Re: Composition vs. inheritance
alexandre_paterson@yahoo.fr wrote:
On Apr 23, 3:27 am, Arne VajhHj <a...@vajhoej.dk> wrote:
...
A very good indication that the class is intended to be extended is if
it is abstract.
:-)
So go ahead and extend abstract classes as you want.
OK, but the OP's question could also be read as 'should I design
my own programs/framework' by creating abstract classes?
And the answer is: yes, if it makes sense.
It's not because the OP is sometimes forced to use badly crafted
frameworks forcing him to extend abstract classes that abstract
classes are a good thing.
In my opinion it's a fundamentally broken design.
As I already pointed out both James Gosling and Bjarne Stroustrup
agree on that one, as do, say, the Spring developers, users, etc.
http://www.artima.com/intv/gosling34.html
(this is really an eye opener).
My other post seems to be lost, so here's Stroustrup's take on this:
http://www.artima.com/intv/modern.html
Stroustrup:
"Since then I have consistently pointed out that one of the major
"ways of writing classes in C++ is without any state, that is,
"just an interface.
Actually they do agree on more then that: they even agree on the
fact that 'concrete inheritance', a.k.a 'implementation inheritance'
is fundamentally broken.
There are even OO languages that explicitely forbids implementation
inheritance (and other that mandates it but that is another story).
And these guys knows way better than me, as do the Spring developer.
I think you read to much into those writings.
That someone thinks a language without extends would be better
does not imply that extend should not be used in a language
that does have it.
That abstract classes can be written with no implementation at
all and that it is a major thing does not imply that abstract
classes with functionality can not be a good thing.
I checked newest Spring. Out of approx. 1600 classes then approx.
300 are abstract.
If there is a comment in the tops saying "extend this class to ...",
then it is probably OK to extend.
Yup... But to me it's also a warning saying: "be very careful when
using this framework, it's probably not been designed by OO gurus".
(I'd rather keep using frameworks like Spring that do the Right Thing
[TM]).
See above.
Arne