java-core-examples/string-format

string-format

Companion code for the ankurm.com post "String.format, formatted() and printf in Java: The Complete Cheat Sheet." Module in java-core-examples, the Java-core series.

Versions this was built and tested against

Component	Version	Notes
JDK	25.0.5+8 (Temurin, LTS)	String.format is Java 5; String.formatted() arrived in Java 15; Formatter parsing was rewritten without regex in Java 17.
JUnit Jupiter	5.11.0
Maven	3.9.11

Quickstart

export JAVA_HOME=/path/to/jdk-25
mvn compile
scripts/run.sh IndianGroupingDemo      # one demo
scripts/run-all.sh                     # regenerate every file in output/

What's in here

File	What it shows
BasicsDemo.java	Every part of %[index$][flags][width][.precision]conversion, changed one at a time.
ReportTableDemo.java	Left/right alignment, zero padding, truncation with %.Ns, a fixed-width invoice, printf.
RoundingDemo.java	%f rounds HALF_UP on the decimal string, so it disagrees with C printf and with BigDecimal(double).
IndianNumberFormat.java + IndianGroupingDemo.java	The main exhibit. Every built-in route to lakh/crore grouping tried and shown to fail, then a 60-line formatter that works.
LocaleDemo.java	The same %,.2f under six locales; the default-locale trap; what is not localized.
FormatErrorsDemo.java	Twelve format strings that compile and then throw (or, for "50% off", throw something surprising), with the real exception text.
FormattedAndPrintfDemo.java	format / formatted / printf / Formatter are one engine; text-block pairing; varargs and arrays.
DateTimeDemo.java	%tF, %tT, %tA on java.time values, localized names, and why %tH on a LocalDate throws.
ConcatBenchmarkDemo.java	Indicative cost of +, StringBuilder, formatted, format. A warmed plain loop, not JMH.
StringFormatTest.java	Pins every claim above as an assertion, 11/11 passing.
scripts/printf_compare.c	The same values through C printf, for the rounding comparison.
output/01-11	Captured runs of everything above. Timing in 08 drifts between runs.

Notes worth knowing before reading the post

• %,d cannot produce lakh/crore grouping in any locale. en_IN and hi_IN both print 123,456,789 (output/04). The , flag takes one grouping size from DecimalFormat.getGroupingSize(), and a DecimalFormat built from #,##,##0 reports a grouping size of 3.
• "Discount 50% off" throws MissingFormatArgumentException, not an unknown-conversion error: % o parses as the space flag plus the octal conversion (output/06).
• %f on a double rounds the decimal string, %f on a BigDecimal rounds the exact value, so 2.675 formats as 2.68 one way and 2.67 the other (output/03).

License

MIT - see the repo-wide LICENSE.