8.4 Handling Local Customs

The second problem of internationalization is the task of following local customs and conventions in areas such as date and time formatting. The java.text package defines classes to help with this duty.

The NumberFormat class formats numbers, monetary amounts, and percentages in a locale-dependent way for display to the user. This is necessary because different locales have different conventions for number formatting. For example, in France, a comma is used as a decimal separator instead of a period, as in many English-speaking countries. A NumberFormat object can use the default locale or any locale you specify. NumberFormat has factory methods for obtaining instances that are suitable for different purposes, such as displaying monetary quantities or percentages. In Java 1.4 and later, the java.util.Currency class can be used with NumberFormat object so that it can correctly print an appropriate currency symbol.

The DateFormat class formats dates and times in a locale-dependent way for display to the user. Different countries have different conventions. Should the month or day be displayed first? Should periods or colons separate fields of the time? What are the names of the months in the language of the locale? A DateFormat object can simply use the default locale, or it can use any locale you specify. The DateFormat class is used in conjunction with the TimeZone and Calendar classes of java.util. The TimeZone object tells the DateFormat what time zone the date should be interpreted in, while the Calendar object specifies how the date itself should be broken down into days, weeks, months, and years. Almost all locales use the standard GregorianCalendar. SimpleDateFormat is a useful subclass of DateFormat: it allows dates to be formatted to or parsed from a date format specified with a simple template string.

The Collator class compares strings in a locale-dependent way. This is necessary because different languages alphabetize strings in different ways (and some languages don't even use alphabets). In traditional Spanish, for example, the letters "ch" are treated as a single character that comes between "c" and "d" for the purposes of sorting. When you need to sort strings or search for a string within Unicode text, you should use a Collator object, either one created to work with the default locale or one created for a specified locale.

The BreakIterator class allows you to locate character, word, line, and sentence boundaries in a locale-dependent way. This is useful when you need to recognize such boundaries in Unicode text, such as when you are implementing a word-wrapping algorithm.

Example 8-3 shows a class that uses the NumberFormat and DateFormat classes to display a hypothetical stock portfolio to the user following local conventions. The program uses various NumberFormat and DateFormat objects to format (using the format( ) method) different types of numbers and dates. These Format objects all operate using the default locale but could have been created with an explicitly specified locale. The program displays information about a hypothetical stock portfolio, formatting dates and numbers and monetary values according to the current or the specified locale. Figure 8-2 shows example output in different locales. The output was produced by running the program in the default locale, with the arguments "en GB" and "ja JP".

