This vs it receiver difference in Kotlin

Introduction

We use this and it to refer to objects inside special blocks of code. They help us work with objects easily without repeating their names.

When you want to access the current object inside a class or function, use <code>this</code>.

When you use a lambda with one parameter and want a short name for it, use <code>it</code>.

When you want to call methods or properties on an object inside a <code>with</code> or <code>apply</code> block, use <code>this</code>.

When you want to refer to the single parameter of a lambda without naming it, use <code>it</code>.

Syntax

Kotlin

fun example() {
    val list = listOf(1, 2, 3)
    list.forEach {
        println(it) // 'it' is the current item
    }

    class Person {
        fun greet() {
            println(this) // 'this' is the current Person object
        }
    }
}

this refers to the current object or receiver in a class or lambda with receiver.

it is the default name for a single parameter in a lambda when no name is given.

Examples

Here, it is each item in the list during the loop.

Kotlin

val numbers = listOf(1, 2, 3)
numbers.forEach {
    println(it) // 'it' is each number
}

this refers to the current Dog object inside the bark function.

Kotlin

class Dog(val name: String) {
    fun bark() {
        println(this.name + " says Woof!")
    }
}

val dog = Dog("Buddy")
dog.bark()

this refers to the string inside the with block.

Kotlin

val result = with("hello") {
    println(this.uppercase()) // 'this' is the string "hello"
    length
}

it is used as the default name for the lambda parameter.

Kotlin

val doubled = listOf(1, 2, 3).map {
    it * 2 // 'it' is each number
}

Sample Program

This program shows how it refers to each item in a list inside a lambda, and how this refers to the current object inside a class method.

Kotlin

fun main() {
    val names = listOf("Anna", "Bob", "Cathy")

    // Using 'it' in a lambda
    names.forEach {
        println("Hello, $it!")
    }

    // Using 'this' inside a class
    class Person(val name: String) {
        fun greet() {
            println("Hi, I am ${this.name}.")
        }
    }

    val person = Person("David")
    person.greet()
}

Important Notes

it is only available when the lambda has exactly one parameter and you don't name it explicitly.

You can use this to clarify which object you mean if there are multiple objects in scope.

Summary

this refers to the current object or receiver.

it is the default name for a single lambda parameter.

Use this inside classes or lambdas with receivers, and it inside simple lambdas with one parameter.

Practice

1. In Kotlin, what does this refer to inside a class method?

easy

A. The current instance of the class

B. The first parameter of a lambda

C. A global variable

D. A static method

Solution

1. Step 1: Understand this in class context

   Inside a class method, this refers to the current object instance that called the method.

2. Step 2: Differentiate from lambda parameter

   this is not a lambda parameter; that role is usually played by it in single-parameter lambdas.

3. Final Answer:

   The current instance of the class -> Option A

4. Quick Check:

   this in class = current object [OK]

Hint: Remember: this means current object in classes [OK]

Common Mistakes:

• Confusing this with lambda parameter it
• Thinking this is a global variable
• Assuming this refers to static context

2. Which of the following is the correct way to refer to the implicit lambda parameter in Kotlin when there is only one parameter?

easy

A. this

B. it

C. self

D. lambda

Solution

1. Step 1: Identify default lambda parameter

   In Kotlin, when a lambda has a single parameter and no explicit name, it is referred to as it.

2. Step 2: Confirm this is not used here

   this refers to the receiver object, not the lambda parameter.

3. Final Answer:

   it -> Option B

4. Quick Check:

   Single lambda parameter = it [OK]

Hint: Single lambda parameter? Use it [OK]

Common Mistakes:

• Using this instead of it in lambdas
• Trying to name the parameter without declaration
• Assuming self is valid in Kotlin

3. What will be the output of the following Kotlin code?

class Box(val value: Int) {
    fun printValue() {
        listOf(1).forEach {
            println(this.value + it)
        }
    }
}

fun main() {
    Box(5).printValue()
}

medium

A. 6

B. 1

C. 5

D. Compilation error

Solution

1. Step 1: Understand this inside lambda with receiver

   Inside printValue, this refers to the Box instance, so this.value is 5.

2. Step 2: Calculate the sum inside lambda

   The lambda parameter it is 1 from the list. So, this.value + it = 5 + 1 = 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   this.value + it = 6 [OK]

Hint: Inside lambda, this is outer object, it is lambda param [OK]

Common Mistakes:

• Thinking this refers to lambda receiver
• Confusing it with this
• Expecting compilation error due to this usage

4. Identify the error in the following Kotlin code snippet:

fun main() {
    val numbers = listOf(1, 2, 3)
    numbers.forEach {
        println(this + it)
    }
}

medium

A. Missing lambda parameter name

B. Incorrect list declaration

C. Using this inside lambda without receiver causes error

D. No error, code runs fine

Solution

1. Step 1: Check this usage inside lambda

   In this lambda, there is no receiver object, so this is not defined and causes an error.

2. Step 2: Confirm lambda parameter usage

   it is valid as the lambda parameter, but this cannot be used without a receiver context.

3. Final Answer:

   Using this inside lambda without receiver causes error -> Option C

4. Quick Check:

   this needs receiver, else error [OK]

Hint: No receiver? Don't use this inside lambda [OK]

Common Mistakes:

• Assuming this always refers to lambda parameter
• Ignoring that this requires receiver context
• Thinking code runs without error

5. Given the following Kotlin code, what will be printed?

class Calculator {
    fun calculate() {
        val numbers = listOf(10, 20)
        numbers.run {
            println(this.sum())
            numbers.forEach {
                println(it + this.sum())
            }
        }
    }
}

fun main() {
    Calculator().calculate()
}

hard

A. 10\n20\n30

B. Compilation error due to ambiguous this

C. 30\n30\n30

D. 30\n40\n50

Solution

1. Step 1: Understand this inside run block

   Inside run, this refers to the numbers list. So, this.sum() is 30.

2. Step 2: Calculate outputs inside forEach

   For each it in numbers (10 and 20), print it + this.sum(): 10+30=40 and 20+30=50.

3. Step 3: List all printed lines

   First print: 30
   Then prints: 40 and 50

4. Final Answer:

   30 40 50 -> Option D

5. Quick Check:

   this in run = list, sum=30 [OK]

Hint: Inside run, this is receiver object [OK]

Common Mistakes:

• Confusing this inside nested lambdas
• Assuming this refers to Calculator instance
• Expecting compilation error due to this