Java Tips - Formatting Numbers

The java.text package includes a Locale class, and, furthermore, the Java runtime automatically sets a default Locale object based on the user's environment; e.g., on the Macintosh and Windows, the user's preferences; on Unix, the user's environment variables. To provide formatters customized for numbers, currencies, and percentages, the NumberFormat class has static factory methods that normally return a DecimalFormat with the correct pattern already instantiated. A DecimalFormat object appropriate to the user's locale can be obtained from the factory method NumberFormat.getInstance( ) and manipulated using set methods. Surprisingly, the method setMinimumIntegerDigits( ) turns out to be the easy way to generate a number format with leading zeros. Here is an example:

import java.text.*;

import java.util.*;



/*

 * Format a number our way and the default way.

 */

public class NumFormat2 {

    /** A number to format */

    public static final double data[] = {

        0, 1, 22d/7, 100.2345678

    };



    /** The main (and only) method in this class. */

    public static void main(String av[]) { 

        // Get a format instance

        NumberFormat form = NumberFormat.getInstance( );



        // Set it to look like 999.99[99]

        form.setMinimumIntegerDigits(3);

        form.setMinimumFractionDigits(2);

        form.setMaximumFractionDigits(4);



        // Now print using it.

        for (int i=0; i<data.length; i++)

            System.out.println(data[i] + "\tformats as " +

                form.format(data[i]));

    }

}

This prints the contents of the array using the NumberFormat instance form:

$ java NumFormat2

0.0     formats as 000.00

1.0     formats as 001.00

3.142857142857143       formats as 003.1429

100.2345678     formats as 100.2346

$

You can also construct a DecimalFormat with a particular pattern or change the pattern dynamically using applyPattern( ). Some of the more common pattern characters are shown in the following.

# = Numeric digit (leading zeros suppressed)
0 = Numeric digit (leading zeros provided)
. = Locale-specific decimal separator (decimal point)
, = Locale-specific grouping separator (comma in English)
- = Locale-specific negative indicator (minus sign)
% = Shows the value as a percentage
; = Separates two formats: the first for positive and the second for negative values
, = Escapes one of the above characters so it appears

The NumFormatTest program uses one DecimalFormat to print a number with only two decimal places and a second to format the number according to the default locale:

// NumFormatTest.java 

/** A number to format */ 

public static final double intlNumber = 1024.25; 

/** Another number to format */ 

public static final double ourNumber = 100.2345678; 

NumberFormat defForm = NumberFormat.getInstance( ); 

NumberFormat ourForm = new DecimalFormat("##0.##"); 

// toPattern( ) shows the combination of #0., etc 

// that this particular local uses to format with 

System.out.println("defForm's pattern is " + 

    ((DecimalFormat)defForm).toPattern( )); 

System.out.println(intlNumber + " formats as " + 

    defForm.format(intlNumber)); 

System.out.println(ourNumber + " formats as " + 

    ourForm.format(ourNumber)); 

System.out.println(ourNumber + " formats as " + 

    defForm.format(ourNumber) + " using the default format");

This program prints the given pattern and then formats the same number using several formats:

defForm's pattern is #,##0.###

1024.25 formats as 1,024.25

100.2345678 formats as 100.23

100.2345678 formats as 100.235 using the default format