Nullable types with ? suffix in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code with nullable types?

Consider the following Kotlin code snippet. What will it print when run?

Kotlin

fun main() {
    val name: String? = null
    println(name?.length ?: "No name")
}

A No name

B 0

C null

D Throws NullPointerException

💡 Hint

Look at the safe call operator ?. and the Elvis operator ?:

The variable name is nullable and set to null. The safe call name?.length returns null because name is null. The Elvis operator ?: then returns the string "No name" instead of null. So the output is No name.

🧠 Conceptual

intermediate

What does the ? suffix mean in Kotlin type declarations?

In Kotlin, what does adding a question mark (?) after a type mean?

A The variable can hold only non-null values of that type

B The variable can hold null or values of that type

C The variable is immutable

D The variable is a constant

💡 Hint

Think about null safety and how Kotlin handles null values.

Adding ? after a type means the variable can hold either a value of that type or null. This is Kotlin's way to explicitly allow nulls and avoid null pointer exceptions.

🔧 Debug

advanced

What error does this Kotlin code raise?

What error will this Kotlin code produce when compiled or run?

Kotlin

fun main() {
    val text: String = null
    println(text.length)
}

A Compilation error: Null can not be a value of a non-null type String

B NullPointerException at runtime

C Compilation error: Unresolved reference 'length'

D Prints 0

💡 Hint

Check the type declaration and the assigned value.

The variable text is declared as a non-nullable String but assigned null. Kotlin does not allow this and will produce a compilation error.

❓ Predict Output

advanced

What is the output of this Kotlin code using safe call and let?

What will this Kotlin program print?

Kotlin

fun main() {
    val number: Int? = 10
    number?.let {
        println(it * 2)
    } ?: println("No number")
}

A Throws NullPointerException

B No number

C null

D 20

💡 Hint

The safe call with let runs the block only if the value is not null.

The variable number is 10, not null. The safe call number?.let executes the lambda, printing 10 * 2 = 20. The Elvis operator is ignored because the value is not null.

🧠 Conceptual

expert

How many items are in the list after filtering nulls with ?. operator?

Given this Kotlin code, how many items does the list nonNullNames contain?

Kotlin

fun main() {
    val names: List<String?> = listOf("Anna", null, "Bob", null, "Cara")
    val nonNullNames = names.mapNotNull { it?.uppercase() }
    println(nonNullNames.size)
}

A 0

B 2

C 3

D 5

💡 Hint

mapNotNull removes null results after applying the lambda.

The list names has 5 items, 2 of which are null. The lambda it?.uppercase() returns uppercase strings for non-null items and null for nulls. mapNotNull filters out the nulls, so nonNullNames contains 3 items.

Practice

1. What does adding a ? after a type in Kotlin mean?

easy

A. The variable is immutable.

B. The variable can hold a null value.

C. The variable is a constant.

D. The variable is a function.

Solution

1. Step 1: Understand Kotlin type suffix

   In Kotlin, adding ? after a type means the variable can hold null.

2. Step 2: Compare with non-nullable types

   Without ?, variables cannot be null and will cause errors if assigned null.

3. Final Answer:

   The variable can hold a null value. -> Option B

4. Quick Check:

   Nullable type = variable can be null [OK]

Hint: Remember: ? means nullable type in Kotlin [OK]

Common Mistakes:

• Thinking ? makes variable immutable
• Confusing ? with constant declaration
• Assuming ? means function type

2. Which of the following is the correct syntax to declare a nullable String variable in Kotlin?

easy

A. var name: String?

B. var name: ?String

C. var name: String!

D. var name: nullable String

Solution

1. Step 1: Recall Kotlin nullable syntax

   Kotlin uses ? after the type name to mark it nullable, e.g., String?.

2. Step 2: Check each option

   var name: String? uses String? correctly. Options A, B, and D use invalid syntax.

3. Final Answer:

   var name: String? -> Option A

4. Quick Check:

   Nullable syntax = type followed by ? [OK]

Hint: Put ? right after the type name for nullable [OK]

Common Mistakes:

• Placing ? before the type
• Using ! instead of ?
• Writing 'nullable' keyword explicitly

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

var text: String? = null
println(text?.length ?: "No length")

medium

A. 0

B. null

C. No length

D. Throws NullPointerException

Solution

1. Step 1: Understand safe call and Elvis operator

   text?.length safely accesses length if text is not null; else returns null. The Elvis operator ?: provides "No length" if left side is null.

2. Step 2: Evaluate given code

   Since text is null, text?.length is null, so the expression prints "No length".

3. Final Answer:

   No length -> Option C

4. Quick Check:

   Safe call + Elvis = fallback value [OK]

Hint: Safe call ?. returns null safely; ?: provides default [OK]

Common Mistakes:

• Expecting NullPointerException on null safe call
• Thinking null?.length returns 0
• Ignoring Elvis operator fallback

4. Identify the error in this Kotlin code snippet:

var number: Int? = 10
var result: Int = number + 5

medium

A. Variable 'result' should be nullable.

B. Variable 'number' cannot be nullable.

C. Missing semicolon after declaration.

D. Cannot add Int? and Int directly.

Solution

1. Step 1: Check type compatibility

   number is nullable Int?, but result expects non-null Int. Adding nullable and non-null directly is not allowed.

2. Step 2: Understand Kotlin's null safety

   You must safely unwrap number before adding, e.g., using number ?: 0 or safe call.

3. Final Answer:

   Cannot add Int? and Int directly. -> Option D

4. Quick Check:

   Nullable + non-null Int needs safe unwrap [OK]

Hint: Nullable Int can't be used directly in arithmetic [OK]

Common Mistakes:

• Assuming nullable Int can be used like Int
• Ignoring need for safe call or Elvis operator
• Thinking semicolon is required in Kotlin

5. You have a nullable list of nullable Strings: val items: List?. How do you safely print the length of the first item or print 0 if the list or first item is null?

hard

A. println(items?.get(0)?.length ?: 0)

B. println(items[0].length)

C. println(items!!.get(0)!!.length)

D. println(items?.get(0).length ?: 0)

Solution

1. Step 1: Analyze nullable types

   items can be null, and its elements can be null. So both list and first item need safe calls.

2. Step 2: Use safe calls and Elvis operator

   items?.get(0)?.length ?: 0 safely accesses first item length or returns 0 if null.

3. Final Answer:

   println(items?.get(0)?.length ?: 0) -> Option A

4. Quick Check:

   Safe calls chain + fallback = safe nullable access [OK]

Hint: Chain ?. for each nullable and use ?: for default [OK]

Common Mistakes:

• Using !! which throws exception if null
• Missing safe call on first item
• Assuming list or item can't be null