Extension functions/properties

This material was written by Aasmund Eldhuset; it is owned by Khan Academy and is licensed for use under CC BY-NC-SA 3.0 US. Please note that this is not a part of Khan Academy's official product offering.

Since you can't modify built-in or third-party classes, you can't directly add functions or properties to them. If you can achieve what you want by only using the public members of a class, you can of course just write a function that takes an instance of the class as a parameter - but sometimes, you'd really like to be able to say x.foo(y) instead of foo(x, y), especially if you want to make a chain of such calls or property lookups: x.foo(y).bar().baz instead of getBaz(bar(foo(x, y))).

There is a nice piece of syntactic sugar that lets you do this: extension functions and extension properties. They look like regular member functions/properties, but they are defined outside of any class - yet they reference the class name and can use this. However, they can only use visible members of the class (typically just the public ones). Behind the scenes, they get compiled down to regular functions that take the target instance as a parameter.

For example, if you work a lot with bytes, you might want to easily get an unsigned byte in the range 0 through 255 instead of the default -128 through 127 (the result will have to be in the form of a Short/Int/Long, though). Byte is a built-in class that you can't modify, but you can define this extension function:

fun Byte.toUnsigned(): Int {
    return if (this < 0) this + 256 else this.toInt()
}

Now, you can do:

val x: Byte = -1
println(x.toUnsigned()) // Prints 255

If you'd rather do x.unsigned, you can define an extension property:

val Byte.unsigned: Int
    get() = if (this < 0) this + 256 else this.toInt()

Keep in mind that this is just syntactic sugar - you're not actually modifying the class or its instances. Therefore, you have to import an extension function/property wherever you want to use it (since it isn't carried along with the instances of the class). For the same reason, you can not override extension members - you can reimplement them for subtypes, but the resolution happens at compile-time based on the static type of the expression you're invoking it on. So if you declare an extension function for Vehicle, and one with the same name and signature for its subclass Car, and you do the following, it's the extension function on Vehicle that will be called, even though v is really a Car:

fun foo(v: Vehicle) = v.extension()
val x = foo(Car())

There are a lot of built-in extension functions/properties in Kotlin - for example, map(), filter(), and the rest of the framework for processing collections in a functional manner is built using extension functions.