Type parameters were added to the Java programming language in the 1.5 release.  While core reflection was updated to provide information about the generic type information, only structural information is provided.  That is, core reflection can tell you that a method in a class has a generic return type and that a class has a type variable.  However, core reflection cannot tell you that a particular Set object is a Set<String> or Set<Number> instead of just a Set.  Currently, such information is not available at runtime because generics in Java and implemented via erasure; roughtly speaking javac transforms a generic Java program to a non-generic program with casts.  Reification would make the type variable information available at runtime too; that information would be useful for certain programming idioms.