Summary

Deprecate String::toLowerCase() and String::toUpperCase(), users should explicitly pass the Locale.

Problem

The String::toLowerCase() and String::toUpperCase() without parameters are locale sensitive, and they are widely misused in places that should be local insensitive.

When they are misused, they may produce unexpected results.

For instance, "TITLE".toLowerCase() in a Turkish locale returns "t\u0131tle" where '\u0131' is the LATIN SMALL LETTER DOTLESS I character.

Solution

Deprecate the two methods and recommend using the corresponding String::toLowerCase(Locale), String::toUpperCase(Locale).

Specification

 * ...
 * @deprecated This method is locale-sensitive, and may produce unexpected
 * results if used for strings that are intended to be interpreted locale
 * independently.
 * <p>
 * Instead, to obtain correct results for locale-insensitive strings, use
 * {@code toLowerCase(Locale.ROOT)}; otherwise, use
 * {@code toLowerCase(Locale.getDefault())}.
 */
@Deprecated(since="21")
public String toLowerCase() { ... }

 * ...
 * @deprecated This method is locale-sensitive, and may produce unexpected
 * results if used for strings that are intended to be interpreted locale
 * independently.
 * <p>
 * Instead, to obtain correct results for locale-insensitive strings, use
 * {@code toUpperCase(Locale.ROOT)}; otherwise, use
 * {@code toUpperCase(Locale.getDefault())}.
 */
@Deprecated(since="21")
public String toUpperCase() { ... }