Win a copy of Mesos in Action this week in the Cloud/Virtualizaton forum!

# converting float to double

Jon Wynett
Greenhorn
Posts: 10
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
Hi Jon,

Joyce

marc weber
Sheriff
Posts: 11343
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