const

Learn to define compile-time constants with const val.

const

Constants declared with const val are computed at compile-time. They must be of primitive types (Int, Double, String, Boolean) and live at the top level or inside an object.

Values computed at runtime cannot be const. Writing const val PI = 3.14 replaces PI with 3.14 everywhere.

const val MAX = 100

fun main() {
    println(MAX)
}

Exercise Declare a constant

At the top level, declare const val MAX = 100. Inside fun main(), print with println(MAX).

Show example solution

const val MAX = 100

fun main() {
    println(MAX)
}

const vs val

The difference between const val and val is when the constant is computed. const val is computed at compile-time and replaced by its value at every use; so it can only be a constant expression.

val is computed once at runtime and holds a reference. For example, val x = readLine() is valid, but const val x = readLine() is a compile error.

Test yourself

Which is a compile-time constant?
- const val MAX = 100
- var y = 10
- lateinit var z
- val x = 5
Show answer

const val MAX = 100 — Constants declared with const val are computed at compile-time.
const: match the terms

const val

Compile-time
val

Runtime, immutable
var

Mutable
lateinit

Deferred
const: fill in the missing part
```
const ____ MAX = 100
```
Show answer
```
const val MAX = 100
```

Next lesson: lateinit

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