This vs it receiver difference in Kotlin - Visual Side-by-Side Comparison

Concept Flow - This vs it receiver difference

Start function with receiver

↓

Use 'this' to refer to receiver object

↓

Use 'it' in lambda for implicit parameter

↓

Perform actions using 'this' or 'it'

↓

Return or end function

Shows how 'this' refers to the receiver object in extension functions, while 'it' is the implicit name for lambda parameters.

Execution Sample

Kotlin

fun String.showThis() {
    println(this)       // 'this' is the String receiver
    listOf(1, 2).forEach {
        println(it)     // 'it' is each element in the list
    }
}

Prints the receiver string using 'this' and each list element using 'it' inside a lambda.

Execution Table

Step	Code Line	Receiver/Object	'this' Value	'it' Value	Output
1	fun String.showThis() {	String receiver	String receiver	N/A	No output
2	println(this)	String receiver	String receiver	N/A	Hello
3	listOf(1, 2).forEach {	List<Int>	String receiver	1	No output
4	println(it)	List<Int>	String receiver	1	1
5	forEach next iteration	List<Int>	String receiver	2	No output
6	println(it)	List<Int>	String receiver	2	2
7	End of function	String receiver	String receiver	N/A	No output

💡 Function ends after printing receiver string and all list elements.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
this (receiver)	N/A	"Hello"	"Hello"	"Hello"	"Hello"
it (lambda param)	N/A	N/A	1	2	N/A

Key Moments - 2 Insights

Why does 'this' refer to the string but 'it' refers to list elements?

'this' refers to the receiver object of the extension function (the string), while 'it' is the implicit name for each element inside the lambda passed to forEach (the list elements). See execution_table rows 2 and 4.

Can 'this' be used inside the lambda instead of 'it'?

No, inside the lambda 'this' still refers to the outer receiver (the string), not the lambda parameter. 'it' is used for the lambda's current element. See execution_table rows 4 and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'this' at step 4?

A 2

B 1

C "Hello"

D N/A

💡 Hint

Check the 'this' Value column at step 4 in execution_table.

At which step does 'it' first get the value 2?

A Step 6

B Step 3

C Step 5

D Step 4

💡 Hint

Look at the 'it' Value column in execution_table rows 4 and 6.

If we replaced 'println(it)' with 'println(this)' inside the lambda, what would print?

A Each list element (1, then 2)

B The receiver string each time

C Nothing

D Compilation error

💡 Hint

Refer to key_moments about 'this' inside lambda and execution_table rows 4 and 6.

Concept Snapshot

In Kotlin extension functions, 'this' refers to the receiver object.
Inside lambdas, 'it' is the implicit name for the single parameter.
'this' inside a lambda still points to the outer receiver.
Use 'it' to access lambda elements.
This distinction helps avoid confusion when nesting lambdas.

Full Transcript

This visual shows how 'this' and 'it' work differently in Kotlin. 'this' refers to the receiver object of an extension function, like a string. When you use a lambda, like in forEach, 'it' is the implicit name for each element in the list. The execution table traces each step: first printing the receiver string with 'this', then printing each list element with 'it'. The variable tracker shows 'this' stays the same, while 'it' changes per element. Key moments explain why 'this' and 'it' are different and why you can't use 'this' to mean the lambda parameter. The quiz tests understanding by asking about values of 'this' and 'it' at different steps and what happens if you swap them. Remember, 'this' is the receiver, 'it' is the lambda 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