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.