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