Apply function behavior and use cases in Kotlin

Introduction

The apply function helps you set up or change an object easily by running code inside it. It makes your code shorter and clearer.

When you want to create an object and set many properties at once.

When you want to configure an object right after creating it.

When you want to run multiple commands on the same object without repeating its name.

When you want to improve code readability by grouping related changes.

When you want to return the object itself after applying changes.

Syntax

Kotlin

val result = obj.apply {
    // code to configure obj
    property1 = value1
    methodCall()
}

The apply function runs the block with this as the object.

It returns the original object after running the block.

Examples

Create a Person and set name and age inside apply.

Kotlin

val person = Person().apply {
    name = "Alice"
    age = 30
}

Create a list and add items inside apply without repeating the list name.

Kotlin

val list = mutableListOf<Int>().apply {
    add(1)
    add(2)
    add(3)
}

Use apply to build a string by appending parts step by step.

Kotlin

val builder = StringBuilder().apply {
    append("Hello")
    append(" World")
}

Sample Program

This program creates a Car object and sets its brand and year using apply. Then it prints the details.

Kotlin

data class Car(var brand: String = "", var year: Int = 0)

fun main() {
    val myCar = Car().apply {
        brand = "Toyota"
        year = 2022
    }
    println("Car brand: ${myCar.brand}")
    println("Car year: ${myCar.year}")
}

Important Notes

apply is useful when you want to initialize or modify an object in one place.

Inside the apply block, you can access the object's properties and methods directly using this.

Because apply returns the object itself, you can chain calls or assign it immediately.

Summary

apply runs code on an object and returns the object itself.

It helps write cleaner and shorter code when setting up objects.

Use it to group changes to an object without repeating its name.