Formatting Numbers with the DecimalFormat class in Java

As you may have read in the article on Locale Number Formats in Java, there are various standard packages that provide you classes and methods for number formatting.

Another class that it's quite useful for this purpose, it's the DecimalFormat class from the java.text package:

 import java.text.DecimalFormat;

The constructor of this class takes as parameter a string which represents a regex (regular expression) that will be used for formatting the number. The special symbols which can be used are:

• 0 - a digit will be added to the string. If a digit does not exist in that place, 0 will be added
• # - if a digit exists in that place, it will be added to the string
• . - signifies the decimal separator
• , - signifies the group separator
• E - the string will format the number according to the e notation (scientific notation
• % - if placed at the end of the regex, the result will be multiplied with 100 and '%' character will be appended to the end of the string
• /uxxxx - the character with the Unicode xxxx will be appended to the formatted string
• 'x' - used for special characters 

Examples:

        double nr1 = 3892.2344, nr2 = 24.832;
        DecimalFormat format1 = new DecimalFormat("00.00");
        System.out.println(format1.format(nr1) + " " + format1.format(nr2));
        //Output: 3892.23 24.83
        DecimalFormat format2 = new DecimalFormat("#000.00##");
        System.out.println(format2.format(nr2) + " " + format2.format(nr2));
        //Output: 024.832 024.832
        DecimalFormat format3 = new DecimalFormat("$0.00");
        System.out.println(format3.format(nr1) + " " + format3.format(nr2));
        //Output: $3892.23 $24.83
        DecimalFormat format4 = new DecimalFormat("00.E0");
        System.out.println(format4.format(nr1) + " " + format4.format(nr2));
        //Output: 39.E2 25.E0
        DecimalFormat format5 = new DecimalFormat("00.00%");
        System.out.println(format5.format(nr1) + " " + format5.format(nr2));
        //Output: 389223.44% 2483.20%
        DecimalFormat format6 = new DecimalFormat("0,0");
        System.out.println(format6.format(nr1) + " " + format6.format(nr2));
        //Output: 3,8,9,2 2,5
        DecimalFormat format7 = new DecimalFormat("\u0045 00.00");
        System.out.println(format7.format(nr1) + " " + format7.format(nr2));
        //Output: E 3892.23 E 24.83
        DecimalFormat format8 = new DecimalFormat("'E' 00.00");
        System.out.println(format8.format(nr1) + " " + format8.format(nr2));
        //Output: E 3892.23 E 24.83

One must note that unlike the NumberFormat, the DecimalFormat will not truncate the integer part and will output a number as is, even if the pattern specifies fewer digits.

Also, you can add any character you want to the format (like I added the dollar sign for format3), as long is not one of the special characters listed above. If you really need to add a character that defined as special, you can specify its Unicode(see format7) or you can put it in inside single quotes ' ' (see format8).

Also one must note that DecimalFormat is a subclass of NumberFormat, so the Locale-related methods are still available.

If your pattern does not respect the defined rules (let's say is like "E.00.00,##" - which is wrong for so many reasons), an IllegalArgumentException will be thrown.