Extension functions

Learn extension functions with fun Receiver.ext().

Extension function

An extension function adds a method to an existing type from outside. The declaration puts the receiver type (ReceiverType.) before the function name. The type itself does not change.

Example: fun String.shout(): String { return uppercase() + "!" }. This function adds shout to String. Calling "hi".shout() returns HI!. Extension functions are resolved statically; they do not touch the type's source. This is useful for extending third-party types.

The shout function

Inside an extension function body, this refers to the receiver object. The receiver's own methods can be called directly.

Example: fun String.shout(): String { return uppercase() + "!" }. Here uppercase() actually means this.uppercase(). Calling "hi".shout() returns HI!. Extension functions are used like in-class methods; they do not modify the class source. They are ideal for extending library types.

Exercise The String.shout extension

Define an extension function fun String.shout(): String whose body returns uppercase() + "!". In main print the result of "hi".shout() with println.

Show example solution

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

fun main() {
    println("hi".shout())
}

Test yourself

How is an <strong>extension function</strong> declared?
- fun Receiver.method()
- function Receiver.method()
- extension Receiver.method()
- class Receiver { method() }
Show answer

fun Receiver.method() — An extension function is written as fun Receiver.method().

Complete the extension function definition

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

Show answer

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

Match the extension function parts

fun String.shout

Receiver and method
uppercase()

Receiver method
"hi".shout()

Call
HI!

Return value

Next lesson: Extension properties

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