Data types and type inference in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Kotlin Type Inference Behavior

What is the inferred type of the variable result in the following Kotlin code?

Android Kotlin

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

A List<Double>

B Array<Int>

C List<Int>

D IntArray

💡 Hint

Think about what the map function returns when applied to a list of integers.

The map function returns a List of the transformed elements. Since it * 2 is an integer operation, the resulting list is of type List.

❓ ui_behavior

intermediate

Kotlin Variable Type and UI Display

Given this Kotlin code snippet in an Android app, what will be displayed in the TextView?

Android Kotlin

val message = "Hello" + 123
textView.text = message

A Hello123

B Hello 123

C Hello+123

D Compilation error

💡 Hint

Consider how Kotlin handles string concatenation with numbers.

Kotlin converts the number 123 to a string and concatenates it directly without spaces, so the TextView shows "Hello123".

❓ lifecycle

advanced

Type Inference Impact on Android Lifecycle

Consider this Kotlin property in an Android Activity:

val count = 0

What is the type of count and what happens if you try to assign count = 5 later in the Activity lifecycle?

A <code>count</code> is inferred as Any and can hold any type.

B <code>count</code> is a nullable Int and can be assigned null.

C <code>count</code> is a mutable Int and can be reassigned without error.

D <code>count</code> is an Int and cannot be reassigned; trying to assign causes a compilation error.

💡 Hint

Look at the keyword used to declare count and what it means in Kotlin.

The val keyword declares an immutable variable. Its type is inferred as Int from the initial value 0. Reassigning it causes a compilation error.

❓ navigation

advanced

Type Inference in Navigation Arguments

In Kotlin Android Navigation, you pass an argument to a destination like this:

val action = HomeFragmentDirections.actionToDetail(itemId = 42)
findNavController().navigate(action)

What is the inferred type of itemId in the generated code?

A Int

B String

C Any

D Long

💡 Hint

Look at the value passed to itemId and how Kotlin infers types from literals.

The literal 42 is an Int by default in Kotlin, so the generated argument type is Int.

🔧 Debug

expert

Debugging Type Inference with Nullable Types

What error will this Kotlin code produce when compiled?

Android Kotlin

val name = null
val length = name.length

A Compilation error: Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver

B Compilation error: Variable 'name' must have a type explicitly specified

C No error, length is 0

D Runtime NullPointerException

💡 Hint

Consider what Kotlin infers when you assign null without a type.

Kotlin requires an explicit type when assigning null without a type. The code causes a compilation error: Variable 'name' must have a type explicitly specified.