Extension Functions: Enhancing Existing Classes

Adding methods to third-party or standard library classes without inheritance

« Collections & Functional Transformations« Previous Kotlin Coroutines & Lightweight Threads »Next »

Extension functions allow you to extend a class with new functionality without having to inherit from the class or use decorator patterns.

Writing Extension Functions

Prefix the function name with the receiver type: fun String.toSlug(): String.

Inside the extension function body, this refers to the receiver object instance.

Extensions do not actually modify the bytecode of the receiver class; they are resolved statically as static utility helper functions under the hood.

String and Date Extension Functions

Try It Yourself »

// Extend the built-in String class with a custom formatting method
fun String.toTitleCase(): String {
    return this.split(" ").joinToString(" ") { word ->
        word.replaceFirstChar { it.uppercase() }
    }
}

fun main() {
    val title = "welcome to codexia android academy"
    // Called as if it were a native method on String!
    println(title.toTitleCase()) // Welcome To Codexia Android Academy
}

Extension functions appear natively in IDE autocomplete.

Google AdSense Slot (inContentBanner)

Set your Publisher ID in src/config/ads.ts to display live ads.

Test Yourself With Exercise

+15 XP

How does Kotlin implement Extension Functions under the hood on the JVM?

« Previous: Collections & Functional Transformations Next: Kotlin Coroutines & Lightweight Threads »