Nullable receiver extensions in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nullable receiver extension in Kotlin?

An extension function where the receiver type can be null, allowing you to call the function on nullable objects safely.

beginner

How do you declare an extension function with a nullable receiver?

Use a question mark after the receiver type, for example: fun String?.myExtension() { ... }.

beginner

Why are nullable receiver extensions useful?

They let you add functions that can be called on null objects without causing errors, making code safer and cleaner.

intermediate

What happens if you call a nullable receiver extension on a null object?

The extension function runs, and inside it, the receiver is null, so you can handle that case safely.

beginner

Example: What does this code print?

fun String?.printLength() {
  if (this == null) println("Null string")
  else println("Length: ${'$'}{this.length}")
}

fun main() {
  val s: String? = null
  s.printLength()
}

It prints: Null string because the receiver is null and the function checks for that.

How do you declare an extension function with a nullable receiver in Kotlin?

A fun String.myFunc() { }

B fun String.myFunc?() { }

C fun ?String.myFunc() { }

D fun String?.myFunc() { }

The question mark goes after the receiver type: String?.

What is the receiver inside a nullable receiver extension when called on null?

A It is null

B It throws an exception

C It is an empty string

D It is a default value

The receiver is null, so you can check for null inside the extension.

Why use nullable receiver extensions instead of normal extensions?

A To override existing functions

B To safely call functions on nullable objects without errors

C To avoid using null checks outside

D To make functions faster

Nullable receiver extensions allow safe calls on null objects.

Which of these is a valid nullable receiver extension function signature?

A fun ?Int.isPositive() = this > 0

B fun Int.isPositive?() = this > 0

C fun Int?.isPositive() = this != null && this > 0

D fun Int.isPositive() = this != null

The question mark goes after the receiver type to make it nullable.

What will this expression evaluate to?

val s: String? = null
s?.length ?: -1

A -1

B 0

C null

D Throws NullPointerException

The safe call returns null, so the Elvis operator returns -1.

Explain what a nullable receiver extension is and why it is useful in Kotlin.

Think about calling functions on variables that might be null.

Write a simple Kotlin extension function with a nullable receiver and explain how it behaves when called on null.

Use fun String?.yourFunction() { ... }