[2071] in java-interest
Re: Fn ptrs & ... callbacks!?
daemon@ATHENA.MIT.EDU (Sal Cataudella)
Fri Sep 22 17:43:16 1995
Date: Fri, 22 Sep 1995 14:25:42 -0400 (EDT)
From: Sal Cataudella <salc@ovid.com>
To: Philip Brown <phil@bolthole.com>
Cc: java-interest@java.sun.com
In-Reply-To: <199509220855.IAA17734@shellx.best.com>
On Fri, 22 Sep 1995, Philip Brown wrote:
> [ Steve Witham writes ]
> >
> > >It's also a problem for handling callbacks from GUI controls.
> >
> > I thought Java got rid of callbacks by having threads...didn't it?
> > Callbacks are just a dumb, inside-out way to implement threads.
>
> No they are not. Callbacks are a way of implementing Event-driven
> programming.
> This should not be confused with threading. They are completely separate
> issues.
Not only that, but method pointers, when used for callback purposes,
allow you to reuse a class's code without having to subclass.
Let's say you need to create 2 sliders, and have each one of them invoke
a different method. With the method ptr callback technique, you can create
2 instances of sliders and simply register the method and object pointers
you wish to have invoked. Thus , you are reusing code (the slider's)
by instantion, not by subclassing. Without the method callback technique,
you are *forced* to subclass *twice*, in order to reuse code.
One of the OO lessons I learned from C++ is that:
"re-use comes thru instantiation, specialization comes thru
subclassing."
Since pointer paranoia abounds in Java (for good reasons),
perhaps one should take a moment and realize that since objects are
really close to C++ reference variables' (except that java's can be
reset anytime ?) anyway, we already have pointers in java!
To implement method pointers, all that is needed
is a method reference type, which can be made to refer to a specific
method.
eg:
class Foo extends Object {
...
void bar( ....) { ....};
} ;
Foo aFoo = new Foo (whatever) ;
Method mRef = aFoo.bar ;//reference to method 'bar' note lack of param
aFoo.mRef( blah, blah ) ; // execution of method 'bar' via reference
Note that this would not be implemented as a *hard* code address,
but could instead be implemented ala Objective C method Id style.
How about it, Arthur?
Sal Cataudella email: salc@ovid.com
Ovid Technologies (formerly CD Plus) phone: 212-563-3006 ext. 406
333 Seventh Ave., 4th Floor fax: 212-563-3784
New York, NY 10001
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