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From Gilles Sadowski <gil...@harfang.homelinux.org>
Subject Re: [Math] What's the problem with interfaces?
Date Wed, 30 Mar 2011 12:15:30 GMT
Hi Jorg.

> > 
> > From comments that were posted to the other thread, I gather the main
> > trend that, because some interfaces needed an upgrade, the "interface"
> > design tool is becoming "evil". Did I get this right?
> > 
> > I guess that you refer to "RandomData" and "RandomDataImpl". This is
> > indeed the typical example of abusing the "interface" tool. When only one
> > implementation is meaningful, an "interface" need not be defined.
> 
> No. It is about adding something to the interface, just like Phil said. If 
> you add a method to an abstract class, the dervied ones are still binary 
> compatible, the same does not apply for interfaces.
> 
> We had discussions in *length* about this and I am not interested in 
> starting over again.

Well, I was not part of those discussions, and CM is _still_ full of Java
"interface"s; so I assume that it's acceptable that I give my point-of-view
now.

As I said, I'm not in favour of creating interface for the sake of having
everything separated in "Foo" and "FooImpl".
I understand that having an abstract class has some definite practical
advantages. And for the things I had in mind and which I'm most concerned
about (coherent design), it might well be a non-issue: "interface" could be
changed to "abstract class".

The problems might come from the one thing which you can do with interface
but cannot with classes, namely, multiple inheritance.

Did someone already analyze the situation in CM?
The issue might well be resolved similarly to the debate about checked
exceptions, i.e. within CM, "interface" may also prove to be an unneccessary
feature of the Java language.

> > The "interface" is not the way (preferred or not) to support alternative
> > implementations. As was already said, this is done with (abstract or not)
> > classes which an alternative implementation can inherit from.
> > Rather, the (Java) "interface" is supposed to represent the abstraction
> > (from the "real" world object) of everything that is needed to interact
> > with that object (i.e. its interface). It makes it possible to treat
> > different objects on a equal footing (from the caller's point-of-view).
> > But you all know that...
> > 
> > So what's the problem? Is it the binary compatibility, again? This is a
> > configuration management issue. When the compatibility is broken, you
> > change the major version number and/or the base package name. That was
> > settled, or not?
> 
> The point is that an interface should not be changed for a long time. 
> Otherwise every new release is a major one.

It is not a question of time but of major (incompatible) and minor
(compatible) releases. As I noted in the reply to Sebb, the real problem
is not supporting older releases. That's a policy issue.

> > It would be a pity to sacrifice a tool aimed at improving the design
> > because of such considerations as keeping backward compatibility with
> > versions that nobody here is going to support.
> 
> It's a pity for the user, because he will *never* be able to use the next 
> version as drop in.

I also answered that in other post.

> > If some user is happy with version "M.m", he can use it forever. If he
> > wants to use a newer version "N.n", he should not expect it to be
> > compatible. It does not have to be! Non-compatible modifications are not
> > performed out of some urge for change but stem from the desire to get a
> > better product, bits by bits.
> 
> If that were true, you would currently have 10 different commons-lang, 10 
> different commons-collections and so on in your classpath.

No, it's the user's choice to select which version he wants.

You cannot have multiple versions of Linux running at the same time, but
nevertheless the kernel developers make releases at a much higher pace than
Commons...

> > Yes, it's not easy to get the interfaces right; so what? If you find that
> > you can improve the design, you do it and bump the major verion number.
> > As someone pointed out, it's not as if we'll run out of numbers.
> 
> And people will stop using it, because they are annoyed when they are 
> depending in the end on n different versions of math, simply because of 
> transitive dependencies.

I think that you reason on the basic assumption that CM is close to
stability. Many problems (some bugs but also design consistency) have shown
that it is not. So, my opinion is that users will prefer a product that
continues to improve rather than something that is backward-compatible. I'm
one of those. If not, I'd be content with what is there and would not feel
the need to contribute.

> > Part of the real problem (as shown by the amazing amount of done and
> > undone work for 2.2) is that you (collectively) want to do too many things
> > at the same time (lots of changes *and* few releases). To be clear, the
> > problem is not the "lots of changes" part (which you would like to "solve"
> > by vetoing future compatibility-breaking improvements). Note that the
> > following is not a criticism of CM (which has many features etc.) but some
> > of the code that I've seen (in the parts which I've more closely looked
> > at) do not make me think that it is mature enough that one can say "That's
> > it!" and stick with the design forever.
> > Again, all this (e.g. removing duplicate code and refactoring the design)
> > can be considered unimportant and swept under the carpet but IMO *any*
> > cleanup is good as it will contribute to the overall robustness and
> > maintainability.
> > Half-baked design will continue to itch.
> 
> Half-baked upgrade strategies also.

Unforunately CM currently does not have the resources of its ambitions
concerning the upgrade strategies! That implies that some expectations
must be dropped: IMHO, code improvement is, at this time, more important
than upgrade easiness.


Regards,
Gilles

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