As a result, adding a new method to an interface will require
clients to change their code, even if they're not interested in
actually using the new method. Cocoa delegate clients couldn't
care less and won't *have* to change.
Or you can just define a new subinterface, as in LayoutManager and
LayoutManager2.
Yes, of course, but that's a big kludge, no?
Ian Joyner wrote:
Delegation is distinct from interfaces & far more powerful. It
is part of the runtime as you mentioned, but Cocoa==Obj-C like
JavaVM==Java. Java actually took the concept of interfaces from
Objective-C where they are called protocols.
I never said that delegation and interfaces are the same thing. I
said that delegation can be implemented using interfaces.
IMHO, Interfaces are a poor way to implement delegation; once
fixed they cannot be easily extended without breaking client code.
Well interfaces are just really the poor man's multiple
inheritance, although the argument for interfaces and against MI is
claimed to be theoretical. Thing is that, like Karl says, if you
want to add something to the interface then every implementing
class has to be changed. In practice, it would be much easier, to
add to a real class a default method, so that implementing classes
would not be affected.
Adding default methods to a real class is precisely what the Adapter
pattern does but then it forces implementers to subclass the Adapter
and you lose the freedom to choose the implementer's superclass.
I'm now convinced that there are real advantages to Obj-C's protocols
as compared to Java's interfaces. Because I'm new to Obj-C and never
gave much detailed thinking to the differences between protocols and
interfaces, I was under the impression that everything you could do
with protocols (in particular, implement Cocoa's delegation
mechanism) could be *easily* done with interfaces.
I've now revised my position. Sure, it's possible to implement the
delegation mechanism using interfaces, but there is at least one
situation which is much more easily handled by Obj-C's protocols than
by Java's interfaces, namely, an after-the-fact addition of a new
method to the delegation protocol. While Java implementations
struggle with kludges or with forcing clients to change their code,
the Obj-C implementation couldn't be any smoother.
Wagner
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Java-dev mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/java-dev/email@hidden
