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.

Practice

1. Which keyword in Kotlin declares a variable whose value cannot be changed after assignment?

easy

A. val

B. var

C. let

D. const

Solution

1. Step 1: Understand Kotlin variable keywords

   Kotlin uses val for immutable variables and var for mutable ones.

2. Step 2: Identify the immutable keyword

   val means the value cannot be changed after assignment.

3. Final Answer:

   val -> Option A

4. Quick Check:

   Immutable variable keyword = val [OK]

Hint: Remember: val = fixed, var = changeable [OK]

Common Mistakes:

• Confusing val and var
• Using let which is not a Kotlin keyword
• Thinking const is used like val

2. Which of the following is the correct way to declare an integer variable with value 10 using type inference in Kotlin?

easy

A. val number: Int = 10

B. val number = 10.0

C. var number = "10"

D. val number = 10

Solution

1. Step 1: Understand type inference in Kotlin

   Kotlin can guess the type from the assigned value, so val number = 10 infers Int.

2. Step 2: Check each option

   val number: Int = 10 declares type explicitly, which is correct but not type inference. val number = 10 uses type inference correctly. var number = "10" assigns a String, not Int. val number = 10.0 assigns a Double, not Int.

3. Final Answer:

   val number = 10 -> Option D

4. Quick Check:

   Type inference assigns Int from 10 [OK]

Hint: Omit type to let Kotlin infer it [OK]

Common Mistakes:

• Adding explicit type when not needed
• Assigning wrong data type
• Confusing Int and Double

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

val x = 5
val y = 2
val result = x / y
println(result)

medium

A. 2.0

B. 2.5

C. 2

D. Compilation error

Solution

1. Step 1: Identify variable types and operation

   Both x and y are Int by type inference. Integer division truncates decimals.

2. Step 2: Calculate integer division

   5 / 2 = 2 (decimal part discarded).

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Integer division truncates decimals = 2 [OK]

Hint: Integer division drops decimals in Kotlin [OK]

Common Mistakes:

• Expecting floating point result
• Using variables as Double without declaration
• Thinking println shows decimal automatically

4. Identify the error in this Kotlin code snippet:

val name = "Alice"
name = "Bob"

medium

A. Missing semicolon after first line

B. Cannot reassign a val variable

C. String values must use single quotes

D. Variable name is a reserved keyword

Solution

1. Step 1: Check variable declaration

   val name = "Alice" declares an immutable variable.

2. Step 2: Check reassignment

   Trying to assign "Bob" to name causes error because val cannot be reassigned.

3. Final Answer:

   Cannot reassign a val variable -> Option B

4. Quick Check:

   val variables are immutable [OK]

Hint: val means no reassignment allowed [OK]

Common Mistakes:

• Thinking val allows reassignment
• Confusing quotes for strings
• Assuming semicolons are mandatory

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

val a = 3
val b = 4.5
val result = a + b

hard

A. Double

B. Int

C. Float

D. String

Solution

1. Step 1: Identify types of operands

   a is Int, b is Double (4.5 is a Double literal).

2. Step 2: Understand type promotion in arithmetic

   When adding Int and Double, Int is promoted to Double, so result is Double.

3. Final Answer:

   Double -> Option A

4. Quick Check:

   Int + Double results in Double [OK]

Hint: Mixed numeric types promote to the wider type [OK]

Common Mistakes:

• Assuming result is Int
• Confusing Float and Double
• Thinking result is String