Code Review Request: 7157893: Warnings Cleanup in java.util.*

[Stuart Marks]

Hi Kurchi, I think we've converged on the code changes. Please prepare and post 
another webrev for a final cross-check before pushing.

What follows is I think merely residual disagreement over the philosophy of how 
to handle generic casts vs reification. :-)

On 4/6/12 3:06 AM, Rémi Forax wrote:
> On 04/05/2012 11:04 PM, Stuart Marks wrote:
>> I'm somewhat skeptical of making code changes now based on potential future
>> benefits when/if generics become reified. This was discussed before; see
>>
>> http://mail.openjdk.java.net/pipermail/jdk8-dev/2011-December/000454.html
>>
>> In that message, John Rose said "If the best practices have to change, then
>> we'll have to change that code again. Or maybe the retrofit strategy will
>> have to take account of the existing code idioms. In any case, we'll cross
>> that bridge when we get to it. (Coping with reification in this case is a
>> decision to make tomorrow, not today.)"
>
> I disagree with John. The main issue with generics nowadays is that
> most of the people doesn't care about a cast to a type variable because
> everybody knows about erasure. So codes are written with an implementation
> glitch in mind.
> Frankly, I don't know if reification will appear (yes it's a kind of magical)
> or not
> but I think it's a sloppy path to not consider all casts as equals.

In order to program effectively with generics, I think you have to understand 
erasure and its implications. It may have been an unfortunate choice, but 
erasure is part of the language and we have to deal with it and in some cases 
rely on it. I don't think it's merely an "implementation glitch."

The difficulty I have with reification is that while there are proposals 
floating around for how it could be done, nobody really knows how it will 
eventually turn out, nor whether it will actually be done. If it is eventually 
done, there will legal and illegal constructs, constructs that generate 
warnings, and perhaps a style guide for how to use reified generics properly.

Right now, we can *imagine* what these future rules might be, but it seems 
untenable to me to try to make today's code conform to those imaginary future 
rules, especially in the absence of tools to help support those rules.

> If unmaskNull return a V, the code of equals will upcast the value from Object
> to V
> to just after downcast it from V to Object,
> I think it's better that unmask to return Object and upcast it to V when it's
> necessary.

Certainly there are cases where there's a redundant downcast and upcast. In a 
reified world, will this be a significant expense? Really, I have no idea.

s'marks