Null safety in collections in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of safe call with nullable list elements

What is the output of this Kotlin code snippet?

Kotlin

val list: List<String?> = listOf("apple", null, "banana")
val result = list.map { it?.length ?: 0 }
println(result)

A [5, -1, 6]

B [5, null, 6]

C [5, 1, 6]

D [5, 0, 6]

💡 Hint

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

The list contains nullable strings. For each element, we get its length if not null; otherwise, 0. So null becomes 0.

❓ Predict Output

intermediate

Filtering nulls from a list

What will be printed by this Kotlin code?

Kotlin

val items: List<String?> = listOf("cat", null, "dog", null, "bird")
val filtered = items.filterNotNull()
println(filtered)

A [cat, dog, bird]

B [cat, null, dog, null, bird]

C [cat, dog, null, bird]

D [null, null]

💡 Hint

filterNotNull() removes all null elements from the list.

filterNotNull() returns a list with only non-null elements, so all nulls are removed.

🔧 Debug

advanced

Identify the error with nullable map keys

What error does this Kotlin code produce?

Kotlin

val map: Map<String?, Int> = mapOf(null to 1, "two" to 2)
println(map[null])

A Compilation error: Null can not be a key in a Map

B Runtime NullPointerException

C Prints 1

D Compilation error: Map keys must be non-nullable

💡 Hint

Kotlin allows nullable keys in maps, but accessing them requires care.

Kotlin permits null keys in maps. The code prints the value for the null key, which is 1.

❓ Predict Output

advanced

Result of safe access on nullable list

What is the output of this Kotlin code?

Kotlin

val list: List<String>? = null
val size = list?.size ?: -1
println(size)

A Throws NullPointerException

B -1

C null

D 0

💡 Hint

The safe call operator ?. returns null if the object is null, then Elvis operator ?: provides a default.

list is null, so list?.size is null, and ?: returns -1 as default.

❓ Predict Output

expert

Count non-null values in a nullable map

What is the output of this Kotlin code?

Kotlin

val map: Map<String, String?> = mapOf("a" to "apple", "b" to null, "c" to "cat")
val count = map.values.count { it != null }
println(count)

A 2

B 3

C 1

D Compilation error

💡 Hint

Count how many values in the map are not null.

Only "apple" and "cat" are non-null values, so count is 2.