[4970] in java-interest
Re: Observable problems
daemon@ATHENA.MIT.EDU (David Geary)
Wed Jan 24 19:52:59 1996
Date: Wed, 24 Jan 1996 16:22:50 -0700
From: David.Geary@Central.Sun.COM (David Geary)
To: chris@telesph.com
Cc: java-interest@java.Eng.Sun.COM
Just thank the good Lord that we don't have to deal with the complexities
of multiple inheritance, otherwise you'd have to do:
class Foo extends Panel, Observable {
...
}
IMHO, disallowing multiple inheritance of implementation results in much
design complexity such as you outline below.
Note Observable methods such as: deleteObserver(Observer), deleteObservers(),
countObservers(), etc. should also be exposed in Foo.
The evils of multiple inheritance are greatly exaggerated.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
David Geary "I'm a loser baby,
geary@rmtc.Central.Sun.COM So why don't you kill me?"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> I'm running into problems using the Observable class and the Observer interface.
> I have a class (call it Foo) which I would like to be Observable (when a button
> is pressed I need to tell two or more observers about it). I can't make Foo
> a sub-class of Observable because Foo already is a sub-class of Panel. I can't
> have Foo use Obserable (have an Observable instance as a member of Foo) because
> Observable.setChanged() and Observable.clearChanged() are protected functions
> and can only be called by sub-classes of Observable.
>
> One solution is to make a class like this:
>
> class MyObservable extends Observable {
>
> ...
>
> public setChanged() {
> super.setChanged();
> }
>
> public clearChanged() {
> super.clearChanged();
> }
> }
>
> And then use it inside my class like this:
>
> class Foo extends Panel {
> MyObservable myObservable;
>
> addObserver(Observer o) {
> myObservable.addObserver(o);
> }
>
> ....
>
> /*
> * When something changes
> */
>
> myObservable.setChanged();
> myObservable.notifyObservers("Something Changed");
>
> ....
> }
>
>
> But, this strikes me as a kludge. Can anyone tell me a better method? Can
> anyone explain why setChanged() and clearChanged() are protected?
>
> Having fun,
> Christopher J. Helck
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