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.

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