11.5 Formatting and Parsing Messages, Numbers, Percentages, and Localized Dates

11.5 Formatting and Parsing Messages, Numbers, Percentages, and Localized Dates

The localization objective asks you to "parse and format messages, dates, times, and numbers including currency and percentage values." Section 11.4 introduced Locale, resource bundles, and the basic NumberFormat and DateTimeFormatter factories. This section completes the objective with MessageFormat, percentage and compact number formats, DecimalFormat patterns, parsing rules, and locale-sensitive date output.

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Formatting Messages with java.text.MessageFormat

Resource bundle values often contain numbered placeholders that are filled in at runtime. MessageFormat replaces {0}, {1}, and so on with arguments in index order, regardless of where they appear in the pattern:

String msg = MessageFormat.format("{0} has {1} items costing {2,number,currency}", "Ana", 3, 9.5);
System.out.println(msg);                          // Ana has 3 items costing $9.50 (en_US default)
System.out.println(MessageFormat.format("{1} before {0}", "A", "B")); // B before A
System.out.println(MessageFormat.format("{0}", 1234567));            // 1,234,567
System.out.println(MessageFormat.format("{0,number,percent}", 0.5)); // 50%

A placeholder can name a format type and style: {n,number}, {n,number,integer}, {n,number,currency}, {n,number,percent}, {n,date,short}, or {n,time,medium}. Numbers are localized even without a type, which is why 1234567 printed with grouping separators.

For a specific locale, create an instance instead of calling the static method:

MessageFormat mf = new MessageFormat("{0,number,currency}", Locale.GERMANY);
System.out.println(mf.format(new Object[] {9.5}));   // 9,50 €

Combining a bundle with MessageFormat is the standard localization pattern:

ResourceBundle rb = ResourceBundle.getBundle("Messages", new Locale("en", "US"));
// Messages_en_US.properties contains: greeting=Hello, {0}! You have {1} new messages.
String text = MessageFormat.format(rb.getString("greeting"), "Ravi", 4);
// Hello, Ravi! You have 4 new messages.

String.format (with %s, %d, %.2f) is a different API that uses the default FORMAT locale. After Locale.setDefault(Locale.Category.FORMAT, Locale.GERMANY), String.format("%.2f", 1234.5) returns 1234,50, while String.format(Locale.US, "%.2f", 1234.5) returns 1234.50.

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Number, Currency, Percentage, and Compact Formats

java.text.NumberFormat provides locale-aware factories. All of them are not thread-safe, unlike DateTimeFormatter.

Remember that a percent format multiplies by 100: the value 0.256 is 25.6%, which rounds to 26% because the percent instance shows no fraction digits by default.

Custom patterns with DecimalFormat

DecimalFormat (a NumberFormat subclass) uses pattern symbols: 0 always prints a digit (padding with zeros), # prints a digit only when needed, , marks grouping, and . marks the decimal separator.

System.out.println(new DecimalFormat("#,##0.00").format(1234.5));  // 1,234.50
System.out.println(new DecimalFormat("000.##").format(3.14159));   // 003.14
System.out.println(new DecimalFormat("0.0").format(2.25));         // 2.2 (HALF_EVEN tie goes to the even digit)

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Parsing Numbers and Currency

NumberFormat.parse(String) returns a Number (Long when the value is integral, otherwise Double) and declares the checked java.text.ParseException:

NumberFormat cf = NumberFormat.getCurrencyInstance(Locale.US);
System.out.println(cf.parse("$1,234.50"));                          // 1234.5
System.out.println(NumberFormat.getPercentInstance(Locale.US).parse("26%")); // 0.26
cf.parse("1234.50");   // throws ParseException: Unparseable number: "1234.50" (no $ prefix)

A currency parser expects the locale's currency symbol, and a percent parser divides by 100. As section 11.4 showed, parsing stops at the first character that cannot belong to a number, so "45.67xyz" parses as 45.67, but text that cannot start a number throws ParseException.

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Locale-Sensitive Dates and Times

DateTimeFormatter styles produce different text for each locale. Use withLocale (or the locale argument of ofPattern) to choose it:

LocalDate d = LocalDate.of(2026, 4, 15);
DateTimeFormatter medium = DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM);
System.out.println(medium.withLocale(Locale.US).format(d));       // Apr 15, 2026
System.out.println(medium.withLocale(Locale.FRANCE).format(d));   // 15 avr. 2026
System.out.println(medium.withLocale(Locale.GERMANY).format(d));  // 15.04.2026
System.out.println(DateTimeFormatter.ofPattern("EEEE d MMMM", Locale.FRANCE).format(d)); // mercredi 15 avril

Month and day names (MMMM, EEEE) come from the formatter's locale, while numeric patterns such as dd/MM/yyyy look the same everywhere. Parsing uses the same formatter: LocalDate.parse("15/04/2026", DateTimeFormatter.ofPattern("dd/MM/yyyy")) succeeds, and a mismatch throws the unchecked DateTimeParseException.