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Basic Types

By · Reviewed & last updated · Part of Kotlin Tutorial

Kotlin’s type system is statically typed with null safety baked in. Every type is either nullable (T?) or not (T). The compiler will reject null wherever non-nullable.

Common types + null safety

EXAMPLE
fun main() {
    // Primitives (boxed when nullable)
    val n: Int         = 42
    val pi: Double     = 3.14
    val ok: Boolean    = true
    val ch: Char       = 'A'
    val msg: String    = "hi"

    // Nullable counterparts
    val maybe: String? = null
    println(maybe?.length ?: 0)

    // Collections (read-only by default; Mutable* for mutation)
    val xs: List<Int>          = listOf(1, 2, 3)
    val mxs: MutableList<Int>  = mutableListOf(1, 2, 3); mxs += 4
    val m: Map<String, Int>    = mapOf("a" to 1, "b" to 2)
    val s: Set<String>         = setOf("red", "green")

    // Pair + Triple — quick tuples
    val (a, b) = "Ada" to 36

    // Any / Nothing / Unit
    val any: Any = 1            // top type
    val nothing: Nothing? = null // bottom — used for fns that never return
    fun crash(): Nothing = throw IllegalStateException("unreachable")
}

Why it matters

The Kotlin standard library’s read-only collection interfaces (List, Map, Set) are views, not guarantees of immutability. For deep immutability use kotlinx.collections.immutable.

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
val n: Int = 7
val f: Double = 3.14
val s: String = "hi"
val b: Boolean = true
Try it Yourself »

Discussion

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