The parameterless versions of String.toUpperCase() and String.toLowerCase() use the default locale of the Java Virtual Machine when transforming strings. This can cause unexpected behavior for certain locales.

Use the corresponding methods with explicit locale parameters to ensure that the results are consistent across all locales. For example:

System.out.println("I".toLowerCase(java.util.Locale.ENGLISH));

prints i, regardless of the default locale.

In the following example, the calls to the parameterless functions may return different strings for different locales. For example, if the default locale is ENGLISH, the function toLowerCase() converts a capital I to i; if the default locale is TURKISH, the function toLowerCase() converts a capital I to the Unicode Character "Latin small letter dotless i" (U+0131).

To ensure that an English string is returned, regardless of the default locale, the example shows how to call toLowerCase and pass locale.ENGLISH as the argument. (This assumes that the text is English. If the text is Turkish, you should pass locale.TURKISH as the argument.)

Java API Specification: String.toUpperCase().