Extension Functions & Properties

Extensions improve syntax without changing a class

An extension adds callable syntax for a type but does not modify that type or gain access to its private members. Resolution is based on the declared receiver type, rather than virtual dispatch among extensions.

Copy codefun String.topicLabel(): String = trim().ifEmpty { "Untitled" }
val String.wordCount: Int
    get() = trim().split(Regex("\\s+")).count { it.isNotEmpty() }

fun main() {
    check(" Kotlin basics ".topicLabel() == "Kotlin basics")
    check("Kotlin basics".wordCount == 2)
}

An extension property cannot introduce a backing field; it computes or delegates through accessible state. A matching actual member takes precedence over an extension. Broad extensions on Any can pollute completion menus and obscure ownership.

Exercise

Write a List<String>.nonBlankTitles() extension returning a normalized new list. Confirm the original collection is unchanged. Compare a top-level helper with the extension version.

Check: use an extension when reading it as an operation on the receiver makes sense, rather than merely to shorten a call.

Reference: Extensions