Kotlin Smart Casting | Android Introduction #29

In this reading, you will learn how to use smart-casting.

You might recall that smart-casting is a Kotlin capability that can change the type of variable for a scope, but only if Kotlin knows that the variable is of that type in the scope.

For example, if you have a parameter a of type Any, and you add the condition if(a is String), then in the scope of such check, the variable a changes its type to String, and then you might ask for its length.

fun consume(a: Any) {
    if (a is String) {
println(a.length) // the type of a is String
    }
    if (a is Int) {
println(a * 10) // the type of a is Int
    }
}

fun main() {
    consume("ABC") // 3
consume(10) // 100
}

Smart-casting can also be used to deal with nullability. When you have a nullable type and a check that it is not null, Kotlin knows that it is not null when the variable is subsequently used.

fun consume(a: String?) {
    if (a != null) {
println(a.length) // the type of a is String
    } else {
println("Cannot handle null")
    }
}

fun main() {
    consume("ABC") // 3
    consume(null) // Cannot handle null
}

Smart-casting is quite powerful and works in a variety of configurations.

fun consumeA(a: String?) {
    if (a == null || a.length == 0) return // after || a is smart-casted to String
println(a.length) // the type of a is String
}

fun main() {
    consume("ABC") // 3
    consume(null)
consume("")
}

However, smart-casting has some limitations. You cannot smart-cast read-write properties. This is due to safety concerns. Kotlin does not have a guarantee that the value of such a property hasn’t been changed between check and usage by some other part of the program.

var a: String? = null

fun main() {
    a = "Sam"
    if (a != null) {
println(a.length) // COMPILATION ERROR, smart-casting not allowed
    }
}

In general, smart-casting is a really powerful mechanism often used in Kotlin applications.

İbrahim Can Erdoğan

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