Essentially, I'm trying to answer this question - is it better for objects to expose raw, unprocessed data OR higher-level, processed information. Think
While I think I know the answer, what I'm really after is some literature on the matter - does this subject have a name?
Yes, I can think of two named concepts involved in this discussion:
First, the “data hiding” part. I like that
IsMoving() doesn’t reveal implementation details compared to
IsMoving() expresses itself at a higher level of abstraction, and abstraction is more stable, so depending on abstractions is generally less risky than depending on details. (Read more at "Demystifying the Dependency Inversion Principle")
Next, the “context dependence” part. I like that I can use
GetSpeed() in other contexts besides deciding
IsMoving(). This makes
GetSpeed() easier to reuse in a variety of situations that have nothing to do with merely deciding whether the vehicle is moving, so I can recombine
GetSpeed() without having to change it in order to build new things. Context-independent code tends to be easier to reuse safely. (Read much more at Growing Object-Oriented Software Guided by Tests)
So, it depends. If I have
GetSpeed() and I want to compute
IsMoving(), then I will eventually introduce
IsMoving() onto Vehicle. (In Java, for example, we have both List.size() and List.isEmpty(), playing the same roles.) If I have only
IsMoving() and I want to use the vehicle’s speed for some other purposes, I might extract
I can’t say that one is universally better than the other.
One last thing: when building toolkits, I want small, context-independent, recombinable pieces; but when building full applications, I want cohesive, abstract, self-documenting things. I can build the second ones by composing the first ones and giving them nice, abstract names.
J. B. Rainsberger, "Demystifying the Dependency Inversion Principle". An overview of the principle.
Steve Freeman and Nat Pryce, Growing Object-Oriented Software Guided by Tests. A tour de force of designing software well; the book I wish I'd written.