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Java in a Nutshell

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Internationalization
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11.4 Handling Local Customs

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

The NumberFormat class is used to format 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 used in many English speaking countries. A NumberFormat object can use the default locale or any locale you specify.

The DateFormat class is used to format 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 be used to 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.

The Collator class is used to compare 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, as when you are implementing a word-wrapping algorithm.

Example 11.3 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. Example 11.2 shows the output of this program in American, Canadian, and French locales. Note the different treatment of dates, numbers, and monetary quantities in these three locales.

Example 11.2: Dates and Numbers Formatted for Three Locales

# American English locale (en_US)
Portfolio value at April 08, 1997 6:57:40 PM PDT:
Symbol  Shares  Bought On       At      Quote   Change
XXX     400     6/15/96         $11.90  $13.00  9%
YYY     1,100   9/14/96         $71.09  $27.25  -61%
ZZZ     6,000   6/27/91         $23.37  $89.12  281%
# Canadian English locale (en_CA)
Portfolio value at April 8, 1997 9:57:40 CDT PM:
Symbol  Shares  Bought On       At      Quote   Change
XXX     400     15/06/96        $11.90  $13.00  9%
YYY     1,100   14/09/96        $71.09  $27.25  -61%
ZZZ     6,000   27/06/91        $23.37  $89.12  281%
# French locale (fr_FR)
Portfolio value at 9 avril 1997 03:57:40 GMT+02:00:
Symbol  Shares  Bought On       At      Quote   Change
XXX     400     15/06/96        11,90 F 13,00 F 9%
YYY     1 100   14/09/96        71,09 F 27,25 F -61%
ZZZ     6 000   27/06/91        23,37 F 89,12 F 281%

Example 11.3: Formatting Dates and Numbers in an Internationalized Program

import java.text.*;
import java.util.Date;
/**
 * A partial implementation of a hypothetical stock portfolio class.
 * We use it only to demonstrate number and date internationalization.
 */
public class Portfolio {
  EquityPosition[] positions;
  Date lastQuoteTime = new Date();
  public void print() {
    // 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.SHORT);
    DateFormat fulldate = DateFormat.getDateTimeInstance(DateFormat.LONG,
                                                         DateFormat.LONG);
    // Print some introductory data.
    System.out.println("Portfolio value at " +
                       fulldate.format(lastQuoteTime) + ":");
    System.out.println("Symbol\tShares\tBought On\tAt\t" +
                       "Quote\tChange");
    // Then display the table using the format() methods of the Format objects.
    for(int i = 0; i < positions.length; i++) {
      System.out.print(positions[i].name + "\t");
      System.out.print(number.format(positions[i].shares) + "\t");
      System.out.print(shortdate.format(positions[i].purchased) + "\t");
      System.out.print(price.format(positions[i].bought) + "\t");
      System.out.print(price.format(positions[i].current) + "\t");
      double change =
        (positions[i].current - positions[i].bought)/positions[i].bought;
      System.out.println(percent.format(change));
    }
  }
  static class EquityPosition {
    String name;             // Name of the stock.
    int shares;              // Number of shares held.
    Date purchased;          // When purchased.
    double bought, current;  // Purchase price and current price (per share).
    EquityPosition(String n, int s, Date when, double then, double now) {
      name = n; shares = s; purchased = when; bought = then; current = now;
    }
  }
}


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