Nullable types with ? suffix in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code with nullable types?

Consider the following Kotlin code snippet. What will it print when run?

Kotlin

fun main() {
    val name: String? = null
    println(name?.length ?: "No name")
}

A No name

B 0

C null

D Throws NullPointerException

💡 Hint

Look at the safe call operator ?. and the Elvis operator ?:

The variable name is nullable and set to null. The safe call name?.length returns null because name is null. The Elvis operator ?: then returns the string "No name" instead of null. So the output is No name.

🧠 Conceptual

intermediate

What does the ? suffix mean in Kotlin type declarations?

In Kotlin, what does adding a question mark (?) after a type mean?

A The variable can hold only non-null values of that type

B The variable can hold null or values of that type

C The variable is immutable

D The variable is a constant

💡 Hint

Think about null safety and how Kotlin handles null values.

Adding ? after a type means the variable can hold either a value of that type or null. This is Kotlin's way to explicitly allow nulls and avoid null pointer exceptions.

🔧 Debug

advanced

What error does this Kotlin code raise?

What error will this Kotlin code produce when compiled or run?

Kotlin

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

A Compilation error: Null can not be a value of a non-null type String

B NullPointerException at runtime

C Compilation error: Unresolved reference 'length'

D Prints 0

💡 Hint

Check the type declaration and the assigned value.

The variable text is declared as a non-nullable String but assigned null. Kotlin does not allow this and will produce a compilation error.

❓ Predict Output

advanced

What is the output of this Kotlin code using safe call and let?

What will this Kotlin program print?

Kotlin

fun main() {
    val number: Int? = 10
    number?.let {
        println(it * 2)
    } ?: println("No number")
}

A Throws NullPointerException

B No number

C null

D 20

💡 Hint

The safe call with let runs the block only if the value is not null.

The variable number is 10, not null. The safe call number?.let executes the lambda, printing 10 * 2 = 20. The Elvis operator is ignored because the value is not null.

🧠 Conceptual

expert

How many items are in the list after filtering nulls with ?. operator?

Given this Kotlin code, how many items does the list nonNullNames contain?

Kotlin

fun main() {
    val names: List<String?> = listOf("Anna", null, "Bob", null, "Cara")
    val nonNullNames = names.mapNotNull { it?.uppercase() }
    println(nonNullNames.size)
}

A 0

B 2

C 3

D 5

💡 Hint

mapNotNull removes null results after applying the lambda.

The list names has 5 items, 2 of which are null. The lambda it?.uppercase() returns uppercase strings for non-null items and null for nulls. mapNotNull filters out the nulls, so nonNullNames contains 3 items.