Type inference by the compiler in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type inference in Kotlin?

Type inference is when the Kotlin compiler automatically figures out the type of a variable or expression without you having to explicitly write it.

beginner

How does Kotlin infer the type of val number = 10?

Kotlin sees the value 10 is an integer, so it infers the type of number as Int.

beginner

Can Kotlin infer the type of a variable declared with var?

Yes, Kotlin can infer the type for var variables just like val, based on the assigned value.

intermediate

What happens if you declare a variable without initializing it, like val x?

Kotlin requires you to initialize variables when declaring them without an explicit type. Otherwise, the compiler cannot infer the type and will show an error.

beginner

Why is type inference helpful for Kotlin programmers?

Type inference makes code shorter and easier to read by removing the need to write types explicitly when they are obvious from the assigned value.

What type does Kotlin infer for val name = "Alice"?

A Int

B String

C Boolean

D Any

The value "Alice" is a text string, so Kotlin infers the type as String.

Which of these declarations will cause a type inference error in Kotlin?

A val x = 5

B var y = true

C val message = "Hi"

D val z

Declaring val z without initialization or explicit type causes an error because Kotlin cannot infer the type.

If you write var count = 10, what type does Kotlin assign to count?

A Double

B String

C Int

D Float

The number 10 is an integer, so Kotlin infers the type Int.

Can Kotlin infer the type of a variable from a function return value?

A Yes, if the function returns a value

B No, you must always specify the type

C Only for built-in functions

D Only if the function is inline

Kotlin can infer the type of a variable from the return value of a function.

What keyword is used to declare a read-only variable with inferred type in Kotlin?

A val

B var

C let

D const

val declares a read-only variable with type inferred by the compiler.

Explain how Kotlin's compiler infers the type of a variable when you assign a value.

Think about how Kotlin guesses the type from the value you give.

Describe a situation where Kotlin's type inference will cause a compilation error.

What if you declare a variable but don't assign a value or type?

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