Platform & Advanced APIs

Internationalization

i18n separates code from locale-sensitive behavior: Locale drives number/date/currency formatting, collation, and message selection; ResourceBundle externalizes translatable text; everything Unicode flows in UTF-8.

Locale = language + region (+ script/variant): Locale.forLanguageTag("sr-Latn-RS")
Format numbers/currency/dates with NumberFormat/DateTimeFormatter.withLocale — never string-build them
ResourceBundle + messages_de_DE.properties selects translations by locale with fallback
MessageFormat/ChoiceFormat handle placeholder order and plural forms per language
Collation (Collator) sorts text per locale rules — String.compareTo is code-point order, not human order
Never default silently: locale-sensitive APIs have overloads taking an explicit Locale

Locale-driven formatting

double amount = 1234567.89;
NumberFormat de = NumberFormat.getCurrencyInstance(Locale.GERMANY);
NumberFormat us = NumberFormat.getCurrencyInstance(Locale.US);
de.format(amount);   // "1.234.567,89 €"
us.format(amount);   // "$1,234,567.89"

var fmt = DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL);
LocalDate.now().format(fmt.withLocale(Locale.FRANCE));   // "mardi 7 juillet 2026"

ResourceBundle + MessageFormat

// messages_en.properties:  cart.items={0,choice,0#no items|1#one item|1<{0} items}
// messages_de.properties:  cart.items={0,choice,0#keine Artikel|1#ein Artikel|1<{0} Artikel}

ResourceBundle msgs = ResourceBundle.getBundle("messages", userLocale);
String text = MessageFormat.format(msgs.getString("cart.items"), itemCount);

The deeper lessons (Core Java II ch. 7): translatable strings must be whole messages with placeholders — concatenating fragments breaks under different word orders; plurals need ChoiceFormat/ICU rules, not if (n == 1); case conversion is locale-sensitive (the Turkish dotless-ı makes "I".toLowerCase() locale-dependent — use toLowerCase(Locale.ROOT) for protocol strings); and sorting user-visible lists needs Collator, which knows that ä sorts differently in German and Swedish.