Type checks and casts

is and !is operators

These operators are used 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

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

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

// 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
}

"Unsafe" cast operator

Usually, the cast operator throws an exception if the cast isn't possible. Thus, it's called unsafe.

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" cast operator

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

val x: String? = y as? String

Last modified: 19 February 2024