Type Checking and Casting

fun describe(value: Any) {
    if (value is String) {
        println("It is a String of length ${value.length}")
    } else {
        println("Not a String")
    }
}

fun main() {
    describe("Kotlin")
    describe(42)
}

fun printLength(value: Any) {
    if (value is String) {
        // value is smart-cast to String here
        println("Length: ${value.length}")
    }
}

fun main() {
    printLength("Hello")
}

fun main() {
    val value: Any = "Kotlin"
    val text = value as String
    println("Cast succeeded: $text")
}

fun main() {
    val value: Any = 42
    val text = value as? String
    println("Safe cast result: $text")
}

1. Using the unsafe cast as on a value that might not actually be that type, causing a ClassCastException at runtime.
2. Not using smart casting after an is check, and casting manually anyway, producing more verbose code than necessary.
3. Forgetting that as? returns null instead of throwing, and not handling that null result.

• is checks whether a value is a certain type at runtime, returning a Boolean.
• Kotlin's smart casts automatically treat a value as the checked type inside an if (x is Type) block, without a manual cast.
• as performs an unsafe cast that throws ClassCastException if the value is not actually that type.
• as? performs a safe cast that returns null instead of throwing when the cast is not possible.