Accessing elements safely in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of safe access with nullable list?

Consider the following Kotlin code that tries to access an element safely from a nullable list. What will be printed?

Kotlin

val list: List<String>? = listOf("apple", "banana", "cherry")
val fruit = list?.getOrNull(1) ?: "none"
println(fruit)

A null

B none

C IndexOutOfBoundsException

D banana

💡 Hint

Remember that getOrNull returns null if index is out of range, and Elvis operator ?: provides a default.

The list is not null and has an element at index 1 which is "banana". So list?.getOrNull(1) returns "banana" and the Elvis operator is not used.

❓ Predict Output

intermediate

What happens when accessing a missing key safely in a map?

Given this Kotlin code, what will be the output?

Kotlin

val map = mapOf("a" to 1, "b" to 2)
val value = map["c"] ?: -1
println(value)

A -1

B null

C 2

D KeyNotFoundException

💡 Hint

Accessing a missing key in a map returns null, so the Elvis operator provides the default.

map["c"] returns null because key "c" is missing. The Elvis operator ?: returns -1 instead.

🔧 Debug

advanced

Why does this safe call cause a NullPointerException?

Look at this Kotlin code snippet. It tries to access the length of a nullable string safely but throws an exception. Why?

Kotlin

val text: String? = null
val length = text!!.length
println(length)

A The !! operator forces a non-null assertion and throws NullPointerException if null

B The variable text is not initialized

C The safe call operator ?. is missing, so length is accessed directly

D The println function cannot print null values

💡 Hint

The !! operator is a way to say 'I am sure this is not null'.

Using !! on a null value throws NullPointerException immediately. Safe call ?. should be used to avoid this.

❓ Predict Output

advanced

What is the output when chaining safe calls with let?

What will this Kotlin code print?

Kotlin

val name: String? = "Kotlin"
val length = name?.let { it.length } ?: 0
println(length)

A Name cannot be null

B 0

C 6

D null

💡 Hint

let runs the block only if the value is not null.

name is "Kotlin", so let runs and returns length 6. Elvis operator is not used.

❓ Predict Output

expert

What is the output of nested safe calls with a null intermediate?

Analyze this Kotlin code and determine the output:

Kotlin

data class Person(val pet: Pet?)
data class Pet(val name: String?)

val person: Person? = Person(null)
val petNameLength = person?.pet?.name?.length ?: -1
println(petNameLength)

A 0

B -1

C NullPointerException

D null

💡 Hint

Each safe call returns null if the previous value is null, so the Elvis operator provides the default.

person is not null, but pet is null, so person?.pet?.name?.length returns null. The Elvis operator returns -1.