Text transformations and search boundaries

The idea

Kotlin String values are immutable, so operations such as trim and substring return values rather than changing the original binding’s text. A StringBuilder is appropriate when many fragments need controlled accumulation. Search functions such as indexOf use a sentinel when no match exists; check that result before using it as a slicing boundary. A failed search is not a valid position.

On the JVM, string indices refer to UTF-16 code units rather than necessarily complete displayed characters. Simple ASCII exercises should state that restriction when they manipulate individual positions. Normalization is also a policy: trimming removes surrounding whitespace, while lowercasing changes case and may affect user-visible spelling. Keep the normalized comparison key separate from the original display value when both responsibilities matter.

fun main(){ val raw="  key:value  "; val text=raw.trim(); val split=text.indexOf(':'); println(text.substring(0,split)); println(raw.length > text.length) }

key
true

The normalized copy is searched and sliced; the original string still includes its surrounding spaces.