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From: blmblm@myrealbox.com <blmblm.myrealbox@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: generics puzzle
Date: 20 Oct 2011 14:14:48 GMT
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In article <ZqDnq.5673$UK6.3114@newsfe06.iad>,
Daniel Pitts  <newsgroup.nospam@virtualinfinity.net> wrote:
> On 10/19/11 6:25 AM, blmblm@myrealbox.com wrote:
> > In article<M%hnq.3083$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!)
> I've used it in the past, so I know it works.
> >
> >> public class SetModifiedGThingVisitor {
> >>        <T>  void visit(GThing<T>  gThing) {
> >> 	gThing.set(gThing.modified());
> >>        }
> >> }
> >
> > Was this meant to implement GThingVisitor?
> Indeed it was. This entire snippet was typed up in my mail program, so I 
> missed a few pieces.
> >> public class GThing<T>  {
> >>      public void accept(GThingVisitor visitor) {
> >> 	visitor.accept(this);
> >>      }
> >> }
> >
> > visitor.visit(this)??
> Yes, that's what I intended :-)

Typos (thinkos?) happen.

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

I can report that this approach works too, though I put the
visitor code directly in CallSite as an anonymous inner class
rather than writing a separate SetModifiedGThingVisitor class.

Overall this approach has for me a certain "can't quite get my head
around it" quality -- not in the sense that I don't understand how it
works, but in the sense that I can't quite sort out how it compares
to the static-method approach.  Thoughts, anyone?

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