public class DecimalFormat extends NumberFormat

DecimalFormat is a concrete subclass of NumberFormat that formats decimal numbers. It has a variety of features designed to make it possible to parse and format numbers in any locale, including support for Western, Arabic, and Indic digits. It also supports different kinds of numbers, including integers (123), fixed-point numbers (123.4), scientific notation (1.23E4), percentages (12%), and currency amounts ($123). All of these can be localized.

To obtain a NumberFormat for a specific locale, including the default locale, call one of NumberFormat's factory methods, such as getInstance(). In general, do not call the DecimalFormat constructors directly, since the NumberFormat factory methods may return subclasses other than DecimalFormat. If you need to customize the format object, do something like this:

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NumberFormat numFormat = NumberFormat.getInstance(loc);
if (numFormat instanceof DecimalFormat decFormat) {
    decFormat.setDecimalSeparatorAlwaysShown(true);
}

A DecimalFormat comprises a pattern and a set of symbols. The pattern may be set directly using applyPattern(), or indirectly using the API methods. The symbols are stored in a DecimalFormatSymbols object. When using the NumberFormat factory methods, the pattern and symbols are read from localized ResourceBundles.

Patterns

DecimalFormat patterns have the following syntax:

 Pattern:
         PositivePattern
         PositivePattern ; NegativePattern
 PositivePattern:
         Prefixopt Number Suffixopt
 NegativePattern:
         Prefixopt Number Suffixopt
 Prefix:
         any Unicode characters except U+FFFE, U+FFFF, and special characters
 Suffix:
         any Unicode characters except U+FFFE, U+FFFF, and special characters
 Number:
         Integer Exponentopt
         Integer . Fraction Exponentopt
 Integer:
         MinimumInteger
         #
         # Integer
         # , Integer
 MinimumInteger:
         0
         0 MinimumInteger
         0 , MinimumInteger
 Fraction:
         MinimumFractionopt OptionalFractionopt
 MinimumFraction:
         0 MinimumFractionopt
 OptionalFraction:
         # OptionalFractionopt
 Exponent:
         E MinimumExponent
 MinimumExponent:
         0 MinimumExponentopt
 

A DecimalFormat pattern contains a positive and negative subpattern, for example, "#,##0.00;(#,##0.00)". Each subpattern has a prefix, numeric part, and suffix. The negative subpattern is optional; if absent, then the positive subpattern prefixed with the minus sign ('-' U+002D HYPHEN-MINUS) is used as the negative subpattern. That is, "0.00" alone is equivalent to "0.00;-0.00". If there is an explicit negative subpattern, it serves only to specify the negative prefix and suffix; the number of digits, minimal digits, and other characteristics are all the same as the positive pattern. That means that "#,##0.0#;(#)" produces precisely the same behavior as "#,##0.0#;(#,##0.0#)".

The prefixes, suffixes, and various symbols used for infinity, digits, grouping separators, decimal separators, etc. may be set to arbitrary values, and they will appear properly during formatting. However, care must be taken that the symbols and strings do not conflict, or parsing will be unreliable. For example, either the positive and negative prefixes or the suffixes must be distinct for DecimalFormat.parse() to be able to distinguish positive from negative values. (If they are identical, then DecimalFormat will behave as if no negative subpattern was specified.) Another example is that the decimal separator and grouping separator should be distinct characters, or parsing will be impossible.

The grouping separator is commonly used for thousands, but in some countries it separates ten-thousands. The grouping size is a constant number of digits between the grouping characters, such as 3 for 100,000,000 or 4 for 1,0000,0000. If you supply a pattern with multiple grouping characters, the interval between the last one and the end of the integer is the one that is used. So "#,##,###,####" == "######,####" == "##,####,####".

Special Pattern Characters

Many characters in a pattern are taken literally; they are matched during parsing and output unchanged during formatting. Special characters, on the other hand, stand for other characters, strings, or classes of characters. They must be quoted, unless noted otherwise, if they are to appear in the prefix or suffix as literals.

The characters listed here are used in non-localized patterns. Localized patterns use the corresponding characters taken from this formatter's DecimalFormatSymbols object instead, and these characters lose their special status. Two exceptions are the currency sign and quote, which are not localized.

Chart showing symbol, location, localized, and meaning.

Symbol	Location	Localized?	Meaning
0	Number	Yes	Digit
#	Number	Yes	Digit, zero shows as absent
.	Number	Yes	Decimal separator or monetary decimal separator
-	Number	Yes	Minus sign
,	Number	Yes	Grouping separator or monetary grouping separator
E	Number	Yes	Separates mantissa and exponent in scientific notation. Need not be quoted in prefix or suffix.
;	Subpattern boundary	Yes	Separates positive and negative subpatterns
%	Prefix or suffix	Yes	Multiply by 100 and show as percentage
U+2030	Prefix or suffix	Yes	Multiply by 1000 and show as per mille value
¤ (U+00A4)	Prefix or suffix	No	Currency sign, replaced by currency symbol. If doubled, replaced by international currency symbol. If present in a pattern, the monetary decimal/grouping separators are used instead of the decimal/grouping separators.
'	Prefix or suffix	No	Used to quote special characters in a prefix or suffix, for example, "'#'#" formats 123 to "#123". To create a single quote itself, use two in a row: "# o''clock".

