DecimalFormat : Java Glossary

2006-02-18 by Roedy Green ©1996-2008 Canadian Mind Products
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DecimalFormat

java.text.DecimalFormat is the class for turning numbers into Strings, nicely decorated with decimal points and commas. It can also be used to parse Strings back in to binary numbers. It works with a String pattern to describe the format of a number. You can use it with doubles, floats, longs and ints. DecimalFormat automatically rounds output to the displayed number of decimal places.

Pattern Characters	Parsing
Forcing +	Learning More
Handling Negatives	Links
Post-Processing

Most Common DecimalFormat Pattern Characters

Most Common DecimalFormat Pattern Characters
Char	Meaning
0	digit or lead zeroes
#	digit or nothing at all. There is no pattern character to provide lead spaces for creating a fixed length pattern.
.	decimal point. Actual character displayed is locale-specific, though the character you specify for the pattern is always a dot. e.g. in Sweden this will display as a comma.
,	comma for grouping digits. Actual character displayed is locale-specific, though the character you specify for the pattern is always a docomma. e.g. in Sweden this will display as a period.
E	exponent separator.
'	used to surround decorative characters, including embedded spaces. e.g. "'$'###,##0.00"
;	Used to separate the pattern for positive numbers from the pattern for negative numbers.
%	display a ratio as a percentage. As in "##0.0%", multiplies the displayed value by 100 and displays a decorative trailing %.

Forcing +

Note there is no + pattern character to force an explicit positive sign. However you can get that effect using decorative + and - this way:

// Getting an explicit + - sign
// Formats 3.0 as "+3.0" and -3.0 as "-3.0".
// The surrounding ' are not needed in this case.
DecimalFormat explicitSign = new DecimalFormat( "'+'0.0;'-'0.0" );

Handling Negatives

Note that - is not used to arrange for a minus sign for negative numbers. That happens automatically.

You can say something like this to arrange for negative numbers to be shown in parentheses:

new DecimalFormat ( "#,##0.0#;(#,##0.0#)" );

Post-Processing

If DecimalFormat won’t do exactly what you want, don’t despair. Just add your own post-processing on the String it produces. Pad to fixed length with StringTools.leftPad or StringTools.rightPad or StringTools.toLZ. You can download the source for StringTools as part of the com.mindprod.common11 package.

Parsing

You can also use DecimalFormat to parse String to extract ints, longs and doubles you can compute with. Here is an example:

Learning More

When you study Sun’s documentation, don’t forget to look at NumberFormat, DecimalFormat’s abstract superclass. That’s where most of its useful methods are. DecimalFormat documents the magic characters you can use in pattern strings.

Sun’s Javadoc on the DecimalFormat class : available:

on the web at java.Sun.com
in the JDK 1.6.0_06 or in the older JDK 1.5.0_15 or the even older JDK 1.4.2_17 on your local J: drive.

Sun’s Javadoc on the NumberFormat class : available:

on the web at java.Sun.com
in the JDK 1.6.0_06 or in the older JDK 1.5.0_15 or the even older JDK 1.4.2_17 on your local J: drive.

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conversion table for Java 1.4-
conversion table for Java 1.5+
Converter Amanuensis
NumberFormat
printf

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