I knew for a while that composition must be used instead of inheritance. I just realized that there is no one example (stackoverflow was asked explicitly) when inheritance would be preferred. Never! The very inheritance, the cornerstone of OOP (Java is OOP), is considered a bad practice! Integer extends Number, FilterInputStream extends InputStream, which extends Closable. Java begins with Object whereby every other object inherits a dozen of fields (lock, hash, class) and 11 methods (wait, getHash, getClass) from it.
Notably however, design pattern experts who make us thinking that inheritance is evil and stone age structural composition must always be used instead, promote OOP and the idea that we must replace conditionals with polymorphism! (which is achievable only through inheritance in OOP languages). For no reason [1], [2], [3]!
Update The last bullet is that tight coupling between functions and data is bad.
Notably however, design pattern experts who make us thinking that inheritance is evil and stone age structural composition must always be used instead, promote OOP and the idea that we must replace conditionals with polymorphism! (which is achievable only through inheritance in OOP languages). For no reason [1], [2], [3]!
Update The last bullet is that tight coupling between functions and data is bad.
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Just to clarify things, so people don't get it wrong:
> It is wonderful moreover, how design pattern experts
> who make us thinking that inheritance is evil
> and stone age structural composition must
> always be used instead, promote OOP and the
> idea that we must replace conditionals with
> polymorphism (which is achievable only
> through inheritance in OOP languages).
First of all, I've never read that composition should be always used instead of inheritance, the wording is somehow different: Prefer Composition over Inheritance. But there are cases where inheritance is better than composition.
Second, just because you compose your classes, doesn't mean that you cannot use polymorphism. One does not exclude the other. I think you are a little bit stuck in type safety. Let's see a language like Ruby. In Ruby you can use polymorphism without even letting the objects know, that they are used the same way! If objects provide the same method you can treat them likewise. In a static typed language like Java, you can also achieve this, by introducing interfaces. So place an interface with only the methods you like to use and let your classes implement that interface and use composition for code-reuse, you will find yourself in a well-structured system, that favors composition over inheritance and makes intensive use of polymorphism.
So guy, please calm down and little bit and study even a bit more. Thanks.
heh
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