Is V? what we need ?

Brian Goetz brian.goetz at
Wed May 1 14:39:59 UTC 2019

This is the point I’ve been failing to get across to Remi as well.  Suppose you have a big value, and you want a sparse ArrayList of that value?  You need a way to say “Arraylist of BigValue, but not flattened.”  And that’s a use-site concern.  ArrayList<BigValue?> gives you that.  

Which is to say: the claim that V? is useless once we have reified generics is simply untrue.  

> On May 1, 2019, at 10:32 AM, Doug Lea <dl at> wrote:
> On 5/1/19 10:12 AM, Brian Goetz wrote:
>> You’re assuming the utility of V? after reification is zero, but
>> that’s not the case. For example, suppose ArrayList is reified.  Then
>> the user gets the choice of erasure or reification by selecting
>> ArrayList<V?> or ArrayList<V>. Erasure has a bad rap, but sometimes
>> it is desirable.
> As I tried but failed to get across in post on sorting, choosing V vs V?
> can lead to 5X cost differences in *either direction* for common
> aggregate (collection and array) operations. So programmers will need to
> learn some of these consequences. (Which is made harder/worse here
> because the cost tradeoffs will probably change over time as VM support
> improves.)
> Which is mainly an argument for Brian's position, except for those who
> believe that Java programmers should not have to encounter these kinds
> of issues that C++, C#, OCAML, etc programmers face all the time;
> betting that somehow VMs will make the right choice.
> (Maybe this is not the right time to mention this, but most other
> languages supporting this range of use have a middle-ground of
> call-by-ref for extracting small parts of large things, as seen in
> comparators, hash-codes, etc that can make a big performance difference.
> I expect people will complain about not supporting this.)
> -Doug

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