Lesson 02 · Formatting & Parsing

Objectives

After this lesson you will be able to:

• Format and parse numbers and currency with NumberFormat.
• Format dates/times for a Locale with a localized DateTimeFormatter.
• Build parameterized messages with MessageFormat.

NumberFormat

NumberFormat formats numbers, currency, and percentages for a Locale.

NumberFormat n = NumberFormat.getInstance(Locale.US);
n.format(1_234.5);                       // "1,234.5"
NumberFormat c = NumberFormat.getCurrencyInstance(Locale.US);
c.format(1234.5);                        // "$1,234.50"
NumberFormat p = NumberFormat.getPercentInstance(Locale.US);
p.format(0.25);                          // "25%"

Formatting and parsing are inverses; parse returns a Number and may throw ParseException:

Number parsed = NumberFormat.getInstance(Locale.US).parse("1,234.5");   // 1234.5

Exam trap

NumberFormat.parse returns a Number (could be Long or Double) and throws the checkedjava.text.ParseException — not NumberFormatException. It also stops at the first unparseable character rather than failing (e.g. parse("12abc") returns 12).

Localized date/time formatting

Use a DateTimeFormatter with a locale and a FormatStyle:

DateTimeFormatter f = DateTimeFormatter
        .ofLocalizedDate(FormatStyle.MEDIUM)
        .withLocale(Locale.US);
LocalDate.of(2026, 6, 19).format(f);     // "Jun 19, 2026"

The same instant renders differently per locale — FormatStyle.FULL/LONG/MEDIUM/SHORT control verbosity.

MessageFormat

MessageFormat builds messages with positional placeholders {0}, {1}, with optional type/ style ({0,number,currency}, {1,date,short}):

String msg = MessageFormat.format(
        "{0} has {1} new messages.", "Alice", 3);     // "Alice has 3 new messages."

Gotcha

MessageFormat placeholders are zero-indexed and reusable. A literal single quote must be doubled (''), and { inside text must be quoted ('{') — a lone quote suppresses formatting of the following segment.

SDET note

When asserting localized output, pin the Locale explicitly (getInstance(Locale.US)) — never rely on the JVM default, which varies by machine and makes tests non-portable. Better still, assert a parse round-trip (format then parse) so CLDR data changes don't break the test on a wording tweak.

Key Takeaways

• NumberFormat (getInstance/getCurrencyInstance/getPercentInstance) formats and parses for a Locale. parse returns a Number, throws checked ParseException, and stops at the first bad char.
• Localize dates with DateTimeFormatter.ofLocalizedDate(style).withLocale(...) and a FormatStyle.
• MessageFormat uses zero-indexed {0} placeholders (with optional ,number,/,date, types); double literal quotes.
• Always pin the Locale in tests; the JVM default is not portable.

Lesson Quiz

Lesson Quiz · Formatting & Parsing0 / 5

1. What does NumberFormat.getCurrencyInstance(Locale.US).format(1234.5) produce?

   • A"1234.5"
   • B"$1,234.50"
   • C"USD 1234.5"
   • D"$1234.5"

2. What exception does NumberFormat.parse throw on bad input?

   • ANumberFormatException
   • BParseException (checked)
   • CIllegalArgumentException
   • DNone

3. What does NumberFormat.getInstance(Locale.US).parse("12abc") return?

   • ANumberFormatException
   • B12
   • C0
   • Dnull

4. MessageFormat placeholders are...

   • A1-indexed {1}
   • B0-indexed {0}
   • Cnamed ${x}
   • D%s style

5. In a test, why pin the Locale explicitly?

   • AIt's faster
   • BThe JVM default Locale varies by machine, making output non-portable
   • CRequired by the compiler
   • DTo avoid ParseException

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Next: Module 09 Mini-Exam. Run the matching code in labs/src/main/java/com/jse21/m09_localization/.