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From: Jake Jarvis <pig_in_shoes@yahoo.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: double == double
Date: Thu, 20 Oct 2016 01:08:18 +0200
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Am 19.10.2016 um 22:50 schrieb Jerry Stuckle:
> On 10/19/2016 10:36 AM, Jake Jarvis wrote:
>> Am 19.10.2016 um 16:26 schrieb Jerry Stuckle:
>>> On 10/19/2016 9:32 AM, Eric Sosman wrote:
>>>> On 10/19/2016 9:03 AM, Mike Amling wrote:
>>>>> On 10/12/16 7:15 AM, Stefan Ram wrote:
>>>>>>   Sometimes it is taught that it makes no sense to compare
>>>>>>   two double values with »==«.
>>>>>>
>>>>>>   But what about this function?
>>>>>>
>>>>>> public static double max( final double a, final double b)
>>>>>> { return
>>>>>>   a == 0.0 && b == 0.0 && java.lang.Double.doubleToRawLongBits( a )
>>>>>>   == Double.doubleToRawLongBits( -0.0d )? b : a >= b ? a : b; }
>>>>>>
>>>>>>   which tries to return positive zero if one argument is
>>>>>>   positive zero and the other one negative zero?
>>>>>>
>>>>>>   It seems that there are special cases when a comparison
>>>>>>   of double values with »==« is appropriate. I have observed
>>>>>>   that this is usually the case when comparing with »0.0« -
>>>>>>   another example would be avoiding to divide by 0.0.
>>>>>
>>>>> If a==0.0 and b==0.0, there's no need to test whether either is -0.0.
>>>>> The function could be simplified to
>>>>>
>>>>> public static double max(final double a, final double b) {
>>>>>   return a==0.0 && b==0.0 ? 0.0 : (a>=b ? a : b);
>>>>> }
>>>>
>>>>     This would return +0.0 for max(-0.0, -0.0).
>>>>
>>>
>>> PMJI, but how do you figure? a==0.0 && b==0.0 would be false
>>
>> No.
>>
>
> And why not?  -0.0 != 0.0.
>

But exactly that is not the case

https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.21.1

| 15.21.1. Numerical Equality Operators == and !=
| [...]
| Floating-point equality testing is performed in accordance with the 
rules of the IEEE 754 standard:
| [...]
| Positive zero and negative zero are considered equal.