string-format: String.format/formatted/printf cheat sheet module (specifier anatomy, rounding vs C printf, locales, Indian lakh/crore grouping Formatter cannot do, format exceptions, %t dates, concat cost), 11 contract tests
Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_017ic4b7Sr2zcoRD2watXv1G
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Claude
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== conversions: what kind of value ==
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text | 42 | 3.141590 | ff | A | true
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== width: minimum characters ==
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[ ab] [ab ] [ 42] [42 ]
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== precision: digits after the point (f), or max chars (s) ==
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[3.14] [4] [ 3.142] [abc]
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== flags ==
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[00042] [+42] [-42] [ 42] [(42)] [1,234,567]
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[0xff] [010] [FF] [-003.142]
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== argument index: reuse and reorder ==
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hello world
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echo, echo and again echo
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== %n and %% ==
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100% sure
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next line
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== %s accepts anything (calls String.valueOf / toString) ==
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[null] [7] [[1, 2]] [SHOUT]
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== %b is not 'is this the string true' ==
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true true false false
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== %c takes a char or a code point ==
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A A 😀
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Item Qty Price Total
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------------------------------------------------
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Notebook 12 45.50 546.00
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Pen (box of 10) 3 120.00 360.00
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Stapler 1 350.00 350.00
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------------------------------------------------
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Grand total 1,256.00
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== the same table with printf: it prints, it does not return a String ==
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Notebook | 12|
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== truncation: %.Ns cuts, width pads ==
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[Pen (b ] [ Pen (b]
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value %.1f %.2f exact binary value BigDecimal(HALF_EVEN,2)
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0.125 0.1 0.13 0.125 0.12
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0.25 0.3 0.25 0.25 0.25
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0.35 0.4 0.35 0.34999999999999997779 0.35
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2.675 2.7 2.68 2.67499999999999982236 2.67
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1.005 1.0 1.01 1.00499999999999989341 1.00
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0.5 0.5 0.50 0.5 0.50
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1.5 1.5 1.50 1.5 1.50
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2.5 2.5 2.50 2.5 2.50
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== %.0f on halves: HALF_UP, away from zero ==
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1 2 3 -3
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== same number, three routes: %f rounds what it is given ==
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double 2.675 -> 2.68
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new BigDecimal("2.675") -> 2.68
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new BigDecimal(2.675) -> 2.67
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== what the JDK gives for 123456789 ==
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String.format(en_IN, "%,d") : 123,456,789
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String.format(hi_IN, "%,d") : 123,456,789
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NumberFormat.getInstance(en_IN) : 123,456,789
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NumberFormat currency (en_IN) : ₹123,456,789.50
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CLDR pattern for en_IN : #,##0.###
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== a DecimalFormat pattern that LOOKS like it should work ==
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pattern #,##,##0 -> grouping size : 3
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pattern #,##,##0 -> output : 123,456,789
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== compact formatting knows crore but drops the digits ==
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compact (default rounding) : 12Cr
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compact (2 fraction digits) : 12.35Cr
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== IndianNumberFormat ==
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0 -> 0
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999 -> 999
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1,000 -> 1,000
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12,345 -> 12,345
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123,456 -> 1,23,456
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1,234,567 -> 12,34,567
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123,456,789 -> 12,34,56,789
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-123,456,789 -> -12,34,56,789
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12,34,567.89
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-1,00,000.00
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Rs. 12,34,567.50
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== plugging it into a format string through %s ==
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Balance 9,87,65,432.10
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default FORMAT locale: en_US
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== %,.2f of 1234567.891 ==
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en-US 1,234,567.89
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de-DE 1.234.567,89
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fr-FR 1^234^567,89
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en-IN 1,234,567.89
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de-CH 1’234’567.89
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und 1,234,567.89
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== the bug: changing the default locale changes %.2f ==
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default (en_US) : 1234.50
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default (de_DE) : 1234,50
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explicit ROOT : 1234.50
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== not localized: hex, NaN, Infinity ==
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ff NaN Infinity
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wrong type for %d -> IllegalFormatConversionException: d != java.lang.String
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too few arguments -> MissingFormatArgumentException: Format specifier '%s'
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unknown conversion -> UnknownFormatConversionException: Conversion = 'q'
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precision on an integer -> IllegalFormatPrecisionException: 2
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flag not valid for %x , -> FormatFlagsConversionMismatchException: Conversion = x, Flags = ,
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left-justify without width -> MissingFormatWidthException: %-d
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zero pad AND left-justify -> IllegalFormatFlagsException: Flags = '-0'
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width on %n -> IllegalFormatWidthException: 5
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double passed to %d -> IllegalFormatConversionException: d != java.lang.Double
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int passed to %f -> IllegalFormatConversionException: f != java.lang.Integer
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a stray % in user text -> MissingFormatArgumentException: Format specifier '% o'
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extra arguments (no error) -> OK: a
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== four spellings, one result ==
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Hi Asha, 3 items, Rs 1499.50
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Hi Asha, 3 items, Rs 1499.50
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Hi Asha, 3 items, Rs 1499.50
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Hi Asha, 3 items, Rs 1499.50
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format == formatted == Formatter: true
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== formatted() on a text block ==
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Invoice for Asha
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items : 3
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total : 1,499.50
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== printf returns the PrintStream, so calls chain ==
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one two three
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== %n vs \n ==
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true on this platform (line.separator is \n)
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== an array argument: spread as the varargs, unless you cast it to Object ==
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x-1
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p-q
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true
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JDK 25.0.5, 2000000 iterations per variant, best of 5 rounds
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PLUS 52.6 ns/op
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BUILDER 66.3 ns/op
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FORMATTED 292.1 ns/op
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FORMAT 279.5 ns/op
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[INFO] Tests run: 11, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.153 s -- in com.ankurm.stringformat.StringFormatTest
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[INFO] Tests run: 11, Failures: 0, Errors: 0, Skipped: 0
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[INFO] BUILD SUCCESS
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C printf %.2f of 2.675 : 2.67
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C printf %.1f of 0.35 : 0.3
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C printf %.2f of 1.005 : 1.00
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C printf %.2f of 0.125 : 0.12
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== %t suffixes on a LocalDateTime ==
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2026-11-12 | 09:05:07 | 09:05 | 11/12/26
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Thursday, 12 November 2026
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09:05 AM
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== the same suffixes, other locales (names are localized, numbers are not) ==
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Donnerstag, 12. November 2026
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jeudi 12 novembre 2026
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== a LocalDate has no hour ==
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IllegalFormatConversionException: H != java.time.LocalDate
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== DateTimeFormatter does the same job with a reusable, immutable object ==
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12 Nov 2026 09:05
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