2. Whether abstract classes are primitive superclass candidates. The static
compiler will check this at compilation time when it sees a superclass of a
primitive class, but the JVM will want to recheck anyway. There are two
sensible ways to handle this in the classfile:
- An attribute that says "I am a primitive superclass candidate." The static
compiler puts it there, and the JVM checks it.
- Infer and tag. If an abstract class is loaded that is not a primitive
superclass candidate, the JVM injects IdentityObject as a superinterface of the
newly loaded class; when we go to load a primitive subclass, this will fail
because primitive classes cannot implement both IdentityObject and
PrimitiveObject.
Reflection probably doesn't have to reflect whether a class is primitive
superclass candidate; it already reflects the things needed to make this
determination.
This one, on the other hand, conveys a core property of a JVM class.
John's notes in the SotV suggests that the JVM is comfortable just
"figuring it out" and not requiring an attribute. So this is the "infer
and tag" option; the VM infers this at runtime. Not clear if there is a
value to having the static compiler capture something that wasn't
explicit in the source and that has to be validated at runtime anyway.
