Number Formatting

Figure 535. Excerpt from java.util.Locale

A Locale object represents a specific geographical, political, or cultural region.

An operation that requires a Locale to perform its task is called locale-sensitive and uses the Locale to tailor information for the user.

For example, displaying a number is a locale-sensitive operation: Numbers should be formatted according to the customs and conventions of the user's native country, region, or culture.

Figure 536. Locale properties

Language
Encoding
Country
Extensible

Figure 537. Get a NumberFormat instance

final NumberFormat standard = new DecimalFormat();
System.out.println(standard.format(1234.5678));

final NumberFormat de =
    DecimalFormat.getInstance(Locale.GERMANY);
System.out.println(de.format(1234.5678));

1234.5678
1.234,568

Figure 538. Create a custom formatter

final DecimalFormatSymbols unusualSymbols =
  new DecimalFormatSymbols(Locale.getDefault());
unusualSymbols.setDecimalSeparator('|');
unusualSymbols.setGroupingSeparator('^');

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

System.out.println(weirdFormatter.format(12345.678));

1^2345|678

exercise No. 176

`Locale` definitions

Each JRE™ ships with a set of available Locale definitions. Write a simple program to get the following list:

160 locales available:
Locale: Arabic(ar_AE)
Locale: Arabic(ar_JO)
Locale: Arabic(ar_SY)
Locale: Croatian(hr_HR)
...
Locale: Indonesian(in_ID)
Locale: English(en_GB)

Tip

Read the documentation of Locale to find appropriate methods.

System.out.println(Locale.getAvailableLocales().length + " locales available:");
for (final Locale l: Locale.getAvailableLocales()) {
  System.out.println("Locale: " + l.getDisplayLanguage() + "(" + l + ")");
}

exercise No. 177

Formatting `int`, `double` and `LocaleDate`

Implement the following three overloaded class methods:

/**
 * Print an int value using the given locale. Example:
 *
 * Using Locale.GERMANY a value of 1234567 should appear as "1.234.567"
 *
 * @param value The value to be formatted.
 * @param locale The locale to be used.
 */
static public void print(final int value,  final Locale locale) {...}

/**
 * Print a double value using the given locale. Example:
 *
 * Using Locale.GERMANY a value of -123.456789 should appear as "-123,457"
 *
 * @param value The value to be formatted.
 * @param locale The locale to be used.
 */
static public void print(final double value,  final Locale locale) {...}

/**
 * Print a {@link LocalDate} value using the given locale. Example:
 *
 * Using Locale.GERMANY the 22-nd of december 2024 should appear as "22. Dezember 2024".
 *
 * @param value The value to be formatted.
 * @param locale The locale to be used.
 */
static public void print(final LocalDate date,  final Locale locale) {...}

You may use them as:

for (final Locale locale : new Locale[]{Locale.GERMANY, Locale.JAPAN, Locale.US}) {
  System.out.println("Locale " + locale.getDisplayCountry());
  print(1234567, locale);
  print(-123.456789, locale);
  print(LocalDate.of(2024, Month.DECEMBER, 22), locale);
  System.out.println("----------------------\n");
}

The expected output reads:

Locale Germany
1.234.567
-123,457
22. Dezember 2024
----------------------

Locale Japan
1,234,567
-123.457
2024/12/22
----------------------

Locale United States
1,234,567
-123.457
December 22, 2024
----------------------

Tip

Read the NumberFormat documentation and its static NumberFormat getInstance(Locale inLocale) in particular. Also note its multiply overloaded String format(...) methods partly suiting your data type requirements.
Regarding dates you may want to consider DateTimeFormatter providing the static DateTimeFormatter ofLocalizedDate(FormatStyle dateStyle) method. Then read about class FormatStyle and the DateTimeFormatter withLocale(Locale locale) method.

Implementing the first two methods is relatively simple:

/**
 * Print an int value using the given locale. Example:
 *
 * Using Locale.GERMANY a value of 1234567 should appear as "1.234.567"
 *
 * @param value The value to be formatted.
 * @param locale The locale to be used.
 */
static public void print(final int value,  final Locale locale) {
  final NumberFormat numberFormat = NumberFormat.getInstance(locale);
  System.out.println(numberFormat.format(value));
}

/**
 * Print a double value using the given locale. Example:
 *
 * Using Locale.GERMANY a value of -123.456789 should appear as "-123,457"
 *
 * @param value The value to be formatted.
 * @param locale The locale to be used.
 */
static public void print(final double value,  final Locale locale) {
  final NumberFormat numberFormat = NumberFormat.getInstance(locale);
  System.out.println(numberFormat.format(value));
}

Processing dates is a bit cumbersome:

/**
 * Print a {@link LocalDate} value using the given locale. Example:
 *
 * Using Locale.GERMANY the 22-nd of december 2024 should appear as "22. Dezember 2024".
 *
 * @param value The value to be formatted.
 * @param locale The locale to be used.
 */
static public void print(final LocalDate date,  final Locale locale) {
  final DateTimeFormatter dateFormat =
    DateTimeFormatter.ofLocalizedDate(FormatStyle.LONG).withLocale(locale);
  System.out.println(dateFormat.format(date));
}

Figure 539. Polymorphic number parsing

final NumberFormat de = NumberFormat.getInstance(Locale.GERMANY),
                       us = NumberFormat.getInstance(Locale.US);
try {
  final Number[] values = {
    de.parse("103.234"), de.parse("400.000,234"),
    us.parse("103.234"), us.parse("400.000,234"),};
  for (final Number n: values) {
    System.out.println(n + "(" + n.getClass().getTypeName() + ")");
  } catch (ParseException e) { ... }

103234(java.lang.Long)
400000.234(java.lang.Double)
103.234(java.lang.Double)
400(java.lang.Long)

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