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.