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() { ... }

Practice

1. What is the main benefit of using nullable receiver extensions in Kotlin?

easy

A. They prevent any null values from being assigned.

B. They make variables non-nullable automatically.

C. They convert nullable types to non-nullable types permanently.

D. They allow calling functions on nullable types without explicit null checks.

Solution

1. Step 1: Understand nullable receiver extensions

   Nullable receiver extensions let you add functions to types that can be null, so you can call these functions safely without extra null checks.

2. Step 2: Identify the benefit

   This means you can write cleaner code by calling functions directly on nullable objects without risking null pointer exceptions.

3. Final Answer:

   They allow calling functions on nullable types without explicit null checks. -> Option D

4. Quick Check:

   Nullable receiver extensions = call on nullable safely [OK]

Hint: Nullable extensions handle null inside function, no extra checks needed [OK]

Common Mistakes:

• Thinking they convert nullable to non-nullable permanently
• Assuming they prevent null assignments
• Confusing with regular extensions on non-null types

2. Which of the following is the correct syntax to define a nullable receiver extension function isNullOrEmpty for String? in Kotlin?

easy

A. fun String.isNullOrEmpty(): Boolean = this == null || this.isEmpty()

B. fun String.isNullOrEmpty?(): Boolean = this == null || this.isEmpty()

C. fun String?.isNullOrEmpty(): Boolean = this == null || this.isEmpty()

D. fun String?.isNullOrEmpty?(): Boolean = this == null || this.isEmpty()

Solution

1. Step 1: Identify nullable receiver syntax

   To define an extension on a nullable type, use String? as the receiver type with a question mark after the type name.

2. Step 2: Check function syntax

   The function name should not have a question mark. So fun String?.isNullOrEmpty() is correct.

3. Final Answer:

   fun String?.isNullOrEmpty(): Boolean = this == null || this.isEmpty() -> Option C

4. Quick Check:

   Nullable receiver = question mark after type [OK]

Hint: Nullable receiver extension uses ? after type, not after function name [OK]

Common Mistakes:

• Putting ? after function name instead of receiver type
• Missing ? on receiver type for nullable
• Using non-nullable receiver for nullable extension

3. What will be the output of the following Kotlin code?

fun String?.describe(): String = this ?: "Empty"

fun main() {
  val text1: String? = "Hello"
  val text2: String? = null
  println(text1.describe())
  println(text2.describe())
}

medium

A. Hello\nEmpty

B. Hello\nnull

C. null\nEmpty

D. Empty\nEmpty

Solution

1. Step 1: Understand the extension function behavior

   The function returns the string itself if not null, otherwise returns "Empty".

2. Step 2: Apply to variables

   text1 is "Hello", so describe() returns "Hello". text2 is null, so describe() returns "Empty".

3. Final Answer:

   Hello\nEmpty -> Option A

4. Quick Check:

   Nullable describe returns value or "Empty" [OK]

Hint: ?: operator returns left if not null, else right [OK]

Common Mistakes:

• Assuming null prints as "null" string
• Confusing ?: with !! operator
• Expecting function to throw on null

4. Identify the error in this Kotlin code using a nullable receiver extension:

fun String?.firstChar(): Char = this[0]

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

medium

A. Compilation error: Extension function syntax incorrect

B. Compilation error: Cannot call get(0) on nullable receiver without null check

C. No error, prints first character or null

D. Runtime exception: NullPointerException when calling firstChar on null

Solution

1. Step 1: Analyze extension function code

   The function calls this[0] on a nullable receiver String? without a null check or safe call.

2. Step 2: Understand compiler behavior

   Kotlin compiler detects the unsafe access on nullable receiver and produces a compilation error.

3. Final Answer:

   Compilation error: Cannot call get(0) on nullable receiver without null check -> Option B

4. Quick Check:

   Unsafe access on nullable receiver = compile error [OK]

Hint: Nullable receiver extensions must check for null before use [OK]

Common Mistakes:

• Assuming nullable receiver is auto-checked
• Thinking it causes runtime exception instead of compilation error
• Thinking it compiles and prints null instead of error

5. You want to create a nullable receiver extension safeLength() for String? that returns the string length or zero if null. Which implementation is correct and safe?

1) fun String?.safeLength() = this.length ?: 0
2) fun String?.safeLength() = this?.length ?: 0
3) fun String.safeLength() = this.length ?: 0
4) fun String?.safeLength() = if (this == null) null else this.length

hard

A. Option 2

B. Option 1

C. Option 3

D. Option 4

Solution

1. Step 1: Check each option for null safety

   Option 1 calls this.length directly on nullable this, causing error. Option 3 uses non-nullable receiver, so not valid for nullable. Option 4 returns null instead of Int zero.

2. Step 2: Identify correct safe call usage

   Option 2 uses safe call this?.length which returns length or null, then ?: 0 returns zero if null. This is safe and returns Int.

3. Final Answer:

   fun String?.safeLength() = this?.length ?: 0 -> Option A

4. Quick Check:

   Safe call ?. with ?: zero fallback is correct pattern [OK]

Hint: Use safe call ?. and ?: to handle nullable safely [OK]

Common Mistakes:

• Calling length directly on nullable without safe call
• Using non-nullable receiver for nullable extension
• Returning null instead of zero for null input