Inheritance is just one of multiple facets of safe code reuse in OOP. Aggregation, composition, encapsulation are as are much as fundamental notions in OOP as inheritance. So i think reducing OOP in general, and java in particular to "inheritance based OOP" is a miss characterization
> are that it fits very few problems well, that it usually causes lots of problems and that many programming languages only have inheritance based OOP in their toolbox.
Do you have any objective way to measure that ?
> Patterns like abstract visitor factories are hacks to express situations that cannot be expressed in an obvious way.
But isnt that the reason to have a pattern ? An easy way to expression a non obvious recurring situation ?
There's a reason why composition is preferred over inheritance. There's also good reason why some programmers will take it to the extreme and say that it ALWAYS causes more problems. Some languages lack of providing an appropriate aggregation alternative generally keeps inheritance alive.
Some people will hold onto their positive notions about inheritance too, and that's fine, but there's a reason why many people advocate strongly against it (and why some modern language designers skip it altogether!)
I might not be understanding well, but are you talking of languages that happen to have no _composition_? But do have inheritance? Sounds like those with inheritance are a strict subset of those with composition (which in turn would be all but fringe languages).
Inheritance is everything else but safe: you have to check if you broke the behavior of all methods and public fields you are inheriting. How often do you go through all classes you are inheriting from? Sometimes you cannot even fix this with overriding methods: You can easily construct a cut off ellipsoid from a regular ellipsoid, but then it stops being a quadric surface invalidating your nice class hierarchy. And you cannot change the class hierarchy because half of your codebase depends on it.
Patterns are like neat useful tricks. Like how to open a bottle with a hammer. Pretty nice to open your beer at the end of the day in a workshop. If you work in a bar and you constantly have to use tricks to use your hammer for your work you should probably rethink if a hammer is the right main tool for you.
That conclusion holds if your primary experience is Java: the Java class library is widely believed to have abused inheritance.
However other languages have avoided that trap. Apple's Cocoa frameworks in ObjC do use inheritance but also delegation, notifications, etc. Also Swift supports inheritance, but here we see Lattner describing inheritance as a "tool in our toolbox," not as the "go-to method for structuring everything."
> Inheritance is everything else but safe: you have to check if you broke the behavior of all methods and public fields you are inheriting.
Designing a class interface intended to be inherited is like any other API design exercise. Your API commits to invariants, and it's the client programmer's responsibility to follow them; if they do the class should not be broken.
If you find yourself checking "all methods and public fields," either the API is bad or you've misunderstood it.
Again, it's just one API design tool. Sometimes the alternative to inheritance is just an ad-hoc, bug-ridden re-implementation of inheritance.
No, it's not. Many places recommend restraint in use of inheritance (and in languages that support only single inheritance, there are sharp limits to what it can do to start with.)
You say that as if these things are not just as easily expressed in FP -- if not even easier.
Aggregation is just records-of-records.
Encapsulation is just "abstract data types", e.g. ML modules with abstract members, or non-exported data constructors in Haskell. Another option would be simply closing over whatever you're trying to hide. Another option would be existential types. (There's some overlap among all of these.)
Composition... well, actually I'm not sure what exactly you mean by "composition" in the context of OO. Can you explain what you mean?
So true! Sadly, most programmers' idea of an “abstract data type” is a class. :-p
No, that's what you read. What i said what was i wrote... OOP is bigger than inheritance.
> -- if not even easier.
Again, this kind of statement really sound like empty FP propaganda to me.
> Encapsulation is just "abstract data types", e.g. ML modules with abstract members, or non-exported data constructors in Haskell. Another option would be simply closing over whatever you're trying to hide. Another option would be existential types. (There's some overlap among all of these.)
Or "abstract data types" is just Encapsulation...
I think you may have misunderstood my intent. I was just trying to say "FP can do these things too".
> Again, this kind of statement really sound like empty FP propaganda to me.
Right, so any type of even very modest support for X is "empty X propaganda". Can we please assume at least a modicum of good faith here?
> Or "abstract data types" is just Encapsulation...
Oh, so "semantics" it is then. Oh, well.
FWIW, I think I'm right in saying that ADTs were invented quite a bit before OOP & "Encapsulation".
I notice that you also didn't answer my question of what Composition actually means in OOP. Do you have an answer? I promise, I wasn't being facetious.
