Safe call operator (?.) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

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

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

Kotlin

data class Person(val name: String?, val age: Int?)

fun main() {
    val person: Person? = Person(null, 25)
    println(person?.name ?: "Unknown")
}

A 25

B null

C Compilation error

D Unknown

💡 Hint

The safe call operator returns null if the object is null, and the Elvis operator provides a default value.

The safe call operator '?.' checks if 'person' is null. Since 'person' is not null but 'name' is null, 'person?.name' evaluates to null. The Elvis operator '?:' then returns "Unknown" as the default.

❓ Predict Output

intermediate

What does this Kotlin code print when using safe call operator with a null object?

Analyze the code below and select the output it produces.

Kotlin

fun main() {
    val text: String? = null
    val length = text?.length ?: -1
    println(length)
}

A -1

B 0

C null

D Compilation error

💡 Hint

Safe call returns null if the object is null, then Elvis operator provides fallback.

Since 'text' is null, 'text?.length' is null. The Elvis operator returns -1 as fallback, so the output is -1.

❓ Predict Output

advanced

What is the output of this Kotlin code with chained safe calls?

Given the following Kotlin code, what will be printed?

Kotlin

class Address(val city: String?)
class User(val address: Address?)

fun main() {
    val user: User? = User(null)
    println(user?.address?.city ?: "No city")
}

A No city

B null

C Compilation error

D Throws NullPointerException

💡 Hint

Safe calls chain returns null if any part is null, then Elvis operator provides default.

The 'user' is not null, but 'address' is null. So 'user?.address?.city' evaluates to null, and the Elvis operator returns "No city".

❓ Predict Output

advanced

What error does this Kotlin code raise when using safe call operator incorrectly?

Examine the code and determine what error it produces when run.

Kotlin

fun main() {
    val list: List<String>? = null
    val first = list?.get(0)!!
    println(first)
}

A Compilation error

B IndexOutOfBoundsException

C KotlinNullPointerException

D No error, prints null

💡 Hint

The '!!' operator forces a non-null assertion and throws if the value is null.

Since 'list' is null, 'list?.get(0)' is null. The '!!' operator then throws KotlinNullPointerException.

🧠 Conceptual

expert

How many items are in the resulting list after this Kotlin code runs?

Consider this Kotlin code using safe call operator and list filtering. How many items does 'filtered' contain?

Kotlin

fun main() {
    val list: List<String?> = listOf("apple", null, "banana", null, "cherry")
    val filtered = list.map { it?.length ?: 0 }.filter { it > 5 }
    println(filtered.size)
}

A 3

B 2

C 1

D 0

💡 Hint

Check the length of each string, replace null with 0, then count how many are greater than 5.

Lengths: "apple"=5, null=0, "banana"=6, null=0, "cherry"=6. Filter >5 keeps 6 and 6, so 2 items.