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Extensions

Kotlin provides the ability to extend a class with new functionality without having to inherit from the class or use design patterns such as Decorator. This is done via special declarations called extensions. For example, you can write new functions for a class from a third-party library that you can't modify. Such functions are available for calling in the usual way as if they were methods of the original class. This mechanism is called extension functions. There are also extension properties that let you define new properties for existing classes.

Extension functions

To declare an extension function, we need to prefix its name with a receiver type, i.e. the type being extended. The following adds a swap function to MutableList<Int>:

fun MutableList<Int>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}

The this keyword inside an extension function corresponds to the receiver object (the one that is passed before the dot). Now, we can call such a function on any MutableList<Int>:

val list = mutableListOf(1, 2, 3)
list.swap(0, 2) // 'this' inside 'swap()' will hold the value of 'list'

Of course, this function makes sense for any MutableList<T>, and we can make it generic:

fun <T> MutableList<T>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}

We declare the generic type parameter before the function name for it to be available in the receiver type expression. See Generic functions.

Extensions are resolved statically

Extensions do not actually modify classes they extend. By defining an extension, you do not insert new members into a class, but merely make new functions callable with the dot-notation on variables of this type.

We would like to emphasize that extension functions are dispatched statically, i.e. they are not virtual by receiver type. This means that the extension function being called is determined by the type of the expression on which the function is invoked, not by the type of the result of evaluating that expression at runtime. For example:

fun main() {
//sampleStart
    open class Shape
    
    class Rectangle: Shape()
    
    fun Shape.getName() = "Shape"
    
    fun Rectangle.getName() = "Rectangle"
    
    fun printClassName(s: Shape) {
        println(s.getName())
    }    
    
    printClassName(Rectangle())
//sampleEnd
}

This example prints "Shape", because the extension function being called depends only on the declared type of the parameter s, which is the Shape class.

If a class has a member function, and an extension function is defined which has the same receiver type, the same name, and is applicable to given arguments, the member always wins. For example:

fun main() {
//sampleStart
    class Example {
        fun printFunctionType() { println("Class method") }
    }
    
    fun Example.printFunctionType() { println("Extension function") }
    
    Example().printFunctionType()
//sampleEnd
}

This code prints "Class method".

However, it's perfectly OK for extension functions to overload member functions which have the same name but a different signature:

fun main() {
//sampleStart
    class Example {
        fun printFunctionType() { println("Class method") }
    }
    
    fun Example.printFunctionType(i: Int) { println("Extension function") }
    
    Example().printFunctionType(1)
//sampleEnd
}

Nullable receiver

Note that extensions can be defined with a nullable receiver type. Such extensions can be called on an object variable even if its value is null, and can check for this == null inside the body. This is what allows you to call toString() in Kotlin without checking for null: the check happens inside the extension function.

fun Any?.toString(): String {
    if (this == null) return "null"
    // after the null check, 'this' is autocast to a non-null type, so the toString() below
    // resolves to the member function of the Any class
    return toString()
}

Extension properties

Similarly to functions, Kotlin supports extension properties:

val <T> List<T>.lastIndex: Int
    get() = size - 1

Note that, since extensions do not actually insert members into classes, there's no efficient way for an extension property to have a backing field. This is why initializers are not allowed for extension properties. Their behavior can only be defined by explicitly providing getters/setters.

Example:

val House.number = 1 // error: initializers are not allowed for extension properties

Companion object extensions

If a class has a companion object defined, you can also define extension functions and properties for the companion object. Just like regular members of the companion object, they can be called using only the class name as the qualifier:

class MyClass {
    companion object { }  // will be called "Companion"
}

fun MyClass.Companion.printCompanion() { println("companion") }

fun main() {
    MyClass.printCompanion()
}

Scope of extensions

Most of the time we define extensions on the top level - directly under packages:

package org.example.declarations
 
fun List<String>.getLongestString() { /*...*/}

To use such an extension outside its declaring package, we need to import it at the call site:

package org.example.usage

import org.example.declarations.getLongestString

fun main() {
    val list = listOf("red", "green", "blue")
    list.getLongestString()
}

See Imports for more information.

Declaring extensions as members

Inside a class, you can declare extensions for another class. Inside such an extension, there are multiple implicit receivers - objects members of which can be accessed without a qualifier. The instance of the class in which the extension is declared is called dispatch receiver, and the instance of the receiver type of the extension method is called extension receiver.

class Host(val hostname: String) {
    fun printHostname() { print(hostname) }
}

class Connection(val host: Host, val port: Int) {
     fun printPort() { print(port) }

     fun Host.printConnectionString(p: Int) {
         printHostname()   // calls Host.printHostname()
         print(":")
         printPort()   // calls Connection.printPort()
     }

     fun connect() {
         /*...*/
         host.printConnectionString(port)   // calls the extension function
     }
}

fun main() {
    Connection(Host("kotl.in"), 443).connect()
    //Host("kotl.in").printConnectionString(443)  // error, the extension function is unavailable outside Connection
}

In case of a name conflict between the members of the dispatch receiver and the extension receiver, the extension receiver takes precedence. To refer to the member of the dispatch receiver you can use the qualified this syntax.

class Connection {
    fun Host.getConnectionString() {
        toString()         // calls Host.toString()
        this@Connection.toString()  // calls Connection.toString()
    }
}

Extensions declared as members can be declared as open and overridden in subclasses. This means that the dispatch of such functions is virtual with regard to the dispatch receiver type, but static with regard to the extension receiver type.

open class Base { }

class Derived : Base() { }

open class BaseCaller {
    open fun Base.printFunctionInfo() {
        println("Base extension function in BaseCaller")
    }

    open fun Derived.printFunctionInfo() {
        println("Derived extension function in BaseCaller")
    }

    fun call(b: Base) {
        b.printFunctionInfo()   // call the extension function
    }
}

class DerivedCaller: BaseCaller() {
    override fun Base.printFunctionInfo() {
        println("Base extension function in DerivedCaller")
    }

    override fun Derived.printFunctionInfo() {
        println("Derived extension function in DerivedCaller")
    }
}

fun main() {
    BaseCaller().call(Base())   // "Base extension function in BaseCaller"
    DerivedCaller().call(Base())  // "Base extension function in DerivedCaller" - dispatch receiver is resolved virtually
    DerivedCaller().call(Derived())  // "Base extension function in DerivedCaller" - extension receiver is resolved statically
}

Note on visibility

Extensions utilize the same visibility of other entities as regular functions declared in the same scope would. For example:

  • An extension declared on top level of a file has access to the other private top-level declarations in the same file;
  • If an extension is declared outside its receiver type, such an extension cannot access the receiver's private members.