# Extension Methods

# Potential Pitfall: Extensions are Resolved Statically

The extension method to be called is determined at compile-time based on the reference-type of the variable being accessed. It doesn't matter what the variable's type is at runtime, the same extension method will always be called.

open class Super

class Sub : Super()

fun Super.myExtension() = "Defined for Super"

fun Sub.myExtension() = "Defined for Sub"

fun callMyExtension(myVar: Super) {


The above example will print "Defined for Super", because the declared type of the variable myVar is Super.

# Top-Level Extensions

Top-level extension methods are not contained within a class.

fun IntArray.addTo(dest: IntArray) {
    for (i in 0 .. size - 1) {
        dest[i] += this[i]

Above an extension method is defined for the type IntArray. Note that the object for which the extension method is defined (called the receiver) is accessed using the keyword this.

This extension can be called like so:

val myArray = intArrayOf(1, 2, 3)
intArrayOf(4, 5, 6).addTo(myArray)

# Lazy extension property workaround

Assume you want to create an extension property that is expensive to compute. Thus you would like to cache the computation, by using the lazy property delegate (opens new window) and refer to current instance (this), but you cannot do it, as explained in the Kotlin issues KT-9686 (opens new window) and KT-13053 (opens new window). However, there is an official workaround provided here (opens new window).

In the example, the extension property is color. It uses an explicit colorCache which can be used with this as no lazy is necessary:

class KColor(val value: Int)

private val colorCache = mutableMapOf<KColor, Color>()

val KColor.color: Color
    get() = colorCache.getOrPut(this) { Color(value, true) }

# Sample extending Java 7+ Path class

A common use case for extension methods is to improve an existing API. Here are examples of adding exist, notExists and deleteRecursively to the Java 7+ Path class:

fun Path.exists(): Boolean = Files.exists(this)
fun Path.notExists(): Boolean = !this.exists()
fun Path.deleteRecursively(): Boolean = this.toFile().deleteRecursively()

Which can now be invoked in this example:

val dir = Paths.get(dirName)
if (dir.exists()) dir.deleteRecursively()

# Sample extending long to render a human readable string

Given any value of type Int or Long to render a human readable string:

fun Long.humanReadable(): String {
    if (this <= 0) return "0"
    val units = arrayOf("B", "KB", "MB", "GB", "TB", "EB")
    val digitGroups = (Math.log10(this.toDouble())/Math.log10(1024.0)).toInt();
    return DecimalFormat("#,##0.#").format(this/Math.pow(1024.0, digitGroups.toDouble())) + " " + units[digitGroups];

fun Int.humanReadable(): String {
    return this.toLong().humanReadable()

Then easily used as:


# Sample extending Java 8 Temporal classes to render an ISO formatted string

With this declaration:

fun Temporal.toIsoString(): String = DateTimeFormatter.ISO_INSTANT.format(this)

You can now simply:

val dateAsString = someInstant.toIsoString()

# Using extension functions to improve readability

In Kotlin you could write code like:

val x: Path = Paths.get("dirName").apply { 
    if (Files.notExists(this)) throw IllegalStateException("The important file does not exist")

But the use of apply is not that clear as to your intent. Sometimes it is clearer to create a similar extension function to in effect rename the action and make it more self-evident. This should not be allowed to get out of hand, but for very common actions such as verification:

infix inline fun <T> T.verifiedBy(verifyWith: (T) -> Unit): T {
    return this

infix inline fun <T: Any> T.verifiedWith(verifyWith: T.() -> Unit): T {
    return this

You could now write the code as:

val x: Path = Paths.get("dirName") verifiedWith {
    if (Files.notExists(this)) throw IllegalStateException("The important file does not exist")

Which now let's people know what to expect within the lambda parameter.

Note that the type parameter T for verifiedBy is same as T: Any? meaning that even nullable types will be able to use that version of the extension. Although verifiedWith requires non-nullable.

# Extension functions to Companion Objects (appearance of Static functions)

If you want to extend a class as-if you are a static function, for example for class Something add static looking function fromString, this can only work if the class has a companion object (opens new window) and that the extension function has been declared upon the companion object:

class Something {
    companion object {}

class SomethingElse {

fun Something.Companion.fromString(s: String): Something = ... 

fun SomethingElse.fromString(s: String): SomethingElse = ... 

fun main(args: Array<String>) {
    Something.fromString("") //valid as extension function declared upon the
                             //companion object

    SomethingElse().fromString("") //valid, function invoked on instance not

    SomethingElse.fromString("") //invalid

# Extensions for easier reference View from code

You can use extensions for reference View, no more boilerplate after you created the views.

Original Idea is by Anko Library (opens new window)

# Extensions

inline fun <reified T : View> View.find(id: Int): T = findViewById(id) as T
inline fun <reified T : View> Activity.find(id: Int): T = findViewById(id) as T
inline fun <reified T : View> Fragment.find(id: Int): T = view?.findViewById(id) as T
inline fun <reified T : View> RecyclerView.ViewHolder.find(id: Int): T = itemView?.findViewById(id) as T

inline fun <reified T : View> View.findOptional(id: Int): T? = findViewById(id) as? T
inline fun <reified T : View> Activity.findOptional(id: Int): T? = findViewById(id) as? T
inline fun <reified T : View> Fragment.findOptional(id: Int): T? = view?.findViewById(id) as? T
inline fun <reified T : View> RecyclerView.ViewHolder.findOptional(id: Int): T? = itemView?.findViewById(id) as? T

# Usage

val yourButton by lazy { find<Button>(R.id.yourButtonId) }
val yourText by lazy { find<TextView>(R.id.yourTextId) }
val yourEdittextOptional by lazy { findOptional<EditText>(R.id.yourOptionEdittextId) }

# Syntax

  • fun TypeName.extensionName(params, ...) { /* body */ } // Declaration
  • fun <T: Any> TypeNameWithGenerics.extensionName(params, ...) { /* body */ } // Declaration with Generics
  • myObj.extensionName(args, ...) // invocation

# Remarks

Extensions are resolved statically. This means that the extension method to be used is determined by the reference-type of the variable you are accessing; it doesn't matter what the variable's type is at runtime, the same extension method will always be called. This is because declaring an extension method doesn't actually add a member to the receiver type.