String Formatting in Java: String.format(), printf() and Formatter - Quiz

They all understand the same template syntax and use java.util.Formatter under the hood. The difference is where the result goes:

• String.format(template, args...) (Java 5) returns a new string — Java's sprintf;
• "template".formatted(args...) (Java 15) does the same but is shorter and works nicely with text blocks; it has no Locale overload and always uses the default locale;
• System.out.printf() and System.out.format() are two names for the same PrintStream method and print straight to the console; they add no line break on their own, so you write %n in the template;
• Formatter accumulates text from several format() calls and can write into a StringBuilder, a file or a stream.

String a = String.format("%s: %d", "Anna", 5);
String b = "%s: %d".formatted("Anna", 5);
System.out.printf("%s: %d%n", "Anna", 5);

StringBuilder sb = new StringBuilder();
Formatter f = new Formatter(sb, Locale.US);
f.format("%-8s%6.2f%n", "Tea", 3.5);

The general form is %[argument_index$][flags][width][.precision]conversion. Only the % sign and the conversion character (d, s, f, x…) are required; everything else is optional:

• argument_index$ — the argument number starting at 1 (%2$s); %<s reuses the argument of the previous specifier;
• flags — alignment, sign, leading zeros, grouping separators (-, +, 0, ,, (, #, space);
• width — the minimum field width: a shorter value is padded with spaces, a longer one is printed in full;
• .precision — digits after the decimal point for floating-point numbers (rounded with HALF_UP), maximum length for strings.

String.format(Locale.US, "[%8.2f]", 1000.0 / 3.0); // [  333.33]
String.format("[%.3s]", "Formatter");               // [For]
String.format("[%3s]", "Formatter");                // [Formatter]

Uppercase variants (%S, %X, %B…) convert the result to upper case; %d, %f and %o have no uppercase forms.

Dates use the %t (or %T) specifier with a suffix: %tY is the year, %tm the month, %td the day, %tB the month name, %tF the ISO date. The argument can be a LocalDate, LocalDateTime, Date, Calendar or long.

Every specifier without an index takes the next argument, so "%tm/%td/%tY" with one date expects three arguments and throws MissingFormatArgumentException. Refer to the same argument with the %1$ index:

LocalDate date = LocalDate.of(2026, 10, 16);
String.format("%tF", date);                             // 2026-10-16
String.format("%1$tm/%1$td/%1$tY", date);               // 10/16/2026
String.format(Locale.US, "%1$tB %1$te, %1$tY", date);   // October 16, 2026
String.format("%tm/%td/%tY", date);                     // MissingFormatArgumentException

For complex date patterns, modern code usually reaches for java.time.format.DateTimeFormatter.

The compiler does not check format strings: mistakes surface only at runtime as unchecked exceptions, subclasses of IllegalFormatException (which extends IllegalArgumentException).

String.format("%d", 3.5);           // IllegalFormatConversionException: d != java.lang.Double
String.format("%s and %s", "A");    // MissingFormatArgumentException: Format specifier '%s'
String.format("%-05d", 7);          // IllegalFormatFlagsException: Flags = '-0'
String.format("%i", 7);             // UnknownFormatConversionException: Conversion = 'i'
String.format("Save 50%");          // UnknownFormatConversionException: Conversion = '%', use 50%%
String.format("%s", "a", "b");      // no exception: "a", the extra argument is ignored
String.format("%d", (Integer) null); // no exception: "null"

The - or 0 flag without a width gives MissingFormatWidthException. Java has no %i or %u from C — integers are printed only with %d.

Both classes live in the java.text package. DecimalFormat formats a number by a pattern such as #,##0.00, and MessageFormat inserts arguments by number in curly braces {0}, {1} instead of % specifiers — the approach used in UI localization bundles.

DecimalFormat df = new DecimalFormat("#,##0.00",
        DecimalFormatSymbols.getInstance(Locale.US));
df.format(1234567.891); // 1,234,567.89

MessageFormat.format("File {0} contains {1} lines", "data.txt", 1500);
// File data.txt contains 1,500 lines
MessageFormat.format("File {0} contains {1,number,#} lines", "data.txt", 1500);
// File data.txt contains 1500 lines

MessageFormat pitfalls: numbers in {1} are formatted with the locale's grouping separators (1500 → 1,500; output shown for Locale.US), and {1,number,#} prints them without separators. A single quote in the pattern is an escape character, so an apostrophe has to be doubled: "Don''t" prints Don't.