Static fields in specialized classes

Brian Goetz brian.goetz at
Tue Oct 14 23:59:42 UTC 2014

> My gut feeling is that this is undesirable and should be disallowed,
> and that perhaps we shouldn't even be able to observe Foo<int>.class
> at all.

If you think about the problem a little more, I think you'll find that 
this is wishful thinking.  (But of course, you're free to have a gut 
feeling without having thought about it at all.)

> After all, if specialization is intended to be viewed primarily as an
> implementation detail

It is an implementation detail in the same sense that erasure is; that 
generics are implemented by erasure almost certainly leaks into the 
user's perception too.  (Erasure is why you can't have 
Foo<String>.class, for example.  Among many other things that the user 
has to pay attention to.)

class Foo<any T> {
     T get() { ... }

Consider the members of various instantiations of Foo.

The signature of get() in Foo<String> is "String get()", but the 
signature of get() in Foo<int> is "int get()".  The erasure of the first 
is "Object get()"; the erasure of the second is "int get()".  While all 
erased reference instantiations can be implemented by the same class, 
the primitive/value instantiations cannot be.

Similarly, Foo<int> can't extend Foo<?> (because that really means Foo<? 
extends Object>, and has to for compatibility reasons), nor can it 
extend raw Foo.  (Which is fine; raw types exist solely to support 
migration from ungenerified codebases to generified ones.)

We could hide the existence of List<int>.class, but that would just make 
life harder for users, who would then have to jump through bigger hoops 
to reflect over the members of List<int> (which are different from the 
members of List<String>, even after erasure.)

> In other words, while the Java language/library changes may be
> expressed by some improvements to reflection, I'd be concerned about
> the many implemented algorithms based on reflection.

It is of course reasonable to be concerned.  Stay tuned and see if your 
concerns are addressed.

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