Idea how to implement VT/VO compatibility in JVM

Vitaly Davidovich vitalyd at
Thu Jan 22 13:35:57 UTC 2015

Yes, I was asking why any T methods wouldn't work in a framework to iterate
over "opaque" types, but I see Stephane's example now to which I replied.

sent from my phone
On Jan 22, 2015 8:27 AM, "Maurizio Cimadamore" <
maurizio.cimadamore at> wrote:

> On 22/01/15 13:19, Vitaly Davidovich wrote:
>> 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?
> In the current prototype, such methods will work, but they will work with
> all reference-parameterizations - example:
> void m(Collection<?> c) { ... }
> List<String> ls = ...
> List<Object> lo = ...
> List<int> li = ...
> List<MyPointValue> lmpv = ...
> m(ls); //ok
> m(lo); //ok
> m(li); //fail, List<int> is not subtype of Collection<?>
> m(lmpv); //fail, List<MyPointValue> is not subtype of Collection<?>
> In other words, Collection<?> means Collection<? extends Object>.
> Therefore, primitives and values are outside the domain supported by the
> unbounded wildcard.
> To fix that, you need to change 'm' as follows:
> <any T> void m2(Collection<T> c) { ... }
> m(ls); //ok
> m(lo); //ok
> m(li); //ok
> m(lmpv); //ok
> Is this what you were asking?
> Maurizio
>> 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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