Java – Which Locale should I specify when I call String#toLowerCase


In Java the String#toLowerCase method uses the default system Locale to determine how to handle lowercasing. If I am lowercasing some ASCII text and want to be sure that this is processed as expected which Locale should I use?

EDIT: I'm mainly concerned about programming identifiers such as table and column names in a schema. As such I want English lower casing to apply.

Locale.ROOT states that it is the language/country neutral locale for the locale sensitive operations

Locale.ENGLISH would presumably also be a safe choice.

Best Solution

Yes, Locale.ENGLISH is a safe choice for case operations for things like programming language identifiers and URL parts since it doesn't involve any special casing rules and all 7-bit ASCII characters in the ENGLISH case-convert to 7-bit ASCII characters.

That is not true for all other locales. In Turkish, the 'I' and 'i' characters are not case-converted to one another.

"Dotted and dotless I" explains:

The Turkish alphabet, which is a variant of the Latin alphabet, includes two distinct versions of the letter I, one dotted and the other dotless.

In Unicode, U+0131 is a lower case letter dotless i (ı). U+0130 (İ) is capital i with dot. ISO-8859-9 has them at positions 0xFD and 0xDD respectively. In normal typography, when lower case i is combined with other diacritics, the dot is generally removed before the diacritic is added; however, Unicode still lists the equivalent combining sequences as including the dotted i, since logically it is the normal dotted i character that is being modified.

Most Unicode software uppercases ı to I and lowercases İ to i, but, unless specifically set up for Turkish, it lowercases I to i and uppercases i to I. Thus uppercasing then lowercasing, or vice versa, changes the letters.

The list of special exceptions is maintained at

# ================================================================================

# Turkish and Azeri

# I and i-dotless; I-dot and i are case pairs in Turkish and Azeri
# The following rules handle those cases.

0130; 0069; 0130; 0130; tr; # LATIN CAPITAL LETTER I WITH DOT ABOVE
0130; 0069; 0130; 0130; az; # LATIN CAPITAL LETTER I WITH DOT ABOVE

# When lowercasing, remove dot_above in the sequence I + dot_above, which will turn into i.
# This matches the behavior of the canonically equivalent I-dot_above

0307; ; 0307; 0307; tr After_I; # COMBINING DOT ABOVE
0307; ; 0307; 0307; az After_I; # COMBINING DOT ABOVE