Idea how to implement VT/VO compatibility in JVM

Vitaly Davidovich vitalyd at
Thu Jan 22 13:19:52 UTC 2015

Can you expand a bit on the part where you say frameworks can't iterate a
Collection<any T> without knowing the instantiation? Do you mean existing
methods that take Collection<?> won't work without change or something else?

sent from my phone
On Jan 22, 2015 8:12 AM, "Stéphane Épardaud" <stef at> wrote:

> On 01/22/2015 12:18 PM, Maurizio Cimadamore wrote:
>> I don't think there's an 'hole' as such in your proposal - but there are
>> 'unknowns'.
> Well, that's already good news. I prefer unknowns to known holes ;)
>  Performance-wise, I think the 'risk' is that if we keep the API the way
>> they are today (i.e. removeAll(Collection<?>) and friends), the boxing path
>> will pretty much be the norm. Now, in a perfect world, as value types are
>> non-polymorphic (or their polymorphism is very restricted) and immutable,
>> that would suggest that VM should have enough of an hint to perform boxing
>> elimination and such, so that the cost you end up paying is negligible. How
>> much of this is reality? As you, I'm not a VM guru - but I think it's a
>> question worth asking. It seems a likely scenario that the JVM will do
>> great in most cases, and have some bad performance cliffs in others - is
>> that something we are willing to sign up for?
> OK, that's fair. By allowing people to use value types boxed we allow them
> to shoot themselves in the foot perf-wise (assuming the VM can't help),
> because we make value types much more accessible. This is mostly due to
> `any T` being opt-in rather than the new default for `T` where `T extends
> Object` is not specified. If we also changed `T` to mean `any T` (source
> only, so for newly compiled code) then all generics code would be
> specializable by default and that means that users of generic code will
> always be able to use the specialised code as long as they can instantiate
> the generic type argument at compile-time (as is already the limitation).
> If they can't, well they already have to use `Object` and boxing and can't
> traverse collections because they're not `List<Object>` (ATM).
> Making it the default has down sides, probably in larger class files, but
> as long as it's not the default, there will always be incompatibilities in
> libraries that will forget to opt-in, which will mean that either (ATM)
> they can't be used with containers of value types, or that the containers
> have to be wrapped to box the value types (unless we fix that as I
> suggest). IMO that's already going to cause compatibility issues and there
> may arise a "coding guideline" (remember them from C++ sore points?) that
> people will be strongly encourage to use `val T` everywhere just in case
> someone wants to use an unboxed value type.
> The rift between primitives and Object is already a famous sore point in
> Java (not criticising, this was a choice made a long time ago, for valid
> reasons, we just have to deal with it) which has been "fixed" in most
> non-Java JVM-languages due to popular demand. I don't think the new default
> with value types should be that generics don't accept value types.
>  On the language-side unknowns, how much code out there is relying on
>> being able to access fields on Foo<?> or raw Foo types? This is perhaps not
>> common, but I think we need to gather data points on this i.e. by looking
>> at existing open source projects (help welcome here!). Other possible weak
>> points are that this doesn't necessarily address all the issue w.r.t.
>> language uniformity - i.e. how is an ArrayList<int> supposed to answer to a
>> question of the kind 'is instance of List<?>' ?
> Fields that don't involve the `any T` are fine. Fields that do may have to
> revert to autoboxing _if_ we feel we _must_ accomodate that to autobox not
> just value types but their containers.
> `ArrayList<int>` is special, mostly due to the fact that I'm not sure we
> can retrofit `Integer` to be a value type, so existing primitives may not
> get the `List<int> === List<Integer>` that I suggest for value types
> (though I'd be very interested in making this work).
> If we take an easier example assuming a value type named `Date`, then yes
> I expect that:
> - `List<val Date> instanceof List<?>` == `true`
> - `List<__Boxed Date> instanceof List<?>` == `true`
>  I'm not denying there's something there worth exploring (as I have in
>> fact already said), but it seems to me that, while you can go a long way
>> with bridges, there are still questions that bridges alone simply do not
>> have an answer for.
> Thanks, and yes I agree there are still questions remaining, and more
> importantly an implementation lacking, but hopefully if the proposal sounds
> decent I can help with the proto.
> Thanks for your answers BTW :)

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