Lambda with receiver concept in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a lambda with receiver in Kotlin?

A lambda with receiver is a special kind of lambda where you can call methods and access properties of a receiver object directly inside the lambda, without using its name.

intermediate

How do you define a lambda with receiver type in Kotlin?

You define it by specifying the receiver type before the lambda parameters, like ReceiverType.() -> ReturnType. For example, String.() -> Int means a lambda that acts on a String receiver and returns an Int.

beginner

What is the benefit of using a lambda with receiver?

It lets you write cleaner and more readable code by calling receiver's methods directly inside the lambda, similar to how you use this in a class, reducing repetition.

intermediate

How do you call a lambda with receiver?

You call it by providing the receiver object and then invoking the lambda on it. For example, val result = receiver.lambda() where lambda is a lambda with receiver.

beginner

Example: What does this Kotlin code print?

val greet: String.() -> String = { "Hello, $this!" }
println("World".greet())

It prints Hello, World! because the lambda uses the receiver String ("World") as this and returns the greeting.

What keyword represents the receiver object inside a lambda with receiver?

A this

B it

C self

D receiver

Inside a lambda with receiver, the receiver object is accessed using the keyword this.

Which of the following is a correct type for a lambda with receiver that acts on Int and returns String?

A () -> String

B (Int) -> String

C Int.() -> String

D String.() -> Int

The correct syntax for a lambda with receiver is ReceiverType.() -> ReturnType, so Int.() -> String is correct.

What is the main advantage of using lambda with receiver?

A Improves performance by compiling to native code

B Allows calling receiver's methods without naming the receiver

C Enables multi-threading automatically

D Prevents null pointer exceptions

Lambda with receiver lets you call methods and properties of the receiver directly, making code cleaner.

Given val action: String.() -> Unit = { println(this) }, how do you invoke it on "Hello"?

A "Hello".action()

B action.invoke("Hello")

C action("Hello")

D action.call("Hello")

You invoke a lambda with receiver by calling it on the receiver object, like "Hello".action().

In a lambda with receiver, what does this refer to?

A The first parameter

B The lambda itself

C The outer class instance

D The receiver object

this inside a lambda with receiver refers to the receiver object on which the lambda is called.

Explain what a lambda with receiver is and how it differs from a regular lambda.

Think about how you can call methods inside the lambda without naming the object.

Describe how to define and invoke a lambda with receiver in Kotlin with a simple example.

Use a String receiver and a lambda that returns a greeting.

Practice

1. What is the main benefit of using a lambda with receiver in Kotlin?

easy

A. It disables access to the outer scope variables.

B. It automatically creates a new object for each lambda execution.

C. It forces all lambdas to return Unit type.

D. It allows you to call methods on an object without repeating the object name inside the lambda.

Solution

1. Step 1: Understand lambda with receiver purpose

   A lambda with receiver lets you write code blocks that behave as if they belong to an object, so you can call its methods directly.

2. Step 2: Identify the benefit

   This means you don't have to repeat the object name inside the lambda, making code cleaner and easier to read.

3. Final Answer:

   It allows you to call methods on an object without repeating the object name inside the lambda. -> Option D

4. Quick Check:

   Lambda with receiver = cleaner calls [OK]

Hint: Remember: receiver means 'this' inside lambda [OK]

Common Mistakes:

• Thinking it creates new objects automatically
• Assuming it restricts return types
• Believing it blocks outer variables

2. Which of the following is the correct syntax to declare a lambda with receiver of type StringBuilder in Kotlin?

easy

A. val lambda: () -> StringBuilder = { append("Hello") }

B. val lambda: (StringBuilder) -> Unit = { append("Hello") }

C. val lambda: StringBuilder.() -> Unit = { append("Hello") }

D. val lambda: StringBuilder -> Unit = { append("Hello") }

Solution

1. Step 1: Recall lambda with receiver syntax

   A lambda with receiver is declared by specifying the receiver type followed by a dot and then the function type, like StringBuilder.() -> Unit.

