typecasts.md

In Kotlin, you can perform type checks to check the type of an object at runtime. Type casts convert objects to a different type.

To learn specifically about generics type checks and casts, for example List<T>, Map<K,V>, see Generics type checks and casts.

{type="tip"}

is and !is operators

Use the is operator or its negated form !is to perform a runtime check that identifies whether an object conforms to a given type:

if (obj is String) {
    print(obj.length)
}

if (obj !is String) { // Same as !(obj is String)
    print("Not a String")
} else {
    print(obj.length)
}

Smart casts

In most cases, you don't need to use explicit cast operators in Kotlin because the compiler tracks the is-checks and explicit casts for immutable values and inserts (safe) casts automatically when necessary:

fun demo(x: Any) {
    if (x is String) {
        print(x.length) // x is automatically cast to String
    }
}

The compiler is smart enough to know that a cast is safe if a negative check leads to a return:

if (x !is String) return

print(x.length) // x is automatically cast to String

Or if it is on the right-hand side of && or || and the proper check (regular or negative) is on the left-hand side:

// x is automatically cast to String on the right-hand side of `||`
if (x !is String || x.length == 0) return

// x is automatically cast to String on the right-hand side of `&&`
if (x is String && x.length > 0) {
    print(x.length) // x is automatically cast to String
}

Smart casts work for when expressions and while loops as well:

when (x) {
    is Int -> print(x + 1)
    is String -> print(x.length + 1)
    is IntArray -> print(x.sum())
}

Note that smart casts work only when the compiler can guarantee that the variable won't change between the check and its usage.

{type="warning"}

Smart casts can be used in the following conditions:

val local variables	Always, except local delegated properties.
val properties	If the property is private, internal, or if the check is performed in the same module where the property is declared. Smart casts can't be used on open properties or properties that have custom getters.
var local variables	If the variable is not modified between the check and its usage, is not captured in a lambda that modifies it, and is not a local delegated property.
var properties	Never, because the variable can be modified at any time by other code.

"Unsafe" cast operator

Usually, the cast operator throws an exception if the cast isn't possible. Thus, it's called unsafe. The unsafe cast in Kotlin is done by the infix operator as.

val x: String = y as String

Note that null cannot be cast to String, as this type is not nullable. If y is null, the code above throws an exception. To make code like this correct for null values, use the nullable type on the right-hand side of the cast:

val x: String? = y as String?

"Safe" (nullable) cast operator

To avoid exceptions, use the safe cast operator as?, which returns null on failure.

val x: String? = y as? String

Note that despite the fact that the right-hand side of as? is a non-nullable type String, the result of the cast is nullable.