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From: Michal Kleczek <kleku75@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: About using assertion
Date: Thu, 19 May 2011 08:34:58 +0200
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Tim McDaniel wrote:

> In article <92r0e9F6lvU1@mid.individual.net>,
> Robert Klemme  <shortcutter@googlemail.com> wrote:
>>On 09.05.2011 18:11, Lew wrote:
>>> public class Foo
>>> {
>>> public double sqrt( double arg )
>>> {
>>> if ( arg < 0.0 )
>>> {
>>> throw new IllegalArgumentException(
>>> "sqrt(): negative argument "+ arg );
>>> }
>>> assert arg >= 0.0; // precondition
>>>
>>> double result;
>>> // rest of algorithm ...
>>> return result;
>>> }
>>> }
>>
>>Lew, I am not sure what you tried to convey with this posting.  I for my
>>part would say that the assert is a tad too much here since the if
>>clause before that gives me enough "confidence" that arg is actually >=
>>0 at that line.  If it isn't then I have bigger problems than
>>calculating square roots of negative numbers. :-)
> 
> Consider
> 
>     Foo.sqrt(Double.NaN)
> 
> The "if" fails, but if assertions are enabled, the "assert" catches
> it.  This might be an illustration that an assertion can be useful
> even if you have "confidence" that it could never fire.
> 
> (As a tangent, I'm not sure whether "Foo.sqrt(-0.0)" should throw an
> exception or not.)
> 

What a great example of why assertions are usefull - even if they look 
redundant and not needed.
Thank You.

-- 
Michal