Scientific Notation

Numbers in scientific notation are expressed as the product of a mantissa and a power of ten, for example, 1234 can be expressed as 1.234 x 10^3. The mantissa is often in the range 1.0 ≤ x < 10.0, but it need not be. DecimalFormat can be instructed to format and parse scientific notation only via a pattern; there is currently no factory method that creates a scientific notation format. In a pattern, the exponent character immediately followed by one or more digit characters indicates scientific notation. Example: "0.###E0" formats the number 1234 as "1.234E3".

• The number of digit characters after the exponent character gives the minimum exponent digit count. There is no maximum. Negative exponents are formatted using the localized minus sign, not the prefix and suffix from the pattern. This allows patterns such as "0.###E0 m/s".
• The maximum integer digits is the sum of '0's and '#'s prior to the decimal point. The minimum integer digits is the sum of the '0's prior to the decimal point. The maximum fraction and minimum fraction digits follow the same rules, but apply to the digits after the decimal point but before the exponent. For example, the following pattern: "#00.0####E0" would have a minimum number of integer digits = 2("00") and a maximum number of integer digits = 3("#00"). It would have a minimum number of fraction digits = 1("0") and a maximum number of fraction digits= 5("0####").
• The minimum and maximum number of integer digits are interpreted together:
  • If the maximum number of integer digits is greater than their minimum number and greater than 1, it forces the exponent to be a multiple of the maximum number of integer digits, and the minimum number of integer digits to be interpreted as 1. The most common use of this is to generate engineering notation, in which the exponent is a multiple of three, e.g., "##0.#####E0". Using this pattern, the number 12345 formats to "12.345E3", and 123456 formats to "123.456E3".
  • Otherwise, the minimum number of integer digits is achieved by adjusting the exponent. Example: 0.00123 formatted with "00.###E0" yields "12.3E-4".
• For a given number, the amount of significant digits in the mantissa can be calculated as such

   Mantissa Digits:
           min(max(Minimum Pattern Digits, Original Number Digits), Maximum Pattern Digits)
   Minimum pattern Digits:
           Minimum Integer Digits + Minimum Fraction Digits
   Maximum pattern Digits:
           Maximum Integer Digits + Maximum Fraction Digits
   Original Number Digits:
           The amount of significant digits in the number to be formatted
   

  This means that generally, a mantissa will have up to the combined maximum integer and fraction digits, if the original number itself has enough significant digits. However, if there are more minimum pattern digits than significant digits in the original number, the mantissa will have significant digits that equals the combined minimum integer and fraction digits. The number of significant digits does not affect parsing.

  It should be noted, that the integer portion of the mantissa will give any excess digits to the fraction portion, whether it be for precision or for satisfying the total amount of combined minimum digits.

  This behavior can be observed in the following example,

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      DecimalFormat df = new DecimalFormat("#000.000##E0");
      df.format(12); // returns "12.0000E0"
      df.format(123456789) // returns "1.23456789E8"
  

• Exponential patterns may not contain grouping separators.

Rounding

DecimalFormat provides rounding modes defined in RoundingMode for formatting. By default, it uses RoundingMode.HALF_EVEN.

Digits

For formatting, DecimalFormat uses the ten consecutive characters starting with the localized zero digit defined in the DecimalFormatSymbols object as digits. For parsing, these digits as well as all Unicode decimal digits, as defined by Character.digit, are recognized.

Special Values

Not a Number(NaN) is formatted as a string, which typically has a single character U+FFFD. This string is determined by the DecimalFormatSymbols object. This is the only value for which the prefixes and suffixes are not used.

Infinity is formatted as a string, which typically has a single character U+221E, with the positive or negative prefixes and suffixes applied. The infinity string is determined by the DecimalFormatSymbols object.

Negative zero ("-0") parses to

• BigDecimal(0) if isParseBigDecimal() is true,
• Long(0) if isParseBigDecimal() is false and isParseIntegerOnly() is true,
• Double(-0.0) if both isParseBigDecimal() and isParseIntegerOnly() are false.

Synchronization

Decimal formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

Example

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// Print out a number using the localized number, integer, currency,
// and percent format for each locale
Locale[] locales = NumberFormat.getAvailableLocales();
double myNumber = -1234.56;
NumberFormat form;
for (int j = 0; j < 4; ++j) {
    System.out.println("FORMAT");
    for (Locale locale : locales) {
        if (locale.getCountry().length() == 0) {
            continue; // Skip language-only locales
        }
        System.out.print(locale.getDisplayName());
        form = switch (j) {
            case 0 -> NumberFormat.getInstance(locale);
            case 1 -> NumberFormat.getIntegerInstance(locale);
            case 2 -> NumberFormat.getCurrencyInstance(locale);
            default -> NumberFormat.getPercentInstance(locale);
        };
        if (form instanceof DecimalFormat decForm) {
            System.out.print(": " + decForm.toPattern());
        }
        System.out.print(" -> " + form.format(myNumber));
        try {
            System.out.println(" -> " + form.parse(form.format(myNumber)));
        } catch (ParseException e) {}
    }
}