ตัวดำเนินการ !!: เมื่อใดและเหตุใดควรหลีกเลี่ยง

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

fun main() {
    val name: String? = null
    try {
        println(name!!.length)
    } catch (e: NullPointerException) {
        println("NPE caught!")
    }
}

fun main() {
    val name: String? = null
    val len = name?.length
    println(len) // null — no crash
}

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

fun main() {
    val name: String? = "Alice"
    if (name != null) {
        println(name.length) // smart-cast to String
    }
}

fun process(name: String?) {
    val safe = requireNotNull(name) { "name is required" }
    println("processing $safe")
}
fun main() {
    process("Ada")
    // process(null) // throws IllegalArgumentException: name is required
}

class Service {
    private var data: String? = null
    fun load() { data = "loaded" }
    fun read(): String = checkNotNull(data) { "load() must be called first" }
}
fun main() {
    val s = Service()
    s.load()
    println(s.read())
}

fun main() {
    val list = listOf(1, 2, 3)
    val firstEven: Int = list.find { it % 2 == 0 }!! // safe: we know list has an even
    println(firstEven)
}

class A(val b: B?)
class B(val c: C?)
class C(val name: String?)
fun main() {
    val a: A? = A(B(C("ok")))
    // BAD: val n = a!!.b!!.c!!.name!!
    val n = a?.b?.c?.name ?: "unknown"
    println(n)
}

fun firstUpper(s: String?): String? = s?.firstOrNull()?.uppercase()
fun main() {
    println(firstUpper("alice")) // A
    println(firstUpper(""))      // null
    println(firstUpper(null))    // null
}