This vs it receiver difference in Kotlin - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the name using 'this' receiver inside the class.

Kotlin

class Person(val name: String) {
    fun printName() {
        println([1].name)
    }
}

fun main() {
    val p = Person("Anna")
    p.printName()
}

Drag options to blanks, or click blank then click option'

Ait

Bthis

Cself

Dthat

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'it' instead of 'this' inside class methods.

In Kotlin, this refers to the current object instance inside a class.

2fill in blank

medium

Complete the lambda expression to print the name using the implicit 'it' receiver.

Kotlin

val names = listOf("Anna", "Bob")
names.forEach { [1] ->
    println([1])
}

Drag options to blanks, or click blank then click option'

Aname

Bit

Cthis

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'it' when parameter is explicitly named.

In a lambda with explicit parameter, you name the parameter (here 'name') and use it inside.

3fill in blank

hard

Fix the error by choosing the correct receiver to access the property inside the lambda with receiver.

Kotlin

class Person(val name: String) {
    fun greet() {
        val printName = with(this) {
            {
                println([1].name)
            }
        }
        printName()
    }
}

fun main() {
    val p = Person("Anna")
    p.greet()
}

Drag options to blanks, or click blank then click option'

Aself

Bit

Cthis

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'it' inside 'with' lambda which uses 'this' receiver.

Inside 'with(this)', 'this' refers to the Person object, so use 'this.name'.

4fill in blank

hard

Fill both blanks to correctly use 'this' and 'it' in nested lambdas.

Kotlin

class Person(val name: String) {
    fun greetAll(names: List<String>) {
        names.forEach [1] ->
            println("Hello, $[2]!")
    }
}

fun main() {
    val p = Person("Anna")
    p.greetAll(listOf("Bob", "Carol"))
}

Drag options to blanks, or click blank then click option'

Aname

Bit

Cthis.name

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this.name' inside the lambda instead of the parameter.

In forEach, the lambda parameter is 'it' by default, representing each name. Inside the string, we use 'name' from the lambda parameter.

5fill in blank

hard

Fill all three blanks to correctly use 'this' and 'it' in a nested scope with 'apply' and 'let'.

Kotlin

class Person(var greeting: String) {
    fun greet(name: String) {
        this.apply {
            greeting = "Hello"
            name.let [1] ->
                println("$[2], $[3]!")
        }
    }
}

fun main() {
    val p = Person("")
    p.greet("Bob")
}

Drag options to blanks, or click blank then click option'

Ait

Bgreeting

Dthis.greeting

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'this' and 'it' inside nested lambdas.

Inside 'let', 'it' is the lambda parameter (the name). Inside 'apply', 'this' refers to the Person object, so use 'this.greeting'.

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