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.

Practice

1. What does the apply function do in Kotlin?

easy

A. Runs code on an object and returns the same object

B. Creates a new object and returns it

C. Runs code and returns the last expression result

D. Runs code without returning anything

Solution

1. Step 1: Understand the purpose of apply

   The apply function executes a block of code on the object it is called on.

2. Step 2: Identify the return value of apply

   It always returns the original object after applying the block, not a new value.

3. Final Answer:

   Runs code on an object and returns the same object -> Option A

4. Quick Check:

   apply returns the object itself [OK]

Hint: Remember: apply returns the original object after changes [OK]

Common Mistakes:

• Thinking apply returns the last expression result
• Confusing apply with let or run
• Assuming apply creates a new object

2. Which of the following is the correct syntax to use apply on an object person to set its name property to "Alice"?

easy

A. person.apply { name = "Alice" }

B. apply(person) { name = "Alice" }

C. person.apply(name = "Alice")

D. person.apply { setName("Alice") }

Solution

1. Step 1: Identify the correct lambda syntax for apply

   apply takes a lambda with receiver where this is the object, so properties can be set directly.

2. Step 2: Check each option's syntax

   person.apply { name = "Alice" } correctly uses person.apply { name = "Alice" }. apply(person) { name = "Alice" } is invalid syntax. person.apply(name = "Alice") wrongly passes a parameter. person.apply { setName("Alice") } assumes a setter method which may not exist.

3. Final Answer:

   person.apply { name = "Alice" } -> Option A

4. Quick Check:

   Use object.apply { property = value } syntax [OK]

Hint: Use curly braces with apply to set properties inside [OK]

Common Mistakes:

• Passing parameters directly to apply
• Using parentheses instead of curly braces
• Calling non-existent setter methods

3. What is the output of this Kotlin code?

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

medium

A. Compilation error

B. []

C. null

D. [1, 2, 3]

Solution

1. Step 1: Understand what apply does on the list

   The apply block adds 1, 2, and 3 to the mutable list.

2. Step 2: Check the returned value of apply

   apply returns the original list after adding the elements, so list contains [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   apply adds elements and returns the list [OK]

Hint: apply returns the object after running code inside [OK]

Common Mistakes:

• Expecting apply to return last added element
• Thinking list is empty after apply
• Confusing apply with run or let

4. Identify the error in this Kotlin code using apply:

val map = mutableMapOf()
map.apply {
    put("a", 1)
    put("b", 2)
}
println(map.get("c"))

medium

A. Syntax error in apply block

B. Compilation error due to missing return

C. No error; output is null

D. Runtime error: key "c" not found

Solution

1. Step 1: Check the apply block usage

   The apply block correctly adds two entries to the map.

2. Step 2: Understand the map.get("c") call

   Since key "c" does not exist, get returns null, which is printed without error.

3. Final Answer:

   No error; output is null -> Option C

4. Quick Check:

   Accessing missing key returns null, no error [OK]

Hint: Missing keys in map.get return null, no crash [OK]

Common Mistakes:

• Expecting runtime error for missing key
• Thinking apply must return a value explicitly
• Confusing map access syntax

5. You want to create and configure a StringBuilder in Kotlin to build the string "Hello World!" using apply. Which code snippet correctly does this?

hard

A. val result = StringBuilder().apply { append("Hello"); append(" World!") }

B. val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString()

C. val result = StringBuilder().apply { append("Hello World!") }

D. val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString

Solution

1. Step 1: Understand apply returns the object itself

   The apply block configures the StringBuilder, but to get the string, toString() must be called after.

2. Step 2: Check each option's correctness

   val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString() calls toString() after apply, returning the built string. val result = StringBuilder().apply { append("Hello World!") } returns the StringBuilder object, not the string. val result = StringBuilder().apply { append("Hello"); append(" World!") } misses toString(). val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString misses parentheses on toString, causing error.

3. Final Answer:

   val result = StringBuilder().apply { append("Hello"); append(" World!") }.toString() -> Option B

4. Quick Check:

   Call toString() after apply to get string [OK]

Hint: Call toString() after apply to get final string [OK]

Common Mistakes:

• Forgetting to call toString() after apply
• Missing parentheses on toString
• Expecting apply to return string automatically