Careeroza

Safe and Unsafe Casts

as and as?

as casts explicitly and throws ClassCastException if the type is wrong. as? is the safe cast: it returns null instead of throwing.

Example

fun main() {
    val values: List<Any> = listOf("42", 7, 3.5)
    for (v in values) {
        val s: String? = v as? String
        println("$v -> ${s ?: "not a String"}")
    }
    val n = (values[1] as? Int)?.plus(1) ?: 0
    println(n)
}

Output

42 -> 42
7 -> not a String
3.5 -> not a String
8

Unsafe Cast Failure

Example

fun main() {
    val data: Any = 123
    val text = data as String
    println(text)
}

Output

Exception: java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')

Tip: Prefer is checks with smart casts or as?. Use plain as only when a wrong type really is a bug.