"Model 2" prototype status

Stephen Colebourne scolebourne at joda.org
Tue Sep 1 06:47:58 UTC 2015

Make sure you've watched the JVMLS videos which may answer some questions:

(they should be considered mandatory for participation in this list!)


On 1 September 2015 at 03:04, John Altidor <jgaltidor at gmail.com> wrote:
> Hi Brian,
> I am new to this mailing list.  My PhD dissertation covered subtyping with
> variance and Java wildcards extensively, so the questions you raised in
> this thread are very interesting to me.  I was wondering how you are
> handling the translational aspects of wildcards and specialized generic
> methods.
> Your earlier post asked how to represent List<any> in bytecode.  Since
> List<any> is a supertype of both List<int> and List<double>, for example,
> type List<any> should only support operations that can be applied to both
> List<int> and List<double>.  One such operation is counting the number of
> elements using method List.size().  Which byte representation would support
> being able to dispatch List.size() on both an instance of List<int> and an
> instance of List<double>?
> It seems such a byte representation would need to be independent of fields.
>   The number of bytes needed to represent an instance differs among
> primitive types (e.g. int and double).  As a result, it seems List<int> and
> List< double> may differ in the number of bytes needed for their fields.
> In that case, one could not know the number of bytes in the instance of
> List<any> returned from the following method:
> List<any> func(int input_num) {
>   if(input_num is odd)
>     return new List<int>();
>   else
>     return new List<double>();
> }
> In addition to type-independent methods such as List.size(), another
> operation that is type safe to allow on an instance of List<any> is
> wildcard capture.  Consider the generic method, swapFirstTwo, below that
> just swaps the order of the first two elements in the input list.  It is
> type safe to pass an instance of List<any> to this method (because no
> runtime type error would occur).
> <any T> void swapFirstTwo(List<T> list) {
>   T first = list.getAndRemoveFirst();
>   T second = list.getAndRemoveFirst();
>   list.addToBeginning(first);
>   list.addToBeginning(second);
> }
> Would two calls to method swapFirstTwo, one with a List<int> as input and
> the other method call with a List<double> as input, result in two
> specialized copies of method swapFirstTwo in byte code?  If that is the
> case, what is the byte representation of method swapFirstTwo when the input
> is an instance of List<any>?
> Thank you,
> John Altidor
> http://jgaltidor.github.io/

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