Peter Veentjer - Anchor Men wrote:
And true subclassing only is possible if every method was subclassable.. that
isn`t the case.. most methods are final.
And that is a /GOOD/ thing. Most methods are an implementation detail,
that need to be called in the framework. As Eelco and Johan and others
already said here: the Component hierarchy has very particular behaviour
that needs to be fixated at several places in the framework in order to
remove the possibility of 'clients' altering that behaviour.
But I think Subclassing is overrated
No, subclassing /used/ to be overrated. Subclassing has its place in an
O.O. design. Look at the design patterns of Gamma et al. they use
inheritence quite to the fullest and no one is calling their designs the
sign of a beginning O.O. developer!
Before another rant about interfaces being the true and only O.O. design
method comes up, please read the blog entry by Mike Spile on
Overreacting
(http://www.pyrasun.com/mike/mt/archives/2005/07/13/21.45.29/index.html).
Putting /everything/ behind an interface falls into this category.
[...] and a sign of a beginning oo developer.. There are other ways... but you
need a seperation of definition and implemenation to realise it.
Talking about a knee jerk reaction! The main architect of Wicket has
over 20 years of O.O. design and development experience, he has worked
on the Swing/JFC team and is considered by his peers as top notch.
Hardly a beginning O.O. developer.
Jonathan can make wrong turns in his designs as he also constantly
learns. Considering the whole, Wicket's design is one of flexibility,
leveraging the correct concepts of Java at the right time: the IModel
interface is a great example of where interfaces are put to good use.
Page would be a wrong place to put an interface into use.
Interfaces have their place and time. Inheritence has its place and
time. Learn both, embrace both, use both and achieve great wonders!
Martijn
Just my two cents...
-Matej
Another reason not to use interfaces here is that there is so much
behaviour in here (in the pages and components), that it is very hard to
extract something usefull out of it. What would the interface of a Page
be? One of the good practices when making interfaces is that they should
be small. Having an interface like
http://jakarta.apache.org/tapestry/tapestry/apidocs/org/apache/tapestry/IPage.html
doesn't really display that. They can be large, like the collection
interfaces are, but they have to be really, really stable and consistent.
Just to be sure, and as I didn't make this decision for Wicket I asked
Jonathan Locke (who is doing movie stuff now, and left the mailing lists
for some time) why he choose for interfaces. Here's a partial transcript
of our chat:
[22:31] jonathan: i don't believe in big interfaces
[22:31] jonathan: no interface should be as large as Page
[22:32] jonathan: it's too fragile
[22:35] jonathan: abstract classes are evolvable
[22:36] jonathan: interfaces should always be a set of methods that you
cannot ever imagine extending
[22:36] jonathan: it encapsulates a very specific set of behaviors
[22:37] jonathan: everyone should feel free to create/extract his own
IPage interface for use with proxies
[22:38] jonathan: there's something fundamentally wrong with the way
that current OO languages try to wire things together with static
interfaces.
[22:39] jonathan: unless i'm missing something, this is really not a
problem with wicket
[22:40] jonathan: in fact i think it's a feature
[22:40] jonathan: interfaces are really most useful for things that are
truly pluggable
[22:43] jonathan: my ideal number of methods in an interface is 1
[22:43] jonathan: ;-)
[22:44] jonathan: 2 is okay in some cases
[22:44] jonathan: 3 often should be re-thought
[22:44] jonathan: the more methods you have, the more likely it is that
you've got a bad design
So, it is a design decission not to work with interfaces. However, as
Jonathan stated, you could extract an interface, let your page class
implement it and off you go. Furthermore, I wonder what it is you want
to do with a proxy that you can't do by simply subclassing? Or AOP?
As a final statement I'd like to add that a framework like Wicket is
very different from a business system, which could have tons of business
methods that must be pluggable. Though even in that case, I think too
that interfaces are overused/ get too much focus with some of the
current popular frameworks. Seems to be getting a knee-jerk reaction
instead of a design decision. But maybe that's just me.
Regards,
Eelco
Peter Veentjer - Anchor Men wrote:
I`m trying to add Spring annotations to the Page, but because there is
no page interface and only a Page class (with final methods) it is not
possible to create a proxy and therefor it is impossible to enhance
objects. This is very inconvenient..
Why are the components not designed from interfaces? Designing from
interfaces is common oo practice.
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