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Re: Using Enumerated Types as Array Indexes

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From	Robert Klemme <shortcutter@googlemail.com>
Newsgroups	comp.lang.java.programmer
Subject	Re: Using Enumerated Types as Array Indexes
Date	Wed, 17 Aug 2011 19:02:07 +0200
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On 17.08.2011 16:17, Lew wrote:
> Andreas Leitgeb wrote:
>> Arved Sandstrom<asandstr...@eastlink.ca>  wrote:
>>> Lew wrote:
>>>> JLS 10: [...] IOW, you never index arrays with a byte, char or
>>>> short. [...]
>>> I'm feeling slow this morning: how do we get problems with
>>> byte->int, short->int, or char->int again?
>
> One gets problems with byte->int and short->int with array indexes
> the way one gets into trouble with those widening conversions
> generally.  The usual suspect is the lack of unsigned versions, so
> widening a (byte)0xA0, for example, would result in a negative
> index.
>
> One gets into trouble generally in programming when one thinks one
> thing is going on ("index takes a byte") whilst ignoring what's
> really going on (there's a widening conversion involved).  You might
> get away with it most of the time, but occasionally such things trip
> you up.

Why had a nice issue recently which exactly fits this bill: after over 
two months of uninterrupted, completely error free production usage the 
application suddenly stopped working giving weird error messages (there 
was an issue with the error reporting as well, but let's ignore that for 
the moment).  Turns out this was the setup:

1. There was an AtomicInteger initialized at startup with 0 which for 
every request coming into that system was incremented in a thread safe 
manner (incrementAndGet()).

2. The result was used to index into an array with the quite obvious
"arr[n % arr.length]".

3. (left as exercise for the reader)

Anybody who now thinks all is fine should stop coding Java immediately 
and go reading the language spec.

> Why think about it imprecisely?  Tell us the advantage of that,
> please.

Some may reason: Someone else might have to pay for the laziness.  But 
then again, the risk is it might come right back haunting the 
originator.  Well, no risk, no fun. :-)

Cheers

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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Using Enumerated Types as Array Indexes KevinSimonson <kvnsmnsn@hotmail.com> - 2011-08-16 07:53 -0700
  Re: Using Enumerated Types as Array Indexes Patricia Shanahan <pats@acm.org> - 2011-08-16 09:30 -0700
  Re: Using Enumerated Types as Array Indexes Tom Anderson <twic@urchin.earth.li> - 2011-08-16 21:37 +0100
  Re: Using Enumerated Types as Array Indexes Gene Wirchenko <genew@ocis.net> - 2011-08-16 14:54 -0700
    Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-16 20:47 -0400
  Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-16 20:38 -0400
    Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-16 17:52 -0700
      Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-16 21:56 -0400
        Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-16 20:20 -0700
          Re: Using Enumerated Types as Array Indexes Lew <lewbloch@gmail.com> - 2011-08-16 20:45 -0700
            Re: Using Enumerated Types as Array Indexes Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-08-17 06:31 -0300
              Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-17 13:28 +0000
                Re: Using Enumerated Types as Array Indexes Lew <lewbloch@gmail.com> - 2011-08-17 07:17 -0700
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-17 19:02 +0200
                Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-17 16:09 -0700
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-17 23:48 +0000
                Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-17 17:34 -0700
                Re: Using Enumerated Types as Array Indexes Patricia Shanahan <pats@acm.org> - 2011-08-17 18:07 -0700
                Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-18 08:22 -0700
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 18:37 +0000
                Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-18 11:50 -0700
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 20:49 +0000
                Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-18 14:48 -0700
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-18 22:50 +0000
                Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-17 21:49 -0400
                Re: Using Enumerated Types as Array Indexes Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-17 21:50 -0400
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-19 14:13 +0200
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-19 20:05 +0000
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-20 00:20 +0200
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-19 23:11 +0000
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-20 11:21 +0200
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-20 11:45 +0000
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-20 15:23 +0200
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-20 16:15 +0000
                Re: Using Enumerated Types as Array Indexes Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-17 21:34 -0400
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-18 07:58 +0200
                Re: Using Enumerated Types as Array Indexes Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-08-18 06:29 -0300
                Re: Using Enumerated Types as Array Indexes RedGrittyBrick <RedGrittyBrick@spamweary.invalid> - 2011-08-18 10:47 +0100
                Re: Using Enumerated Types as Array Indexes Lew <lewbloch@gmail.com> - 2011-08-18 11:35 -0700
                Re: Using Enumerated Types as Array Indexes Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-08-18 18:30 -0300
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-19 14:57 +0200
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-19 20:10 +0000
                Re: Using Enumerated Types as Array Indexes Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-08-19 19:12 -0300
                Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-20 00:26 +0200
                Re: Using Enumerated Types as Array Indexes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-08-19 22:59 +0000
          Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-17 21:34 -0400
            Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-17 21:46 -0400
              Re: Using Enumerated Types as Array Indexes markspace <-@.> - 2011-08-17 19:51 -0700
    Re: Using Enumerated Types as Array Indexes David Lamb <dalamb@cs.queensu.ca> - 2011-08-17 08:29 -0400
      Re: Using Enumerated Types as Array Indexes Robert Klemme <shortcutter@googlemail.com> - 2011-08-17 16:04 +0200
      Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-17 21:51 -0400
  Re: Using Enumerated Types as Array Indexes Niklas Holsti <niklas.holsti@tidorum.invalid> - 2011-08-17 11:24 +0300
    Re: Using Enumerated Types as Array Indexes Arne Vajhøj <arne@vajhoej.dk> - 2011-08-17 21:55 -0400
      Re: Using Enumerated Types as Array Indexes Niklas Holsti <niklas.holsti@tidorum.invalid> - 2011-08-18 10:59 +0300
  Re: Using Enumerated Types as Array Indexes Roedy Green <see_website@mindprod.com.invalid> - 2011-08-17 22:09 -0700

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