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])

Drag options to blanks, or click blank then click option'

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)

Drag options to blanks, or click blank then click option'

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)

Drag options to blanks, or click blank then click option'

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"]

Drag options to blanks, or click blank then click option'

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]

Drag options to blanks, or click blank then click option'

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.

Practice

1. What does Kotlin's null safety feature ensure when working with collections like List or Set?

easy

A. It prevents storing null values in collections declared without nullable types.

B. It automatically converts all null values to empty strings in collections.

C. It allows null values in any collection without restrictions.

D. It disables the use of collections if they contain null values.

Solution

1. Step 1: Understand Kotlin's null safety in collections

   Kotlin distinguishes between nullable and non-nullable types. Collections declared with non-nullable types cannot hold null values.

2. Step 2: Apply this to collection elements

   If a collection is declared as List<String>, it cannot contain null. To allow null, it must be declared as List<String?>.

3. Final Answer:

   It prevents storing null values in collections declared without nullable types. -> Option A

4. Quick Check:

   Null safety = no null in non-nullable collections [OK]

Hint: Non-nullable collections disallow null elements [OK]

Common Mistakes:

• Assuming all collections allow null by default
• Confusing null safety with automatic null replacement
• Thinking null safety disables collections with nulls

2. Which of the following is the correct way to declare a list that can hold nullable integers in Kotlin?

easy

A. val numbers: List<Int> = listOf(1, 2, null)

B. val numbers: List<Int> = listOf(null, null, null)

C. val numbers: List = listOf(1, 2, 3)

D. val numbers: List = listOf(1, 2, null)

Solution

1. Step 1: Identify nullable type declaration

   To allow null in a list, the element type must be nullable, written as Int?.

2. Step 2: Check list initialization

   listOf(1, 2, null) is valid only if the list type is List<Int?>. val numbers: List = listOf(1, 2, null) matches this.

3. Final Answer:

   val numbers: List<Int?> = listOf(1, 2, null) -> Option D

4. Quick Check:

   Nullable type uses Int? for null elements [OK]

Hint: Use Type? to allow nulls in collections [OK]

Common Mistakes:

• Declaring list as List<Int> but adding nulls
• Using List<Int> with null elements causes error
• Confusing nullable element type with nullable list

3. What will be the output of the following Kotlin code?

val list: List = listOf("apple", null, "banana")
val filtered = list.filterNotNull()
println(filtered)

medium

A. ["apple", null, "banana"]

B. [null]

C. ["apple", "banana"]

D. Compilation error

Solution

1. Step 1: Understand filterNotNull() behavior

   This function removes all null elements from a collection, returning a list of non-null elements.

2. Step 2: Apply filterNotNull() on the list

   Original list has "apple", null, "banana". After filtering, only "apple" and "banana" remain.

3. Final Answer:

   ["apple", "banana"] -> Option C

4. Quick Check:

   filterNotNull() removes nulls [OK]

Hint: filterNotNull() removes all nulls from collections [OK]

Common Mistakes:

• Expecting nulls to remain after filterNotNull()
• Thinking filterNotNull() causes compile error
• Confusing filterNotNull() with filter()

4. Identify the error in this Kotlin code snippet:

val list: List<Int> = listOf(1, 2, null, 4)

medium

A. The list declaration syntax is incorrect.

B. The list cannot contain null because Int is non-nullable.

C. The listOf function cannot be used with integers.

D. There is no error; the code compiles fine.

Solution

1. Step 1: Check element type and values

   The list is declared as List<Int>, which means elements cannot be null.

2. Step 2: Identify the presence of null

   The list contains a null value, which is not allowed for non-nullable types, causing a compilation error.

3. Final Answer:

   The list cannot contain null because Int is non-nullable. -> Option B

4. Quick Check:

   Non-nullable list disallows null elements [OK]

Hint: Non-nullable types disallow null values in collections [OK]

Common Mistakes:

• Assuming null is allowed in List<Int>
• Thinking listOf syntax is wrong
• Ignoring Kotlin's null safety rules

5. Given a list of nullable strings val items: List = listOf("a", null, "b", null, "c"), how can you create a map where keys are the strings and values are their lengths, excluding null values safely?

hard

A. items.filterNotNull().associateWith { it.length }

B. items.associateWith { it!!.length }

C. items.associate { it to it.length }

D. items.map { it.length }.toMap()

Solution

1. Step 1: Remove null values safely

   Use filterNotNull() to exclude null elements before mapping.

2. Step 2: Create map with strings as keys and lengths as values

   Use associateWith { it.length } on the filtered list to map each string to its length.

3. Final Answer:

   items.filterNotNull().associateWith { it.length } -> Option A

4. Quick Check:

   Filter nulls first, then associate [OK]

Hint: Filter nulls before mapping to avoid errors [OK]

Common Mistakes:

• Using !! operator risking NullPointerException
• Trying to associate null keys or values
• Mapping lengths without filtering nulls first