This vs it receiver difference in Kotlin - Quick Revision & Key Differences

Recall & Review

beginner

What does the this keyword refer to in Kotlin lambdas?

<p><code>this</code> refers to the instance of the class where the lambda is defined, not the lambda itself.</p>

beginner

What is the role of it in Kotlin lambdas?

it is the implicit name of the single parameter passed to a lambda when no explicit name is given.

intermediate

How does this differ from it in Kotlin lambda expressions?

<p><code>this</code> refers to the outer class or receiver object, while <code>it</code> refers to the lambda's single input parameter.</p>

beginner

In Kotlin, when do you use it instead of naming the lambda parameter?

When the lambda has exactly one parameter and you don't explicitly name it, Kotlin uses it as the default name.

intermediate

Can this be used inside a lambda to refer to the lambda itself?

<p>No, <code>this</code> always refers to the outer receiver or class instance, not the lambda. To refer to the lambda parameter, use <code>it</code> or name it explicitly.</p>

In a Kotlin lambda with one parameter, what does it represent?

A The single parameter passed to the lambda

B The outer class instance

C The lambda itself

D A keyword to refer to the receiver object

it is the implicit name for the single parameter of a lambda when no explicit name is given.

What does this refer to inside a Kotlin lambda expression?

A The lambda's single parameter

B A local variable inside the lambda

C The lambda itself

D The outer class or receiver object

this refers to the outer class or receiver object, not the lambda parameter.

If you want to refer to the lambda's parameter explicitly, what should you do?

A Use <code>it</code> or name the parameter explicitly

B Use <code>self</code>

C Use <code>this</code>

D Use <code>lambda</code>

You can use it if the lambda has one parameter and no explicit name, or you can name the parameter explicitly.

Which keyword refers to the instance of the class where the lambda is defined?

A it

B this

C self

D lambda

this refers to the instance of the class or the receiver object where the lambda is defined.

When is it NOT available in a Kotlin lambda?

A When the lambda has exactly one parameter

B When the lambda is inside a class

C When the lambda has no parameters

D When <code>this</code> is used

it is only available when the lambda has exactly one parameter and no explicit name is given.

Explain the difference between this and it in Kotlin lambdas.

Think about what each keyword points to inside and outside the lambda.

When should you use it and when should you use this inside Kotlin lambda expressions?

Consider the context of the lambda and the class it is in.

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