Evolving the wrapper classes

[Tobi Ajila]

Thanks for the example Dan, this "Object[] objs = arr; // just like Point[]
<: Object[]" makes it very clear. Brian's response makes more sense to me
now.


> From: Dan Smith <daniel.smith at oracle.com>
> To: Tobi Ajila <Tobi_Ajila at ca.ibm.com>
> Cc: Brian Goetz <brian.goetz at oracle.com>, valhalla-spec-experts
> <valhalla-spec-experts at openjdk.java.net>, valhalla-spec-experts
> <valhalla-spec-experts-bounces at openjdk.java.net>
> Date: 2020/06/19 01:32 PM
> Subject: [EXTERNAL] Re: Evolving the wrapper classes
>
>
> > On Jun 19, 2020, at 11:07 AM, Tobi Ajila <Tobi_Ajila at ca.ibm.com> wrote:
> >
> > I am still a little unclear as to what the motivation is for this.
> Is this solely for specialized generics?
> >
> > In Dan's examples with `I` and `java/lang/Integer$val`, the only
> places where conversions are needed are when primitives are used as
> type parameters or to call instance methods on them, both of which
> can already be done with primitive arrays. So in the LW3 - LW20
> timeframe would we have any need for these conversions? If so, could
> you provide some examples?
>
> I think it comes down to specialization and subtyping.
>
> Pre-specialization, here's one example that uses subtyping:
>
> int[] arr = { 1 };
> Object[] objs = arr; // just like Point[] <: Object[]
> Object obj = objs[0];
> Integer i = (Integer) obj;
>
> This would compile to something like:
>
> iconst_1
> newarray T_INT
> dup
> iconst_0
> iconst_1
> iastore
> astore_0
>
> aload_0
> astore_1
>
> aload_1
> iconst_0
> aaload
> astore_2
>
> aload_2
> checkcast java/lang/Integer
> astore_3
>
> Going in the other direction—allocating a [Qjava/lang/Integer; and
> then using iaload/iastore on it—may not be necessary unless/until
> the language supports "new T[]" in specialized code, but it
> tentatively makes sense to support now anyway, rather than having to
> come back and fix it up later.
>
> > In the case of specialized generics, is the intention that `[I`
> (and I suppose `I` as well) will appear in generic code?
>
> If you mean "can '[<T>' be specialized to '[I'?", the answer is no.
> The primitive types cannot act as type arguments.