Type inference by the compiler in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code with type inference?

Consider the following Kotlin code snippet. What will be printed when it runs?

Kotlin

fun main() {
    val x = listOf(1, 2, 3).map { it * 2 }
    println(x)
}

A [2, 4, 6]

B [1, 2, 3]

C [1, 4, 9]

D Compilation error

💡 Hint

The map function applies the lambda to each element, doubling it.

The listOf creates a list of integers. The map function multiplies each element by 2, so the output list is [2, 4, 6].

❓ Predict Output

intermediate

What type does the compiler infer for this variable?

Given the Kotlin code below, what is the inferred type of variable result?

Kotlin

val result = listOf("apple", "banana", "cherry").filter { it.startsWith('b') }

A MutableList<String>

B List<String>

C Set<String>

D Array<String>

💡 Hint

The filter function returns the same collection type as the original list but filtered.

The filter function returns a List because the original collection is a List and filter preserves the type.

🔧 Debug

advanced

What error does this Kotlin code produce due to type inference?

Examine the Kotlin code below. What error will the compiler report?

Kotlin

fun main() {
    val x = null
    println(x)
}

A Type inference error: Null can not be a value of a non-null type

B Runtime NullPointerException

C Compilation succeeds and prints 'null'

D Syntax error: missing type declaration

💡 Hint

Kotlin requires explicit type for null without context.

The compiler cannot infer the type of x because null has no type by itself. Kotlin requires explicit type declaration for null values.

❓ Predict Output

advanced

What is the output of this Kotlin code using type inference with lambdas?

What will this Kotlin program print when executed?

Kotlin

fun main() {
    val numbers = listOf(1, 2, 3, 4)
    val doubled = numbers.map { it * 2 }
    val filtered = doubled.filter { it > 4 }
    println(filtered)
}

A [1, 2, 3, 4]

B [2, 4]

C [6, 8]

D Compilation error due to type mismatch

💡 Hint

First double each number, then keep only those greater than 4.

The map doubles each number: [2,4,6,8]. The filter keeps numbers > 4: [6,8].

🧠 Conceptual

expert

Which statement about Kotlin's type inference is correct?

Choose the correct statement about how Kotlin infers types in the following scenario.

Kotlin

val x = listOf(1, 2, 3).map { if (it % 2 == 0) it else "odd" }

A The inferred type of x is List<String> because of the else branch

B The inferred type of x is List<Int> because all elements are numbers

C The code causes a compilation error due to mixed types in the lambda

D The inferred type of x is List<Any> because the lambda returns Int or String

💡 Hint

Kotlin infers the closest common supertype when lambda returns different types.

Since the lambda returns Int for even numbers and String for odd numbers, Kotlin infers List as the common supertype.

Practice

1. What does type inference mean in Kotlin?

easy

A. The compiler automatically figures out the variable's type.

B. You must always specify the variable type manually.

C. The program runs slower because types are guessed at runtime.

D. It means variables cannot change their values.

Solution

1. Step 1: Understand type inference concept

   Type inference means the compiler looks at the value assigned and decides the variable's type automatically.

2. Step 2: Compare options with definition

   Only The compiler automatically figures out the variable's type. correctly describes this automatic type detection by the compiler.

3. Final Answer:

   The compiler automatically figures out the variable's type. -> Option A

4. Quick Check:

   Type inference = automatic type detection [OK]

Hint: Type inference means no need to write types explicitly [OK]

Common Mistakes:

• Thinking types are guessed at runtime
• Believing you must always write types
• Confusing type inference with variable mutability

2. Which of the following Kotlin variable declarations uses type inference correctly?

easy

A. val number: String = 10

B. val number: Int = "10"

C. val number = 10

D. val number =

Solution

1. Step 1: Check each declaration for correctness

   val number = 10 assigns 10 to 'number' without specifying type, so compiler infers Int type correctly.

2. Step 2: Identify errors in other options

   val number: Int = "10" assigns a String to Int type explicitly - error. val number: String = 10 assigns Int to String type - error. val number = is incomplete syntax.

3. Final Answer:

   val number = 10 -> Option C

4. Quick Check:

   Correct syntax with inferred type = val number = 10 [OK]

Hint: Look for variable with value but no explicit type [OK]

Common Mistakes:

• Mixing types in assignment
• Leaving declaration incomplete
• Assigning wrong type to explicit type

3. What is the inferred type of result in this Kotlin code?

val result = listOf(1, 2, 3).map { it * 2 }

medium

A. Array<Int>

B. List<Int>

C. Int

D. List<String>

Solution

1. Step 1: Analyze the expression assigned to result

   listOf(1, 2, 3) creates a List<Int>. The map function multiplies each element by 2, so the result is a List<Int>.

2. Step 2: Confirm the inferred type

   Since map returns a List of the transformed elements, the inferred type is List<Int>.

3. Final Answer:

   List<Int> -> Option B

4. Quick Check:

   Mapping Int list with Int operation = List<Int> [OK]

Hint: map keeps list type, changes element type if needed [OK]

Common Mistakes:

• Confusing List with Array
• Thinking map returns a single Int
• Assuming type changes to String

4. Identify the error in this Kotlin code snippet related to type inference:

val x = null
val y = x + 5

medium

A. Cannot add Int to null value

B. x cannot be null without explicit type

C. Type inference infers x as Int automatically

D. No error, code runs fine

Solution

1. Step 1: Understand type inference for null assignment

   Assigning null without type makes x's type Nothing?, which cannot be used in arithmetic.

2. Step 2: Check the operation y = x + 5

   Adding 5 to null is invalid; Kotlin does not allow arithmetic with null values.

3. Final Answer:

   Cannot add Int to null value -> Option A

4. Quick Check:

   Arithmetic with null causes error [OK]

Hint: Null needs explicit type; can't do math with null [OK]

Common Mistakes:

• Assuming null has Int type
• Ignoring null safety rules
• Thinking code compiles without error

5. Given this Kotlin code, what is the inferred type of data?

val data = mapOf("a" to 1, "b" to 2).mapValues { it.value * 10 }

hard

A. List>

B. List<Int>

C. Map

D. Map

Solution

1. Step 1: Analyze mapOf and mapValues usage

   mapOf creates a Map. mapValues transforms each value by multiplying by 10, keeping keys same.

2. Step 2: Determine the resulting type

   mapValues returns a Map with the same key type and transformed value type, so Map.

3. Final Answer:

   Map -> Option D

4. Quick Check:

   mapValues keeps Map type, changes values [OK]

Hint: mapValues keeps Map type, changes only values [OK]

Common Mistakes:

• Confusing mapValues with map returning List
• Assuming values become Strings
• Thinking result is a List instead of Map