This vs it receiver difference in Kotlin - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of 'this' vs 'it' in Kotlin lambda

What is the output of the following Kotlin code snippet?

Kotlin

class Example {
    fun printThis() {
        val numbers = listOf(1, 2, 3)
        numbers.forEach {
            println("it: $it")
        }
        numbers.forEach { number ->
            println("number: $number")
        }
    }
}

fun main() {
    val example = Example()
    example.printThis()
}

A

it: 1
it: 2
it: 3
number: 1
number: 2
number: 3

B

this: 1
this: 2
this: 3
number: 1
number: 2
number: 3

C

it: 1
it: 2
it: 3
this: 1
this: 2
this: 3

D

this: 1
this: 2
this: 3
this: 1
this: 2
this: 3

💡 Hint

Remember that 'it' is the implicit name of the single parameter in a lambda when no explicit name is given.

In Kotlin, inside a lambda with one parameter, 'it' refers to that parameter implicitly. When you name the parameter explicitly (like 'number'), you use that name instead of 'it'. 'this' refers to the instance of the enclosing class, not the lambda parameter.

🧠 Conceptual

intermediate

Difference between 'this' and 'it' in Kotlin lambdas

Which statement correctly describes the difference between 'this' and 'it' inside a Kotlin lambda expression?

A 'it' refers to the enclosing class instance, while 'this' refers to the lambda's parameter.

B 'this' and 'it' both refer to the lambda's single parameter but in different contexts.

C 'this' refers to the lambda's receiver object, while 'it' refers to the lambda's single parameter.

D 'this' is always null inside lambdas, and 'it' is the lambda's receiver.

💡 Hint

Think about what 'this' usually means in Kotlin and what 'it' is used for in lambdas.

'this' inside a lambda with receiver refers to the receiver object of the lambda. 'it' is the implicit name of the single parameter passed to a lambda without an explicit parameter name.

❓ Predict Output

advanced

Output of nested lambdas using 'this' and 'it'

What is the output of this Kotlin code?

Kotlin

class Outer {
    val name = "Outer"
    fun run() {
        val inner = Inner()
        inner.action {
            println(this.name)
            println(it)
        }
    }
    inner class Inner {
        val name = "Inner"
        fun action(block: Inner.(String) -> Unit) {
            this.block("Parameter")
        }
    }
}

fun main() {
    val outer = Outer()
    outer.run()
}

A

Outer
this

B

Outer
Parameter

C

Inner
this

D

Inner
Parameter

💡 Hint

Remember that 'this' inside the lambda with receiver refers to the receiver object, and 'it' is the lambda's parameter.

The lambda is an extension function on Inner, so 'this' refers to the Inner instance. 'it' is the single parameter passed to the lambda, which is the string "Parameter".

🔧 Debug

advanced

Identify the error in 'this' vs 'it' usage

What error will this Kotlin code produce?

Kotlin

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

A Error: Unresolved reference: this

B Prints the list elements

C Error: 'this' cannot be used inside lambda

D Prints the list object reference

💡 Hint

Consider what 'this' refers to inside a lambda without a receiver.

Inside a lambda without a receiver, 'this' refers to the enclosing class or function. In this case, 'main' is a top-level function, so 'this' is not defined, causing an unresolved reference error.

🚀 Application

expert

Using 'this' as receiver and 'it' as parameter in Kotlin

Given the following Kotlin code, what will be the output?

Kotlin

class Greeter(val greeting: String) {
    fun greet() {
        val printGreeting: Greeter.(String) -> Unit = {
            println("$greeting, $it!")
        }
        this.printGreeting("World")
    }
}

fun main() {
    val greeter = Greeter("Hello")
    greeter.greet()
}

A World, Hello!

B Hello, World!

C Hello, it!

D Error: Cannot access 'it' in this context

💡 Hint

Remember that in a lambda with receiver, 'this' refers to the receiver object and 'it' is the single parameter.

The lambda 'printGreeting' is an extension function on Greeter with a single String parameter. Inside it, 'greeting' is accessed via 'this' (the Greeter instance), and 'it' is the parameter passed when calling the lambda.

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