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Using Predefined Formats

By invoking the methods provided by the NumberFormat class, you can format numbers, currencies, and percentages according to Locale. The material that follows demonstrates formatting techniques with a sample program called NumberFormatDemo.

Numbers

You can use the NumberFormat methods to format primitive-type numbers, such as double, and their corresponding wrapper objects, such as Double.

The following code example formats a Double according to Locale. Invoking the getNumberInstance method returns a locale-specific instance of NumberFormat. The format method accepts the Double as an argument and returns the formatted number in a String.

Double amount = new Double(345987.246);
NumberFormat numberFormatter;
String amountOut;

numberFormatter = NumberFormat.getNumberInstance(currentLocale);
amountOut = numberFormatter.format(amount);
System.out.println(amountOut + " " + 
                   currentLocale.toString());

The output from this example shows how the format of the same number varies with Locale:

345 987,246	 fr_FR
345.987,246	 de_DE
345,987.246	 en_US

Currencies

If you're writing business applications, you'll probably need to format and to display currencies. You format currencies in the same manner as numbers, except that you call getCurrencyInstance to create a formatter. When you invoke the format method, it returns a String that includes the formatted number and the appropriate currency sign.

This code example shows how to format currency in a locale-specific manner:

Double currency = new Double(9876543.21);
NumberFormat currencyFormatter;
String currencyOut;

currencyFormatter = NumberFormat.getCurrencyInstance(currentLocale);
currencyOut = currencyFormatter.format(currency);
System.out.println(currencyOut + " " + 			
                   currentLocale.toString());

The output generated by the preceding lines of code is as follows:

9 876 543,21 €	 fr_FR
9.876.543,21 €	 de_DE
$9,876,543.21	 en_US

At first glance this output may look wrong to you, because the numeric values are all the same. Of course, 9 876 543,21 € is not equivalent to $9.876.543,21. However, bear in mind that the NumberFormat class is unaware of exchange rates. The methods belonging to the NumberFormat class format currencies but do not convert them.

Version note: As of release JDK7, you can supersede the currency at runtime by creating a currency.properties file. See the Currency class specification for more information.

Percentages

You can also use the methods of the NumberFormat class to format percentages. To get the locale-specific formatter, invoke the getPercentInstance method. With this formatter, a decimal fraction such as 0.75 is displayed as 75%.

The following code sample shows how to format a percentage.

Double percent = new Double(0.75);
NumberFormat percentFormatter;
String percentOut;

percentFormatter = NumberFormat.getPercentInstance(currentLocale);
percentOut = percentFormatter.format(percent);

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