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From: blmblm@myrealbox.com <blmblm.myrealbox@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: generics puzzle
Date: 18 Oct 2011 14:49:51 GMT
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In article <45bfae98-a142-469b-9b8b-9aa8a59391f1@n13g2000vbv.googlegroups.com>,
Robert Klemme  <shortcutter@googlemail.com> wrote:
> On Oct 17, 12:41 pm, blm...@myrealbox.com <blmblm.myreal...@gmail.com>
> wrote:
> > I'm having a bit of trouble with generics, and while I've come
> > up with ways of getting done what I want to get done, I wonder
> > whether there's some nice solution that isn't occurring to me.
> >
> > The basic situation is this:
> >
> > I have an abstract generic class (call it GThing) with type
> > parameter T, which has a method (call it set()) that takes a
> > parameter of type T and another method (call it modified()) that
> > returns a result of type T.  I also have some classes that extend
> > GThing and specify its type parameter, and code that is meant
> > to operate on lists of instances of these various subclasses.
> > What I would *like* to be able to do is construct a list of
> > GThing<?> objects and then do to each element of it something
> > like the following:
> >
> >   element.set(element.modified())
> >
> > but the compiler objects to that, which I find reasonable enough,
> > though I'm not enough of a generics expert to really articulate
> > what the problem is.  (I have a feeling that this "type erasure"
> > thing, which I understand dimly at best, comes into it.  :-)? )
> 
> The reason is the ? in the type: the compiler does not know the type.
> The very moment you declare a List<GThing<?>> the type of T is unknown
> even though it's the same instance.  Unknown types cannot be
> compatible yet you try to do invoke a method with argument type ? with
> a parameter of type ? and since "? incompatible to ?" it is not
> allowed.

Nice job of putting into words my vague idea about what's wrong;
thanks!

> You are basically doing this (list1) but bounding does not help
> (neither way, list2 and list3); only concrete types are able to ensure
> type compatibility:
> 
> import java.util.ArrayList;
> import java.util.List;
> 
> public class GenericsProblemRK {
> 
>     public static void main(String[] args) {
> 	final List<List<?>> list1 = new ArrayList<List<?>>();
> 
> 	for (final List<?> l : list1) {
> 	    l.add(l.get(0)); // error
> 	    final Object x = l.get(0); // OK
> 	    l.add(x); // error
> 	}
> 
> 	final List<List<? extends String>> list2 = new ArrayList<List<?
> extends String>>();
> 
> 	for (final List<? extends String> l : list2) {
> 	    l.add(l.get(0)); // error
> 	    final Object x = l.get(0); // OK
> 	    l.add(x); // error
> 	}
> 
> 	final List<List<? super String>> list3 = new ArrayList<List<? super
> String>>();
> 
> 	for (final List<? super String> l : list3) {
> 	    l.add(l.get(0)); // error
> 	    final Object x = l.get(0); // OK
> 	    l.add(x); // error
> 	}
> 
> 	final List<List<String>> list4 = new ArrayList<List<String>>();
> 
> 	for (final List<String> l : list4) {
> 	    l.add(l.get(0)); // OK
> 	}
>     }
> 
> }
> 
> > One fix is to just introduce a method setFromModified() in GThing,
> > but that doesn't appeal to me.  An ugly-hack fix is to add to
> > GThing a set() method that takes an Object and typecasts it,
> > but -- yuck, right?  
> 
> In your case since apparently you want to update internally why not
> just introduce
> 
> public void update() {
>   set(modified());
> }

I'm not sure I can say why this doesn't appeal to me -- something
about not wanting to clutter up GThing with unnecessary methods.

> This will safely compile.  Basically this is what you did with
> setModified() in the wrapper class.  All other approaches will be
> hacky (i.e. involve explicit casting, using Object parameters or
> return values etc.).

Yeah, maybe ....  In my "real" code (quotation marks because it's
a toy project embarked on for entertainment and a bit of education)
the method in the wrapper class does more than just call modified()
and then set().  I don't know that that matters much, though.  I do
have a couple of different "call sites" (to use another poster's term)
that need similar functionality, so maybe it *does* go in GThing.
Well, like I said, "entertainment and a bit of education", and for
me that seems to involve -- the coding equivalent of periodically
rearranging the furniture, maybe.

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