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Trail: Learning the Java Language
Lesson: Object Basics and Simple Data Objects

Formatting Numbers with Custom Formats

You can use the DecimalFormat class to format decimal numbers into 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 (in a .java source file).

Constructing Patterns

You specify the formatting properties of DecimalFormat with a pattern String. The pattern determines what the formatted number looks like. 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.

Altering the Formatting Symbols

You can use the DecimalFormatSymbols (in the API reference documentation) 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);

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

String bizarre = weirdFormatter.format(12345.678);

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


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