Beware Kotlin's Hidden Localization

Recently, I had a fun bug at work which ultimately stemmed from a very simple problem — hidden localization. I needed to interpolate a String in Kotlin but didn’t want to use Kotlin’s string literal interpolation feature.¹ Therefore I opted for Java’s String.format method; Kotlin provides a handy fun String.format(vararg args: Any?): String extension for this.

val string = "%d dollars".format(2)
println(string) // Prints "2 dollars" as expected

However, I had missed an important part of the documentation:

Uses this string as a format string and returns a string obtained by substituting the specified arguments, using the default locale.

Locale.setDefault(Locale("ar")) // Set locale to Arabic
val string = "%d dollars".format(2)
println(string) // Prints "٢ dollars"

I had forgotten that some languages have non-latin numerals! This caused a bug in some other code that expected latin numerals. The fix is trivial to tell Kotlin to not use any localization:

Locale.setDefault(Locale("ar")) // Set locale to arabic
val string = "%d dollars".format(2, null) // Format without localization
println(string) // Prints "2 dollars" as expected

Next time you reach for Kotlin’s String.format, make sure you double check whether you need localization or not!