Any and Unit

Learn Any and Unit types.

Any and Unit

Any is the supertype of all non-nullable types in Kotlin. If a variable can hold values of different types, you can use Any: val x: Any = 5 then x = "text" is valid.

Unit indicates that a function returns no value; it is Kotlin's equivalent of Java's void but is actually a type with a single instance.

fun hello(): Unit {
    println("Hello")
}

Exercise Unit function

At the top level, write fun hello(): Unit with a println("Hello") inside. Inside fun main(), call hello().

Show example solution

fun hello(): Unit {
    println("Hello")
}

fun main() {
    hello()
}

The Nothing type

Nothing is Kotlin's bottom type: it represents computations that never complete. For example, an infinite loop or a function that always throws returns Nothing.

In practice, the TODO() function returns Nothing; this makes it useful to mark code not yet written. Don't confuse it with Unit: Unit does return a value (its single instance), Nothing never returns.

Test yourself

Return type

fun hello(): ____ {
    println("Hello")
}

Show answer

fun hello(): Unit {
    println("Hello")
}

Of which types is Any the supertype?
- All non-nullable types
- Only Int
- Only String
- Only classes
Show answer

All non-nullable types — Any is the supertype of all non-nullable types.
Any and Unit: match the terms

Any

Supertype
Unit

No return
Nothing

Never returns
Int

Integer

Next lesson: Summary - Data types

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