2. Step 2: Match syntax with options

   val lambda: StringBuilder.() -> Unit = { append("Hello") } matches this syntax and uses append("Hello") inside the lambda, which is valid for StringBuilder.

3. Final Answer:

   val lambda: StringBuilder.() -> Unit = { append("Hello") } -> Option C

4. Quick Check:

   Receiver syntax = Type.() -> ReturnType [OK]

Hint: Receiver lambdas use Type.() -> ReturnType syntax [OK]

Common Mistakes:

• Confusing parameter lambdas with receiver lambdas
• Missing the dot after the receiver type
• Using arrow without parentheses

3. What is the output of the following Kotlin code?

val buildString: StringBuilder.() -> Unit = {
    append("Hi")
    append(", ")
    append("there!")
}

val result = StringBuilder().apply(buildString).toString()
println(result)

medium

A. Compilation error

B. Hi, there!

C. Hi there!

D. StringBuilder@<hashcode>

Solution

1. Step 1: Understand the lambda with receiver usage

   The lambda buildString is a StringBuilder receiver lambda that appends "Hi", ", ", and "there!" to the StringBuilder.

2. Step 2: Apply lambda and convert to string

   The apply function runs the lambda on a new StringBuilder instance, then toString() converts it to the string "Hi, there!".

3. Final Answer:

   Hi, there! -> Option B

4. Quick Check:

   apply + receiver lambda = built string [OK]

Hint: apply runs lambda on object, returns object [OK]

Common Mistakes:

• Expecting default toString output of StringBuilder
• Missing commas in output
• Thinking lambda returns string directly

4. Identify the error in this Kotlin code using a lambda with receiver:

val printLength: String.() -> Unit = {
    println(length())
}

"Hello".printLength()

medium

A. Cannot call length() as a function; length is a property.

B. Lambda with receiver must return a value.

C. String cannot be a receiver type.

D. Missing parentheses when calling printLength.

Solution

1. Step 1: Check usage of length inside lambda

   In Kotlin, length is a property of String, not a function, so calling length() causes an error.

2. Step 2: Verify other parts

   The lambda correctly uses String as receiver and returns Unit, and calling printLength without parentheses is valid syntax.

3. Final Answer:

   Cannot call length() as a function; length is a property. -> Option A

4. Quick Check:

   Properties accessed without () [OK]

Hint: Remember: properties have no parentheses [OK]

Common Mistakes:

• Using parentheses with properties
• Thinking receiver type can't be String
• Forgetting lambda return type can be Unit

5. You want to create a function buildHtml that uses a lambda with receiver to build an HTML string with nested tags. Which approach correctly uses lambda with receiver to achieve this?

fun buildHtml(block: StringBuilder.() -> Unit): String {
    val sb = StringBuilder()
    sb.block()
    return sb.toString()
}

val html = buildHtml {
    append("")
    append("")
    append("Hello")
    append("")
    append("")
}
println(html)

hard

A. This code correctly uses a lambda with receiver to build the HTML string.

B. The lambda should have a receiver of type HtmlBuilder, not StringBuilder.

C. You must return the lambda result explicitly inside buildHtml.

D. The lambda cannot be called with sb.block(); use block(sb) instead.

Solution

1. Step 1: Analyze the function signature and call

   The function buildHtml takes a lambda with receiver StringBuilder.() -> Unit and calls it on sb using sb.block(), which is correct syntax.

2. Step 2: Check the lambda body and return

   The lambda appends HTML tags to StringBuilder, and the function returns the built string with sb.toString(), which is the expected behavior.

3. Final Answer:

   This code correctly uses a lambda with receiver to build the HTML string. -> Option A

4. Quick Check:

   Lambda with receiver called on object = correct [OK]

Hint: Call receiver lambda with object.lambda() syntax [OK]

Common Mistakes:

• Confusing lambda with receiver and lambda with parameter
• Forgetting to call lambda on the object
• Expecting lambda to return string directly