Extension Functions: Syntax und Dispatch-Regeln

Was sind Extension Functions?

Grundlegende Syntax von Extension Functions

fun String.shout(): String = uppercase() + "!"
println("hello".shout())  // HELLO!
fun Int.isEven() = this % 2 == 0
println(4.isEven())  // true

this in Extensions

fun List<Int>.secondOrNull(): Int? {
    return if (size >= 2) this[1] else null
}
println(listOf(10, 20, 30).secondOrNull())  // 20
println(listOf(5).secondOrNull())           // null

Statisches Dispatching: Extensions werden nicht überschrieben

open class Base
class Child : Base()
fun Base.greet() = "Hello from Base"
fun Child.greet() = "Hello from Child"

val obj: Base = Child()
println(obj.greet())  // "Hello from Base" (static dispatch!)

Extensions vs. Member Functions

class Greeter {
    fun hello() = "member hello"
}
fun Greeter.hello() = "extension hello"  // shadowed!
println(Greeter().hello())  // "member hello"

Extensions für Nullable-Receiver

fun String?.orEmpty() = this ?: ""
fun Any?.toStringSafe() = this?.toString() ?: "null"
println(null.orEmpty())  // ""
println(42.toStringSafe())  // "42"

Sichtbarkeit und Gültigkeitsbereich

// Top-level extension:
fun List<String>.joinUppercase() = joinToString { it.uppercase() }
// Private to file:
private fun String.clean() = trim().lowercase()

Extension Functions für Companion Objects

class User(val name: String) {
    companion object
}
fun User.Companion.guest() = User("Guest")
val guest = User.guest()

Extensions für Standardbibliothekstypen

fun Double.roundTo(decimals: Int): Double {
    val factor = Math.pow(10.0, decimals.toDouble())
    return Math.round(this * factor) / factor
}
println(3.14159.roundTo(2))  // 3.14

Extensions organisieren

// StringExtensions.kt
fun String.titleCase() = split(" ")
    .joinToString(" ") {
        it.replaceFirstChar { c -> c.uppercase() }
    }
println("hello world".titleCase())  // Hello World

Extensions können nicht auf private Member zugreifen

class Secret {
    private val key = "hidden"
    val name = "public"
}
fun Secret.reveal() {
    println(name)  // OK
    // println(key)  // Error: key is private
}

Kurztest

Zusammenfassung