Swap case

The practical use is fixing text typed with Caps Lock on, which comes out as the exact inverse of what was intended — hELLO instead of Hello, because the shift key you pressed for the capital produced a lowercase letter. Swapping the case restores it exactly, which no other conversion does: uppercasing loses the original capitals and lowercasing loses them too.

Swapping is its own inverse — do it twice and you have the original text back. That makes it safe to try, unlike converting to upper or lower case, which throws information away permanently. If you have already uppercased a paragraph, no tool can tell you where the capitals were.

A few characters do not round-trip, and they are worth knowing about. The German ß uppercases to SS — two characters — and SS lowercases to ss, so ß does not survive. Turkish has a dotted and a dotless i that map differently from English, which is why case conversion is locale-sensitive and why "I".toLowerCase() gives ı rather than i in a Turkish locale. This uses the default locale rules, which is right for English and wrong for Turkish.