Figure 8-2. Stock portfolios formatted for U.S., British, and French locales

Example 8-3. Portfolio.java

package je3.i18n; import java.text.*; import java.util.*; import java.io.*; /**  * A partial implementation of a hypothetical stock portfolio class.  * We use it only to demonstrate number and date internationalization.  **/ public class Portfolio {     EquityPosition[  ] positions;        // The positions in the portfolio     Date lastQuoteTime = new Date( );   // Time for current quotes     // Create a Portfolio     public Portfolio(EquityPosition[  ] positions, Date lastQuoteTime) {         this.positions = positions;         this.lastQuoteTime = lastQuoteTime;     }          // A helper class: represents a single stock purchase     static class EquityPosition {         String name;             // Name of the stock.         int shares;              // Number of shares held.         Date purchased;          // When purchased.         Currency currency;       // What currency are the prices expressed in?         double bought;           // Purchase price per share         double current;          // Current price per share         // Format objects like this one are useful for parsing strings as well         // as formatting them.  This is for converting date strings to Dates.         static DateFormat dateParser = new SimpleDateFormat("yyyy-MM-dd");         EquityPosition(String n, int s, String date, Currency c,                        double then, double now) throws ParseException         {             // Convert the purchased date string to a Date object.             // The string must be in the format yyyy-mm-dd             purchased = dateParser.parse(date);             // And store the rest of the fields, too.             name = n; shares = s; currency = c;             bought = then; current = now;         }     }     // Return a localized HTML-formatted string describing the portfolio     public String toString( ) {         StringBuffer b = new StringBuffer( );         // Obtain NumberFormat and DateFormat objects to format our data.         NumberFormat number = NumberFormat.getInstance( );         NumberFormat price = NumberFormat.getCurrencyInstance( );         NumberFormat percent = NumberFormat.getPercentInstance( );         DateFormat shortdate = DateFormat.getDateInstance(DateFormat.MEDIUM);         DateFormat fulldate = DateFormat.getDateTimeInstance(DateFormat.LONG,                                                              DateFormat.LONG);         // Print some introductory data.         b.append("<html><body>");         b.append("<i>Portfolio value at ").             append(fulldate.format(lastQuoteTime)).append("</i>");         b.append("<table border=1>");         b.append("<tr><th>Symbol<th>Shares<th>Purchased<th>At<th>" +                  "Quote<th>Change</tr>");                  // Display the table using the format( ) methods of the Format objects.         for(int i = 0; i < positions.length; i++) {             b.append("<tr><td>");             b.append(positions[i].name).append("<td>");             b.append(number.format(positions[i].shares)).append("<td>");             b.append(shortdate.format(positions[i].purchased)).append("<td>");             // Set the currency to use when printing the following prices             price.setCurrency(positions[i].currency);             b.append(price.format(positions[i].bought)).append("<td>");             b.append(price.format(positions[i].current)).append("<td>");             double change =                 (positions[i].current-positions[i].bought)/positions[i].bought;             b.append(percent.format(change)).append("</tr>");         }         b.append("</table></body></html>");         return b.toString( );     }          /**      * This is a test program that demonstrates the class      **/     public static void main(String[  ] args) throws ParseException {         Currency dollars = Currency.getInstance("USD");         Currency pounds = Currency.getInstance("GBP");         Currency euros = Currency.getInstance("EUR");         Currency yen = Currency.getInstance("JPY");         // This is the portfolio to display.         EquityPosition[  ] positions = new EquityPosition[  ] {             new EquityPosition("WWW", 400, "2003-01-03", dollars, 11.90,13.00),             new EquityPosition("XXX", 1100, "2003-02-02", pounds, 71.09,27.25),             new EquityPosition("YYY", 6000, "2003-04-17", euros, 23.37,89.12),             new EquityPosition("ZZZ", 100, "2003-8-10", yen, 100000,121345)         };         // Create the portfolio from these positions         Portfolio portfolio = new Portfolio(positions, new Date( ));         // Set the default locale using the language code and country code         // specified on the command line.         if (args.length == 2) Locale.setDefault(new Locale(args[0], args[1]));         // Now display the portfolio.         // We use a Swing dialog box to display it because the console may         // not be able to display non-ASCII characters like currency symbols         // for Pounds, Euros, and Yen.         javax.swing.JOptionPane.showMessageDialog(null, portfolio,                                Locale.getDefault( ).getDisplayName( ),                                javax.swing.JOptionPane.INFORMATION_MESSAGE);         // The modal dialog starts another thread running, so we have to exit         // explictly when the user dismisses it.         System.exit(0);     } }

8.4.1 Setting the Locale

Example 8-3 contains code that explicitly sets the locale using the language code and the country code specified on the command line. If these arguments are not specified, it uses the default locale for your system. When experimenting with internationalization, you may want to change the default locale for the entire platform so you can see what happens. How you do this is platform-dependent. On Unix platforms, you typically set the locale by setting the LANG environment variable. For example, to set the locale for Canadian French, using a Unix csh-style shell, use this command:

% setenv LANG fr_CA

Or, to set the locale to English as spoken in Great Britain when using a Unix sh-style shell, use this command:

$ export LANG=en_GB

To set the locale in Windows, use the Regional Settings control on the Windows Control Panel.