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.