
Discussed on meta: http://meta.stackoverflow.com/questions/278657/arent-we-too-hasty-in-closing-questions-without-code — BartoszKP, Dec 07 '14 at 11:23
Someone explain the downvote? I did my research first and i felt this question isn't that broad because the term was used in a specified context. — committedandroider, Dec 07 '14 at 20:48
@committedandroider See the meta discussion I've given link to. Some people agree with me, that this question is OK, however it seems that most of the community doesn't like it (I don't understand why though). — BartoszKP, Dec 08 '14 at 13:19

BartoszKP · Accepted Answer · 2014-12-07T11:05:37.440

In this particular context, defined in the site you quote (cohesion), "single abstraction" means one concept. So it is strongly related to Single Responsibility Principle - a class should deal with "one thing". If a class deals with more than one thing, it will often have many different methods and variables that don't necessarily belong together, thus it would have low cohesion.

This not a common usage of this word for this purpose, from my experience it is more likely to be used in reasoning about levels/layers of abstraction, like in the other SO questions you link. They relate to a different rule, namely the Single Layer of Abstraction Principle.

Thanks, that one thing idea makes sense – committedandroider Dec 07 '14 at 02:17 — committedandroider, Dec 07 '14 at 02:17

What does "abstraction" mean in the context of "class cohesion" or "class cohesiveness" code metrics?

1 Answers1