Reified generics in Java

Neal Gafter post a blog entry about adding reified generics to Java. I likethat proposal mostly because developpers will have the choice betweeen reified or not reified generics.

About the syntax, i think it's better to use an anotation than to re-use the keyword class. Annotating a type variable is not currently allowed but it seems that the upcoming JSR 308 will allow that.

 class ArrayList<@Reified E> {
   ...
 }

 class ArrayList<class E> {
   ...
 }

The big question is why tagging the type variable and not the parametrized type.
I'm not sure that allowing to declare on a same parametrized type refied type variable and non refied is a good idea, because i don't see a use case.

 class HashMap<@Reified K,V> {

 }

Here, HashMap is not refiable because V is not refiable. So what is the need of half reified type ?

Else, i not sure about the fact that allowing reified generics requires to change collection interfaces. Allowing reified interfaces will just allow safe cast/instanceof, i wonder if it worth all problems linked to the introduction of a new collection hierarchy. It's perhaps up to a collection implementor to decide if a collection implentation will use or not reified generics.

To end, i've recently read a paper of Mirko Viroli about how to represent wildcard types at compiler level but i'am not aware of any work about that at VM level.

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