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Another Day, Another JVM Bug

Posted February 23, 2005 – 3:00 pm by Yakov Shafranovich in Programming

I am not 100% sure if it is a bug, but the consensus within my company seems to indicate that it is. Here is a copy of the bug report filed with Sun:

synopsis: DecimalFormat with a Lot of “#” Returns Incorrect Results for Floats

FULL PRODUCT VERSION :

java version “1.5.0″

Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0-b64)

Java HotSpot(TM) Client VM (build 1.5.0-b64, mixed mode)

ADDITIONAL OS VERSION INFORMATION :

Redhat Fedora Core 2 / Linux 2.6.10-1.9_FC2 #1 i686 athlon i386 GNU/Linux

A DESCRIPTION OF THE PROBLEM :

When using DecimalFormat with a lot of “#”, putting too many “#” causes conversion problems with floats. For example, using “############0.0#######” on “0.3f” will produce “0.30000001″ instead of “0.3″. Converting the float to string first via “String test = “” + 0.3f” doesn’t have this problem. Doesn’t happen with doubles.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :

DecimalFormat df = new DecimalFormat(”############0.0#######”);

System.out.println(df.format(0.3f));

ACTUAL -

0.30000001

REPRODUCIBILITY :

This bug can be reproduced always.
---------- BEGIN SOURCE ----------
import java.text.DecimalFormat;
public class DecimalFormatTester {
public static void main(String[] args) {
	DecimalFormat df1 = new DecimalFormat("#0.0#");
	DecimalFormat df2 =
		new DecimalFormat("############0.#######");
	float test_value = 0.3f;
	System.out.println("Short: " + df1.format(test_value));
	System.out.println("Long: " + df2.format(test_value));
}
}
---------- END SOURCE ----------
CUSTOMER SUBMITTED WORKAROUND :

Use “#0.0#” instead.