Nullable receiver extensions in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nullable receiver extension function call

What is the output of this Kotlin code snippet?

Kotlin

fun String?.greet(): String = this?.let { "Hello, $it!" } ?: "Hello, Guest!"

fun main() {
    val name: String? = null
    println(name.greet())
}

A Compilation error

B Hello, !

C Hello, null!

D Hello, Guest!

💡 Hint

Remember that the extension function handles null receivers with the Elvis operator.

The extension function is defined on a nullable String receiver. When the receiver is null, the Elvis operator returns "Hello, Guest!". Since name is null, the output is "Hello, Guest!".

❓ Predict Output

intermediate

Nullable receiver extension with safe call

What will this Kotlin program print?

Kotlin

fun Int?.doubleOrZero(): Int = this?.times(2) ?: 0

fun main() {
    val number: Int? = 5
    println(number.doubleOrZero())
}

A 5

B 10

C 0

D Compilation error

💡 Hint

Check how the extension function handles non-null receivers.

The receiver is 5 (non-null). The function multiplies it by 2, so the output is 10.

🔧 Debug

advanced

Identify the runtime error in nullable receiver extension

What error will this Kotlin code produce when run?

Kotlin

fun String?.firstChar(): Char = this!!.first()

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

A NullPointerException

B No error, prints first character

C IndexOutOfBoundsException

D Compilation error

💡 Hint

Look at the use of the not-null assertion operator (!!) on a null receiver.

The not-null assertion operator (!!) throws NullPointerException if the receiver is null. Here, text is null, so the program crashes with NullPointerException.

📝 Syntax

advanced

Which option correctly defines a nullable receiver extension?

Which of the following Kotlin function definitions correctly declares an extension function with a nullable receiver?

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

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

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

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

💡 Hint

Nullable receiver syntax places the question mark after the type name.

Option A correctly places the question mark after String to indicate a nullable receiver. Other options have invalid syntax.

🚀 Application

expert

Result of chaining nullable receiver extensions

Given these Kotlin extension functions, what is the output of the program?

Kotlin

fun String?.trimOrNull(): String? = this?.trim()?.takeIf { it.isNotEmpty() }

fun String?.exclaim(): String = this?.let { "$it!" } ?: "No input!"

fun main() {
    val input: String? = "   "
    println(input.trimOrNull().exclaim())
}

A +

B !

C No input!

D Compilation error

💡 Hint

Consider how trimOrNull returns null for blank strings and how exclaim handles null receivers.

trimOrNull returns null because the trimmed string is empty. Then exclaim returns "No input!" for null receivers.

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