実際のコードで使う安全呼び出し?.とElvis ?:

大きな効果をもたらす2つの演算子

セーフコールの基本

fun main() {
    val name: String? = "Kotlin"
    val len: Int? = name?.length
    println(len) // 6
    val nope: String? = null
    println(nope?.length) // null
}

Elvis 演算子の基本

fun main() {
    val name: String? = null
    val display = name ?: "Anonymous"
    println(display) // Anonymous
}

セーフコールと Elvis の組み合わせ

fun main() {
    val name: String? = null
    val length = name?.length ?: 0
    println("length = $length") // 0
}

セーフコールの連結

class Address(val city: String?)
class User(val address: Address?)
fun main() {
    val u: User? = User(Address(null))
    val city = u?.address?.city ?: "Unknown"
    println(city) // Unknown
}

右辺での Elvis による例外送出

fun loadConfig(path: String?): String {
    val p = path ?: throw IllegalArgumentException("path required")
    return "loaded $p"
}
fun main() {
    println(loadConfig("/etc/app"))
    // loadConfig(null) // throws
}

メソッドでのセーフコール

fun main() {
    val raw: String? = "  hello  "
    val cleaned = raw?.trim()?.uppercase() ?: ""
    println(cleaned) // HELLO
}

条件式でのセーフコール

fun main() {
    val name: String? = "Alice"
    if (name?.startsWith("A") == true) {
        println("Starts with A")
    }
}

return と Elvis

fun process(input: String?) {
    val safe = input ?: return
    println("processing $safe")
}
fun main() {
    process("abc")
    process(null) // returns silently
}

continue/break と Elvis

fun main() {
    val items: List<String?> = listOf("a", null, "b", null, "c")
    for (item in items) {
        val v = item ?: continue
        print("$v ")
    }
    println() // a b c
}

実践：設定の検索

class Config(val timeoutMs: Int?, val host: String?)
fun main() {
    val cfg: Config? = Config(null, "prod.example.com")
    val timeout = cfg?.timeoutMs ?: 5000
    val host = cfg?.host ?: "localhost"
    println("$host @ $timeout ms")
}

簡単な確認

まとめ