Trail: Internationalization
Lesson: Formatting
Section: Numbers and Currencies

# Customizing Formats

You can use the `DecimalFormat` class to format decimal numbers into locale-specific strings. This class allows you to control the display of leading and trailing zeros, prefixes and suffixes, grouping (thousands) separators, and the decimal separator. If you want to change formatting symbols, such as the decimal separator, you can use the `DecimalFormatSymbols` in conjunction with the `DecimalFormat` class. These classes offer a great deal of flexibility in the formatting of numbers, but they can make your code more complex.

The text that follows uses examples that demonstrate the `DecimalFormat` and `DecimalFormatSymbols` classes. The code examples in this material are from a sample program called `DecimalFormatDemo`.

## Constructing Patterns

You specify the formatting properties of `DecimalFormat` with a pattern `String`. The pattern determines what the formatted number looks like. For a full description of the pattern syntax, see Number Format Pattern Syntax.

The example that follows creates a formatter by passing a pattern `String` to the `DecimalFormat` constructor. The `format` method accepts a `double` value as an argument and returns the formatted number in a `String`:

```DecimalFormat myFormatter = new DecimalFormat(pattern);
String output = myFormatter.format(value);
System.out.println(value + " " + pattern + " " + output);
```

The output for the preceding lines of code is described in the following table. The `value` is the number, a `double` , that is to be formatted. The `pattern` is the `String` that specifies the formatting properties. The `output`, which is a `String`, represents the formatted number.

Output from `DecimalFormatDemo` Program
`value` `pattern` `output` Explanation
123456.789 ###,###.### 123,456.789 The pound sign (#) denotes a digit, the comma is a placeholder for the grouping separator, and the period is a placeholder for the decimal separator.
123456.789 ###.## 123456.79 The `value` has three digits to the right of the decimal point, but the `pattern` has only two. The `format` method handles this by rounding up.
123.78 000000.000 000123.780 The `pattern` specifies leading and trailing zeros, because the 0 character is used instead of the pound sign (#).
12345.67 \$###,###.### \$12,345.67 The first character in the `pattern` is the dollar sign (\$). Note that it immediately precedes the leftmost digit in the formatted `output`.
12345.67 \u00A5###,###.### ¥12,345.67 The `pattern` specifies the currency sign for Japanese yen (¥) with the Unicode value 00A5.

## Locale-Sensitive Formatting

The preceding example created a `DecimalFormat` object for the default `Locale`. If you want a `DecimalFormat` object for a nondefault `Locale`, you instantiate a `NumberFormat` and then cast it to `DecimalFormat`. Here's an example:

```NumberFormat nf = NumberFormat.getNumberInstance(loc);
DecimalFormat df = (DecimalFormat)nf;
df.applyPattern(pattern);
String output = df.format(value);
System.out.println(pattern + " " + output + " " + loc.toString());
```

Running the previous code example results in the output that follows. The formatted number, which is in the second column, varies with `Locale`:

```###,###.###      123,456.789     en_US
###,###.###      123.456,789     de_DE
###,###.###      123 456,789     fr_FR
```

So far the formatting patterns discussed here follow the conventions of U.S. English. For example, in the pattern ###,###.## the comma is the thousands-separator and the period represents the decimal point. This convention is fine, provided that your end users aren't exposed to it. However, some applications, such as spreadsheets and report generators, allow the end users to define their own formatting patterns. For these applications the formatting patterns specified by the end users should use localized notation. In these cases you'll want to invoke the `applyLocalizedPattern` method on the `DecimalFormat` object.

## Altering the Formatting Symbols

You can use the DecimalFormatSymbols class to change the symbols that appear in the formatted numbers produced by the `format` method. These symbols include the decimal separator, the grouping separator, the minus sign, and the percent sign, among others.

The next example demonstrates the `DecimalFormatSymbols` class by applying a strange format to a number. The unusual format is the result of the calls to the `setDecimalSeparator`, `setGroupingSeparator`, and `setGroupingSize` methods.

```DecimalFormatSymbols unusualSymbols = new DecimalFormatSymbols(currentLocale);
unusualSymbols.setDecimalSeparator('|');
unusualSymbols.setGroupingSeparator('^');

String strange = "#,##0.###";
DecimalFormat weirdFormatter = new DecimalFormat(strange, unusualSymbols);
weirdFormatter.setGroupingSize(4);

String bizarre = weirdFormatter.format(12345.678);
System.out.println(bizarre);
```

When run, this example prints the number in a bizarre format:

```1^2345|678
```

## Number Format Pattern Syntax

You can design your own format patterns for numbers by following the rules specified by the following BNF diagram:

```pattern    := subpattern{;subpattern}
subpattern := {prefix}integer{.fraction}{suffix}
prefix     := '\\u0000'..'\\uFFFD' - specialCharacters
suffix     := '\\u0000'..'\\uFFFD' - specialCharacters
integer    := '#'* '0'* '0'
fraction   := '0'* '#'*
```

The notation used in the preceding diagram is explained in the following table:

Notation Description
`X*` 0 or more instances of X
`(X | Y)` either X or Y
`X..Y` any character from X up to Y, inclusive
`S - T` characters in S, except those in T
`{X}` X is optional

In the preceding BNF diagram, the first subpattern specifies the format for positive numbers. The second subpattern, which is optional, specifies the format for negative numbers.

Although not noted in the BNF diagram, a comma may appear within the integer portion.

Within the subpatterns, you specify formatting with special symbols. These symbols are described in the following table:

Symbol Description
0 a digit
# a digit, zero shows as absent
. placeholder for decimal separator
, placeholder for grouping separator
E separates mantissa and exponent for exponential formats
; separates formats
- default negative prefix
% multiply by 100 and show as percentage
? multiply by 1000 and show as per mille
¤ currency sign; replaced by currency symbol; if doubled, replaced by international currency symbol; if present in a pattern, the monetary decimal separator is used instead of the decimal separator
X any other characters can be used in the prefix or suffix
' used to quote special characters in a prefix or suffix

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