Type inference

Learn type inference and explicit type declarations.

Type inference

Kotlin is statically typed; every variable has a type known at compile time. But you usually don't need to write the type explicitly. The compiler infers it from the assigned value.

For example, when you write val n = 5, Kotlin infers n is Int. To state the type explicitly, write val n: Int = 5. Both forms are valid.

val n = 5          // Int is inferred
val pi: Double = 3.14  // type written explicitly

Exercise Write the type explicitly

Write a fun main(). Declare val pi: Double = 3.14 and print with println(pi).

Show example solution

fun main() {
    val pi: Double = 3.14
    println(pi)
}

When to write the type explicitly?

Type inference is enough most of the time; sometimes writing the type explicitly is more readable. Examples include a function's return type, a class property, or cases where the expected type is unclear.

Also, writing val x: Double = 5 makes Kotlin convert 5 from Int to Double, so x becomes 5.0. Adding the type makes the code's intent explicit.

Test yourself

Write the type

val pi: ____ = 3.14

Show answer

val pi: Double = 3.14

What is the type of name in `val name = "Alex"`?
- Int
- Boolean
- Char
- String
Show answer

String — A value in double quotes is a String.
Type inference: match the terms

5

Int
3.14

Double
true

Boolean
'A'

Char

Next lesson: const

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