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Re: About using assertion

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Tim McDaniel wrote:
> Patricia Shanahan wrote:
>> Negative zero is a strange beast. As far as I can tell, it exists
>> mainly to produce negative infinity when it is used as a divisor. For
>> example, it is equal to zero for comparison purposes.

> Hm, I thought it didn't, but the authoritative-looking but old
> <http://www.concentric.net/~ttwang/tech/javafloat.htm>  says otherwise.

Authoritative-looking is not authoritative, though it is correct.

Authoritative is the official documentation, which your source elucidates.

> However, Float/Double apparently acts differently from float/double:

Maybe because they *are* different?

For example, the mechanism of comparison differs.  Primitives don't use method 
calls, object types do.

>      Comparing two Java "Float" objects can have different semantics
>      than comparing two Java "float" values. Recall the "float" class
>      is a Java primitive class, while java.lang.Float is a subclass of
>      "Object".
>
>      A "NaN" value is not equal to itself. However, a "NaN" Java
>      "Float" object is equal to itself. The semantic is defined this
>      way, because otherwise "NaN" Java "Float" objects cannot be
>      retrieved from a hash table.
>
>      (new Float(0.0 / 0.0)).equals(new Float(0.0 / 0.0))  ->   true
>
>      For the class java.lang.Float, objects are ordered from lowest to
>      highest: -Infinity, negative numbers, -0.0, 0.0, positive numbers,
>      Infinity, NaN. "java.lang.Double" objects are identically ordered.
>
>      (new Float(0.0)).equals(new Float(-0.0))  ->   false

The primitives are ordered thus:
"Except for NaN, floating-point values are ordered; arranged from smallest to 
largest, they are negative infinity, negative finite nonzero values, positive 
and negative zero, positive finite nonzero values, and positive infinity."
JLS §4.2.3

It goes on to say that NaN can be represented by multiple different bit 
patterns that can be distinguished programmatically.

As with other primitives, 'double' is ordered via the ==, >, <, and related 
operators.

With the reference type 'Double' comparison is via the 'compareTo()' method, 
which of course is different from and independent of the primitive operators. 
  Since the purposes of a reference type differ from those of a primitive type 
in Java, for example, 'Comparable#compareTo()' should be consistent with 
'equals()' (as mentioned in the docs for 'Double'), it makes sense that the 
semantics might differ.  Unfortunate, perhaps, but sensible.

<http://download.oracle.com/javase/6/docs/api/java/lang/Double.html#compareTo(java.lang.Double)>

"Compares two Double objects numerically. There are two ways in which 
comparisons performed by this method differ from those performed by the Java 
language numerical comparison operators (<, <=, ==, >= >) when applied to 
primitive double values:

"- Double.NaN is considered by this method to be equal to itself and greater 
than all other double values (including Double.POSITIVE_INFINITY).

"- 0.0d is considered by this method to be greater than -0.0d.

"This ensures that the natural ordering of Double objects imposed by this 
method is consistent with equals."

This doesn't quite assure me that all forms of 'NaN' (by bit pattern) would 
box to 'Double.valueOf( Double.NaN )'.  I'll have to run an example program.

Anyway, it is an interesting point of comparison between the very different 
'double' and 'Double' types (and their narrower counterparts) and a good thing 
to watch out for in the mapping between them.  Thanks for bringing it up.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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Re: About using assertion tmcd@tmcd-p4-linux.austin.tx.us (Tim McDaniel) - 2011-05-19 00:32 -0500
  Re: About using assertion Michal Kleczek <kleku75@gmail.com> - 2011-05-19 08:34 +0200
    Re: About using assertion Lew <noone@lewscanon.com> - 2011-05-19 08:46 -0400
      Re: About using assertion Michal Kleczek <kleku75@gmail.com> - 2011-05-19 15:16 +0200
        Re: About using assertion Lew <noone@lewscanon.com> - 2011-05-19 09:38 -0400
          Re: About using assertion Robert Klemme <shortcutter@googlemail.com> - 2011-05-19 07:41 -0700
            Re: About using assertion Lew <noone@lewscanon.com> - 2011-05-19 11:00 -0400
  Re: About using assertion Patricia Shanahan <pats@acm.org> - 2011-05-19 05:52 -0700
    Re: About using assertion tmcd@tmcd-p4-linux.austin.tx.us (Tim McDaniel) - 2011-05-21 18:37 -0500
      Re: About using assertion Patricia Shanahan <pats@acm.org> - 2011-05-21 19:00 -0700
      Re: About using assertion Lew <noone@lewscanon.com> - 2011-05-22 01:19 -0400

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