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Mastering Kotlin Essentials

Smart-Casting and the Elvis Operator

Explore smart casting for null checks and efficiently set default values with the Elvis operator.

We'll cover the following...

Smart-casting

Smart-casting also works for nullability. Therefore, in the scope of a non-nullability check, a nullable type is cast to a non-nullable type.

Kotlin 1.5
fun printLengthIfNotNull(str: String?) {
    if (str != null) {
        println(str.length) // str smart-casted to String
    }
}
fun main() {
    val nullableString: String? = "Hello, Kotlin"
    val nullString: String? = null
    
    printLengthIfNotNull(nullableString) // Output: 13
    printLengthIfNotNull(nullString) // No output (null)
}

Smart-casting also works when we use return or throw if a value is not null.

Kotlin 1.5
fun printLengthIfNotNull(str: String?) {
    if (str == null) return
    println(str.length) // str smart-casted to String
}
fun main() {
    val nullableString: String? = "Hello, Kotlin"
    val nullString: String? = null
    
    printLengthIfNotNull(nullableString) // Output: 13
    printLengthIfNotNull(nullString) // No output (null)
}

Utilizing smart-casting, this code demonstrates ...