On 04/07/2012 02:35 AM, Stuart Marks wrote:
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. :-)
I fully agree.
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."
Erased casts are an implementation glitch, not the erasure by itself.
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.
There are several implementations that exist,
here is the publicly available papers.
M. Cimadamore and M. Viroli.
Reifying wildcards in Java using the EGO approach.
In SAC '07: Proceedings of the 2007 ACM symposium on
Applied computing, pages 1315{1322, New York, NY, USA,
2007. ACM.
M. Cimadamore and M. Viroli.
On Rei cation of Java Wildcards.
Science of Computer Programming, 2008
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.
I don't think so, the equivalent cast with a reified type is cheap.
s'marks
Rémi
