Nullable types in Kotlin

null is in the type system: String? admits it, String never will.

fun main() {
    // String? admits null; plain String never does.
    val radioCheck: String? = null
    val relay: String? = "Windy Col repeater"

    // ?. stops at null; ?: supplies the fallback.
    println(radioCheck?.length ?: "no signal")
    println(relay?.uppercase() ?: "no signal")

    val beds: Int? = mapOf("Larkspur" to 18)["Stone"]
    val planned = beds ?: 0
    println("Planning on $planned beds")
}

How it works

1. val radioCheck: String? may hold null; plain String cannot.
2. ?. stops the chain at null instead of throwing.
3. ?: — the Elvis operator — supplies the value for the null case.

Keywords and builtins used here

• fun
• main
• val

The run, in numbers

Lines

13

Characters to type

382

Tokens

78

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 67 seconds.

Type this snippet

Step 1 of 3 in Null safety, step 16 of 27 in Language basics.

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Nullable types in other languages

• Dart