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Kotlin Extension Functions

An extension function adds a call to a type without changing the type itself. The new function is called with a dot, just like a member function.

An extension on String

shout is written outside the String type and called on a string value. this inside the body is that string.

fun String.shout(): String {
    return this.uppercase() + "!"
}

fun main() {
    val word = "hello"
    println(word.shout())
}

An extension on Int

squared multiplies the receiver by itself. Even a plain number can stand before the dot as the receiver.

fun Int.squared(): Int {
    return this * this
}

fun main() {
    println(7.squared())
    val n = 5
    println(n.squared())
}

An extension on a list type

total adds up the numbers in the receiver list. The call reads like a member of the list.

fun List<Int>.total(): Int {
    return this.sum()
}

fun main() {
    val prices = listOf(10, 25, 15)
    println(prices.total())
}

The declared type decides

The variable pet is declared as Animal, even though it holds a Dog. The print shows the Animal extension, because the declared type picks the extension.

open class Animal(val name: String)

class Dog(name: String) : Animal(name)

fun Animal.label(): String {
    return "Animal named " + this.name
}

fun Dog.label(): String {
    return "Dog named " + this.name
}

fun main() {
    val pet: Animal = Dog("Rex")
    println(pet.label())
}

Example

fun String.withDots(): String {
    return "..." + this + "..."
}

fun Int.isBig(): Boolean {
    return this >= 100
}

fun main() {
    println("news".withDots())
    println(150.isBig())
    println(40.isBig())
}

Key points

  • Write the type name before the function name to make an extension.
  • The value before the dot is the receiver of the call.
  • Inside the body, this stands for the receiver.
  • An extension can sit on a type from the standard library, such as String or Int.
  • The compiler picks an extension from the declared type of the value.
  • An extension does not replace a member function with the same name.

In this example, withDots wraps its string receiver in dots. The isBig extension compares its number receiver with 100. Both calls use a dot and read like member functions.

Note

Extension functions add calls from outside a class, and they do not change the class.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Kotlin Playground

Exercise

Inside an extension function body, what does this stand for?