Type Checks and Casts: 'is' and 'as'

is and !is Operators

We can check whether an object conforms to a given type at runtime by using the is operator or its negated form !is:

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 many cases, one does not need to use explicit cast operators in Kotlin, because the compiler tracks the is-checks for immutable values and inserts (safe) casts automatically when needed:

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

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

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

or in the right-hand side of && and ||:

    // 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
    }

Such 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 do not work when the compiler cannot guarantee that the variable cannot change between the check and the usage. More specifically, smart casts are applicable according to the following rules:

• val local variables - always;
• val properties - if the property is private or internal or the check is performed in the same module where the property is declared. Smart casts aren't applicable to open properties or properties that have custom getters;
• var local variables - if the variable is not modified between the check and the usage and is not captured in a lambda that modifies it;
• 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 is not possible. Thus, we call it unsafe. The unsafe cast in Kotlin is done by the infix operator as (see operator precedence):

val x: String = y as String

Note that null cannot be cast to String as this type is not nullable, i.e. if y is null, the code above throws an exception. In order to match Java cast semantics we have to have nullable type at cast right hand side, like

val x: String? = y as String?

"Safe" (nullable) cast operator

To avoid an exception being thrown, one can use a safe cast operator as? that 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-null type String the result of the cast is nullable.