Type inference by the compiler in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with type inferred by the compiler.

Kotlin

val number = [1]

Drag options to blanks, or click blank then click option'

Atrue

B"Hello"

C3.14

D42

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string or boolean value when an integer is expected.

The compiler infers the type of number as Int because the value 42 is an integer.

2fill in blank

medium

Complete the code to declare a variable with type inferred as String.

Kotlin

val greeting = [1]

Drag options to blanks, or click blank then click option'

A"Welcome"

B100

Cfalse

D2.718

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a number or boolean instead of a string.

The compiler infers the type of greeting as String because the value is a text inside quotes.

3fill in blank

hard

Fix the error by completing the code to declare a variable with type inferred as Boolean.

Kotlin

val isActive = [1]

Drag options to blanks, or click blank then click option'

Atrue

B"true"

C1

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string "true" instead of boolean true.

Using numbers or null instead of boolean.

The compiler infers isActive as Boolean because the value true is a boolean literal, not a string.

4fill in blank

hard

Fill both blanks to create a list with type inferred by the compiler.

Kotlin

val numbers = listOf([1], [2])

Drag options to blanks, or click blank then click option'

A1

B"two"

C3

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing types like string and integer in the list elements.

The list contains integers 1 and 3, so the compiler infers the list type as List<Int>.

5fill in blank

hard

Fill all three blanks to create a map with type inferred by the compiler.

Kotlin

val map = mapOf([1] to [2], [3] to 42)

Drag options to blanks, or click blank then click option'

A"one"

B1

C"two"

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using integers as keys and strings as values inconsistently.

The map has keys as strings "one" and "two", and values as integers 1 and 42. The compiler infers the type as Map<String, Int>.

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