Extension Functions

Extension functions let you add new methods to existing classes (like String, Int, etc.). If you’ve used implicit classes in Scala, extension functions are just like those.

Example

In the years before Kotlin, I wanted an Android method that would let me write code like this:

if (x.between(1, 10)) ...

I couldn’t do that with Java, so I had to settle for less-readable code that looked like this:

if (between(x, 1, 10)) { ...

But these days with Kotlin I can get what I want with an extension function:

fun Int.between(a: Int, b: Int): Boolean {
    return if (this >= a && this <= b) true else false
}

The REPL shows how to use that function:

>>> 5.between(1, 10)
true

>>> 22.between(1, 10)
false

How to declare an extension function

The key to writing an extension function is that you declare that you want to add a function to a specific data type:

fun Int.between(a: Int, b: Int): Boolean {
    ---

You also specify that you want the function to have this name:

fun Int.between(a: Int, b: Int): Boolean {
        -------

these parameters:

fun Int.between(a: Int, b: Int): Boolean {
                ------  ------

and this return type:

fun Int.between(a: Int, b: Int): Boolean {
                                 -------

In fact, an extension function is just like a normal function, except that it’s preceded by the name of the data type you want the function to be added to, as shown in this example.

An extension function on Any

Here’s an extension function that I add to the Any type:

fun Any.printMe(): Unit = println("I am ${this}")

Because Any is the root of all Kotlin objects — like java.lang.Object in Java — it works on all Kotlin types:

1.printMe()
"Hello".printMe()
true.printMe()

Here’s what those examples look like in the REPL:

>>> 1.printMe()
I am 1

>>> "Hello".printMe()
I am Hello

>>> true.printMe()
I am true