Re: Call-Super antipattern
On Wed, 7 May 2008, Christian wrote:
Tom Anderson schrieb:
But a language feature which means something like "when you send this
message to an object, invoke all the matching methods in the hierarchy,
not just the most-overriding one" would do it. I'm not aware of any
language having a feature like that; i think you could get the effect
in python using a metaclass, and you could probably do the same in any
other language that supports deep metamagic, like LISP, but nothing
lets you do it directly.
I think that would be the real antipattern.
when you call super in a class that you override that is ok and often needed.
As you are having a good look at that class and therefore know if a method
needs to call super or not to keep a consistent state for itself.
If one allowed calling all methods of all super type that would make you
responsible for all objects in the hirarchie ..and like that breaking
the encapsulation the object you extend represents.
I look at it as making each class in the hierarchy responsible for making
sure that their method can be called by subclasses. I don't see that it
would break encapsulation. Indeed, by doing it declaratively, rather than
by requiring super calls everywhere, you reduce the burden of
responsibility on subclasses, and reduce the coupling between levels.
It would need to be done in a form that was obvious to the programmer,
though, so they couldn't fail to notice that it was happening. Perhaps by
introducing a keyword in the declaration, and requiring that it be in any
'overriding' declaration as well, so if a programmer didn't realise, and
omitted it, the compiler would tell them. So:
public class Vehicle
{
public super void start()
{
unlockDoors() ;
}
}
public class Car
{
public void start() // not declared 'super'
{
startEngine() ;
}
}
Would be a compile-time error.
I should add that i'm not seriously advocating this as a feature for java,
or any other language. I just think it's an interesting idea to kick
around!
tom
--
Ensure a star-man is never constructed!