Type checks and casts

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

is and !is operators

To perform a runtime check that identifies whether an object conforms to a given type, use 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 most cases, you don't need to use explicit cast operators because the compiler automatically casts objects for you. This is called smart-casting. The compiler tracks the type checks and explicit casts for immutable values and inserts implicit (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 even 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

Control flow

Smart casts work not only for if conditional expressions but also for when expressions and while loops:

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

If you declare a variable of Boolean type before using it in your if, when, or while condition, then any information collected by the compiler about the variable will be accessible in the corresponding block for smart-casting.

This can be useful when you want to do things like extract boolean conditions into variables. Then, you can give the variable a meaningful name, which will improve your code readability and make it possible to reuse the variable later in your code. For example:

class Cat {
    fun purr() {
        println("Purr purr")
    }
}

fun petAnimal(animal: Any) {
    val isCat = animal is Cat
    if (isCat) {
        // The compiler can access information about
        // isCat, so it knows that animal was smart-cast
        // to the type Cat.
        // Therefore, the purr() function can be called.
        animal.purr()
    }
}

fun main(){
    val kitty = Cat()
    petAnimal(kitty)
    // Purr purr
}

Logical operators

The compiler can perform smart casts on the right-hand side of && or || operators if there is a type check (regular or negative) 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
}

If you combine type checks for objects with an or operator (||), a smart cast is made to their closest common supertype:

interface Status {
    fun signal() {}
}

interface Ok : Status
interface Postponed : Status
interface Declined : Status

fun signalCheck(signalStatus: Any) {
    if (signalStatus is Postponed || signalStatus is Declined) {
        // signalStatus is smart-cast to a common supertype Status
        signalStatus.signal()
    }
}

Inline functions

The compiler can smart-cast variables captured within lambda functions that are passed to inline functions.

Inline functions are treated as having an implicit callsInPlace contract. This means that any lambda functions passed to an inline function are called in place. Since lambda functions are called in place, the compiler knows that a lambda function can't leak references to any variables contained within its function body.

The compiler uses this knowledge, along with other analyses to decide whether it's safe to smart-cast any of the captured variables. For example:

interface Processor {
    fun process()
}

inline fun inlineAction(f: () -> Unit) = f()

fun nextProcessor(): Processor? = null

fun runProcessor(): Processor? {
    var processor: Processor? = null
    inlineAction {
        // The compiler knows that processor is a local variable and inlineAction()
        // is an inline function, so references to processor can't be leaked.
        // Therefore, it's safe to smart-cast processor.
      
        // If processor isn't null, processor is smart-cast
        if (processor != null) {
            // The compiler knows that processor isn't null, so no safe call 
            // is needed
            processor.process()
        }

        processor = nextProcessor()
    }

    return processor
}

Exception handling

Smart cast information is passed on to catch and finally blocks. This makes your code safer as the compiler tracks whether your object has a nullable type. For example:

//sampleStart
fun testString() {
    var stringInput: String? = null
    // stringInput is smart-cast to String type
    stringInput = ""
    try {
        // The compiler knows that stringInput isn't null
        println(stringInput.length)
        // 0

        // The compiler rejects previous smart cast information for 
        // stringInput. Now stringInput has the String? type.
        stringInput = null

        // Trigger an exception
        if (2 > 1) throw Exception()
        stringInput = ""
    } catch (exception: Exception) {
        // The compiler knows stringInput can be null
        // so stringInput stays nullable.
        println(stringInput?.length)
        // null
    }
}
//sampleEnd
fun main() {
    testString()
}

Smart cast prerequisites

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

To explicitly cast an object to a non-nullable type, use the unsafe cast operator as:

val x: String = y as String

If the cast isn't possible, the compiler throws an exception. This is why it's called unsafe.

In the previous example, if y is null, the code above also throws an exception. This is because null can't be cast to String, as String isn't nullable. To make the example work for possible null values, use a 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.

Last modified: 25 September 2024