Kotlin Extension Functions - Adding New Features

Extension functions in Kotlin allow you to add new features or behavior to existing classes without modifying their source code. In this guide, we'll explore extension functions, how to create them, and their practical use cases.

Creating an Extension Function

To create an extension function, define a function outside the class and use the class name as the receiver type. Here's an example that adds a function to the `String` class:

fun String.removeSpaces(): String {
return this.replace(" ", "")

In this example, we've added an `removeSpaces` extension function to the `String` class.

Using Extension Functions

You can use extension functions on objects of the extended class as if they were regular member functions:

val sentence = "This is a sample sentence."
val cleanedSentence = sentence.removeSpaces()

The `removeSpaces` extension function is called on the `sentence` string, removing spaces from it.

Extension Functions for Existing Classes

You can create extension functions for classes you don't own or can't modify. For example, you can add functions to the `List` class:

fun List<Int>.customSum(): Int {
var sum = 0
for (number in this) {
sum += number
return sum

Now, you can use the `customSum` extension function on lists of integers:

val numbers = listOf(1, 2, 3, 4, 5)
val sum = numbers.customSum()
println("Custom Sum: $sum")

Extensions for Libraries and Frameworks

Extension functions are widely used to add utility methods to libraries and frameworks, enhancing their functionality without modifying their source code. This makes them a powerful tool for code organization and reuse.


Kotlin extension functions are a versatile way to add new features and behavior to existing classes, whether you own the code or not. They help keep your code clean and organized, allowing you to extend functionality without cluttering class hierarchies or modifying external libraries.

Happy coding!