Null safety in collections in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a list that cannot hold null values.

Kotlin

val numbers: List<Int> = listOf(1, 2, 3, [1])

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A4

Bnull

C"5"

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to add null to a non-nullable list

Using a string instead of an integer

The list is declared to hold only non-null Int values, so adding null is not allowed.

2fill in blank

medium

Complete the code to declare a list that can hold null values.

Kotlin

val nullableNumbers: List<[1]> = listOf(1, null, 3)

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AInt

BAny

CString

DInt?

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-nullable type Int which disallows nulls

Using unrelated types like String

Using Int? allows the list to hold null values along with integers.

3fill in blank

hard

Fix the error in the code to safely access the first element which might be null.

Kotlin

val first: Int? = nullableNumbers.[1](0)

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Aget

BgetOrNull

Cfirst

DelementAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using get which throws an exception if index is invalid

Using first which throws exception if list is empty

getOrNull safely returns the element at the index or null if out of bounds, preventing exceptions.

4fill in blank

hard

Fill both blanks to create a map that holds nullable values and safely access a value.

Kotlin

val map: Map<String, [1]> = mapOf("a" to 1, "b" to null)
val value: [2]? = map["b"]

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AInt?

BInt

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring map values as non-nullable Int when nulls are present

Using wrong types for the value variable

The map holds nullable integers (Int?), and accessing a key returns a nullable Int value.

5fill in blank

hard

Fill all three blanks to create a list of nullable strings and filter out nulls safely.

Kotlin

val items: List<[1]> = listOf("apple", null, "banana")
val filtered: List<[2]> = items.filterNotNull()
val firstItem: [3] = filtered[0]

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AString?

BString

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-nullable type for the original list

Not filtering nulls before accessing elements

The original list holds nullable strings (String?), filtering removes nulls resulting in a list of non-null String, so the first item is a non-null String.