Extension function in Kotlin. Allows you to add a new function to the system (List, ..) or my (Book, ...) class

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Extension function in Kotlin.

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обновлено: 6 Augusta 2026

Extension functions allow you to add new functions to a system class, such as List, ... or to my own class Book, etc.
An extension function is defined as a regular function, with the class name at the beginning:

fun class_name.function_name(parameters): type
{
...
}

Example

Let's add a new function to a class that can't be changed.
That is, we will add a new function myAverage to the List class of the library Kotlin.

Kotlin

fun List<Float>.myAverage(): Float
{
     return this.sum() / this.count()
}

fun main() {
     val values:List<Float> = listOf(7.0f, 9.0f)

     val value = values.myAverage() // call a new function

     println(value)
     // on the screen we will see 8
}

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fun() { ... } is a function in Kotlin. The function is needed to perform an action.

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Lambda function in Kotlin. Example 1: var myFunc1 : (a:Int, b:Int) -> Int = { p1, p2 -> p1 + p2 };

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Объявления

Объявления

• What is Kotlin?

• Why is IntelliJ IDEA the most popular development environment for Kotlin?

• Download and install IntelliJ IDEA to explore Kotlin

• Create a new console project to explore Kotlin | IntelliJ IDEA

Setting the value

• Declare the variable and set the value in Kotlin | val, var, lateinit, constant: const val, global variable: companion object

null

• null value, use the symbol ? and ?. and !! and ?: and !!. and ?. in Kotlin

Int, Float, Boolean, Char ...

• Integer numbers in Kotlin are: Byte, UByte, Short, UShort, Int, UInt, Long, ULong

• Decimal numbers in Kotlin are: Float, Double

• A flag with values of true or false in Kotlin is: Boolean

• The symbol in Kotlin is: Char

• Convert number to text in Kotlin | Int → String

String

• The string, the text in Kotlin is: String

• Interpolate strings in Kotlin. Example: val address:String = "${street}, ${country}"

• What is the difference between String and StringBuilder ?

Enum

• What is an enumeration (enum) in Kotlin ?

• How do I find enum by value in Kotlin ?

Collections and Arrays

• Collections are lists, dictionaries, and other classes for adding, removing items, searching for items, sorting items. | Kotlin

• Lists in Kotlin. interface List , interface MutableList , class ArrayList

• Create collections list, set, map and an array array in Kotlin

Function

• fun() { ... } is a function in Kotlin. The function is needed to perform an action.

Extension function

• Extension function in Kotlin. Allows you to add a new function to the system (List, ..) or my (Book, ...) class

Lambda function

• Lambda function in Kotlin. Example 1: var myFunc1 : (a:Int, b:Int) -> Int = { p1, p2 -> p1 + p2 };

Class

• What is class in Kotlin? Example: class MyBook { ... } | Primary and secondary constructor | block init

• lateinit is a late initialization for the class field | Kotlin

• class that inherits from interface in Kotlin | Example: class MyBook : IBook { ...}

• Unnamed class that inherits from interface in Kotlin | Example: val book1 = object : IBook { ...}

• Generic, template class in Kotlin | Example: class MyBook[<]T[>] { ... }

Flow

• flow is the data flow in Kotlin. These are the substitution of LiveData in Android Kotlin and the substitution of Observable in RxJava

Writing a program

• The Kotlin console app works like Android Jetpack Compose

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