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# Double.Nan question.plz help

bnkiran kumar
Ranch Hand
Posts: 176
The following code will print

1: Double a = new Double(Double.NaN);
2: Double b = new Double(Double.NaN);
3:
4: if( Double.NaN == Double.NaN )
5: System.out.println("True");
6: else
7: System.out.println("False");
8:
9: if( a.equals(b) )
10: System.out.println("True");
11: else
12: System.out.println("False");

A) True
True

B) True
False

C) False
True

D) False
False
c is the answer, plz anyone answer.

Edisandro Bessa
Ranch Hand
Posts: 584
Hi All,

I am getting prepared to write SCJP5 exam. Should I worry about Nan use for the exam or it's only for SCJP 1.4 ?

mambe nanje
Ranch Hand
Posts: 31
well Double.NAN is not actually defined any where as a precise value, so any test about it will never be true

Tilo Hemp
Ranch Hand
Posts: 91
interestingly,
Double.NaN != Double.NaN
gives true!

marc weber
Sheriff
Posts: 11343
Here's what I had in my own notes regarding NaN...

NaN is non-ordinal, so relational comparisons involving NaN always result in false. The only exception is NaN != NaN, which returns true. Wrapper methods Float.isNaN(float) and Double.isNaN(double) can be used to test for NaN. Alternatively, a float or double is NaN if it is not equal to itself (e.g., x != x).

Furthermore...
• Math.round(Float.NaN) results in an int zero, and Math.round(Double.NaN) results in a long zero.
• String literals "NaN" and "Infinity" are acceptable for Float and Double constructors.
• float or double division by zero results in +/- Infinity; except dividing zero by zero, which results in NaN.
•
Tilo Hemp
Ranch Hand
Posts: 91
Hi Marc,

thanks a lot for the interesting information!

I've got one remark and one question:

NaN is non-ordinal, so relational comparisons involving NaN always result in false. The only exception is NaN != NaN, which returns true.

--> the exception is not limited to NaN != NaN, but can also applied to say Double.NaN != 12.34, which also gives true. I guess != simply inverts the output of ==.

And my question is: what does "Cuius rei demonstrationem mirabilem sane detexi hanc marginis exiguitas non caperet." mean? I tried to find an automated latin-translator, but this did not work out

Regards

marc weber
Sheriff
Posts: 11343
Originally posted by Tilo Hemp:
...--> the exception is not limited to NaN != NaN, but can also applied to say Double.NaN != 12.34, which also gives true. I guess != simply inverts the output of ==...

You're correct! I guess it would be more accurate to say something like, "the exception is in applying the != comparison to NaN, which returns true -- even when comparing two NaNs."

As for the quote, see Fermat's Last Theorem (and note the enigmatic quality that made Fermat's words so intriguing in the first place ).

Bert Bates
author
Sheriff
Posts: 8900
5
And to answer an earlier question, this topic is on the 1.4 exam, but NOT the 1.5 exam.

Tilo Hemp
Ranch Hand
Posts: 91
thanks a lot marc, this is quite funny!
especially, putting this quotation in a small margin under short and to-the-point statements

bnkiran kumar
Ranch Hand
Posts: 176
marc weber ,thank you for your valuable material.