Win a copy of Mesos in Action this week in the Cloud/Virtualizaton forum!
  • Post Reply
  • Bookmark Topic Watch Topic
  • New Topic

converting float to double

 
Jon Wynett
Greenhorn
Posts: 10
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
How can I convert a float to a double without have the number "adjusted". For example, if I create a Float with value 18.4 and then either get the doubleValue() or create a new Double with that Float, it comes out as 18.399999618530273.

I've tried this with many different numbers and all get the same type of problem. Also, casting a float as a double gets the same problem.

Here's some sample code:
public static void testFloatToDouble() {
Float f = new Float(18.4);
Double d = new Double(f.floatValue());
System.out.println("Float: " + f);
System.out.println("Float as double: " + f.doubleValue());
System.out.println("Float as Double: " + d);
}
And the results:
Float: 18.4
Float as double: 18.399999618530273
Float as Double: 18.399999618530273

Jon
 
Joyce Lee
Ranch Hand
Posts: 1392
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
Hi Jon,

How about this?


Joyce
 
marc weber
Sheriff
Posts: 11343
Java Mac Safari
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
This is a consequence of the way floating-point numbers are stored in computers (per IEEE 754 standards). If the decimal portion cannot be expressed as a sum of reciprocal powers of 2 within a particular "window," then the number loses precision.

If you want precision, then you should use the BigDecimal class in java.math. But be sure to use the String constructor, or you'll get similar errors. Compare the String and double constructor descriptions in the API...

http://java.sun.com/j2se/1.4.2/docs/api/java/math/BigDecimal.html

For details on IEEE 754, follow the links here...

http://www.public.iastate.edu/~sarita/ieee754/homepage.html
[ October 04, 2004: Message edited by: marc weber ]
 
Ilja Preuss
author
Sheriff
Posts: 14112
  • Mark post as helpful
  • send pies
  • Quote
  • Report post to moderator
With other words, you can't really store the value 18.4 in a float, because in binary doesn't have a finite number of decimal places (similar to 0.1 ternary = 1*3^(-1) = 1/3 doesn't have a finite number of decimal places in the decimal system). What gets stored is a close approximation.
 
  • Post Reply
  • Bookmark Topic Watch Topic
  • New Topic