On Thu, 2008-10-16 at 09:48 -0700, Robert Greayer wrote: > > On Thu, 2008-10-16 at 15:02 +1300, Richard O'Keefe > > wrote: > > > On 16 Oct 2008, at 12:09 pm, Jonathan Cast wrote: > > > >> I am not sure how say in a Java language a > > constructor can "conjure > > > >> up > > > >> a value of an unknown type". > > > > > > > > Well, that's the point. It can't, in > > Haskell or in Java. If you > > > > understand that --- that you can't call the > > default constructor of a > > > > class that is not statically known at compile > > time > > > > > > If you understand that about Java, then you don't > > understand Java. > > > > God, I hope never to understand Java. *shudder* > > > > > Java reflection means that compile-time types are > > backed up by > > > runtime objects belonging to Type in general, to Class > > if they > > > are class types. It also means that you can discover > > the > > > default constructor by using aClass.getConstructor(), > > and you > > > can invoke it by using .newInstance(). > > > > Wait, what? Why can't Java use this to keep template > > parameters around > > at run time? Or is the article (as per which > > Set<Integer> and > > Set<Double> are identical at run time) full of it? > > > > The article (whichever it was) wasn't full of it... Set<Integer> and > Set<Double> are identical at runtime. You cannot, given a Class > object at runtime that happens to be the Class object corresponding to > Set<?>, conjure up an instance of Set... but simply for the reason > that Set has no constructors (it is an interface).
I think we've found our difference between Haskell and Java... So Set<Double> is an interface, thus you can't copy construct it. Makes sense --- but does it belong in an article about gotchas with *generics*? > You can, however, given a class object that happens to be the class > object corresponding to (say) HashSet, Can I have HashSet<Integer>? Could I construct HashSet<?>, if I did? > conjure up an instance of a HashSet, and assign it to a variable of > the (static) type Set<Integer> or Set<Double>... i.e. > > Set<Integer> foo = (Set<Integer>) hashSetClass.newInstance(); > Set<Double> bar = (Set<Double>) hashSetClass.newInstance(); > > which will generate warnings about unsafe casts, but nevertheless can > compile, and won't cause any exceptions at runtime. jcc _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
