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From: blmblm@myrealbox.com <blmblm.myrealbox@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: generics puzzle
Date: 19 Oct 2011 13:25:46 GMT
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In article <M%hnq.3083$Oz5.1691@newsfe16.iad>,
Daniel Pitts  <newsgroup.nospam@virtualinfinity.net> wrote:
> On 10/18/11 2:45 AM, Steven Simpson wrote:
> > On 17/10/11 16:58, Daniel Pitts wrote:
> >> On 10/17/11 5:14 AM, Steven Simpson wrote:
> >>> On 17/10/11 11:41, blmblm@myrealbox.com wrote:
> >>>> One fix is to just introduce a method setFromModified() in GThing,
> >>>> but that doesn't appeal to me.
> >>>
> >>> Instead of adding it to GThing, create a static method:
> >>>
> >>> private static<T> void setModified(GThing<T> t) {
> >>> t.set(t.modified());
> >>> }
> >>
> >>
> >> What about the simpler solution:
> >>
> >> in the GThing class:
> >>
> >> public void setModified() {
> >> set(modified());
> >> }
> >
> > It "doesn't appeal" to the OP? Nor to me. Not sure I can put my finger
> > on it, but if I were the maintainer of GThing, I'd consider its
> > specification to be complete without this method. It doesn't increase
> > the value of the class as an abstraction. Adding it would certainly just
> > be a convenience for the caller. It solves a problem generated by the
> > design of the call site, not the design of the class's contract. The
> > caller can write his own routine as necessary.
> >
> Possibly, but if it is something that happens frequently, then it seems 
> more likely to belong to GThing than externally. Otherwise your 
> call-site is suffering from the code smell "feature envy"
> 
> One other approach is a "visitor" pattern:
> 

I know the "visitor" pattern (vaguely anyway), but ....  I'm not sure
I understand your version of it here, which -- does this compile?
because ....

> 
> public interface GThingVisitor {
>     <T> void visit(GThing<T> gThing);
> };

(Aside:  Hm, a generic method in an interface?!  well, why not,
maybe, but I'm not sure it would have occurred to me to try!)

> public class SetModifiedGThingVisitor {
>       <T> void visit(GThing<T> gThing) {
> 	gThing.set(gThing.modified());
>       }
> }

Was this meant to implement GThingVisitor?

> public class GThing<T> {
>     public void accept(GThingVisitor visitor) {
> 	visitor.accept(this);
>     }
> }

visitor.visit(this)??

> public class CallSite {
>     public void setAllModified(Iterable<? extends GThing<?>> things) {
>   	GThingVisitor visitor = new SetModifiedGThingVisitor();
>          for (GThing<?> thing: things) {
>              thing.accept(visitor);
>          }
>     }
> }
> 

-- 
B. L. Massingill
ObDisclaimer:  I don't speak for my employers; they return the favor.