Proyeksi Bintang dan Kapan Menggunakan *

Apa Itu Proyeksi Bintang

fun printList(list: List<*>) {
    for (item in list) println(item)
}
fun main() {
    printList(listOf(1, 2, 3))
    printList(listOf("a", "b"))
    printList(listOf(true, 42, "mixed"))
}

Membaca dari Tipe yang Diproyeksikan dengan Bintang

fun process(list: List<*>) {
    val first: Any? = list.firstOrNull()
    println("First: $first, Size: ${list.size}")
    // list.add("x") // compile error: can't write to List<*>
}
fun main() {
    process(listOf(10, 20, 30))
}

Bintang vs out Any

fun sumStars(list: List<*>): Int = list.size
fun sumOut(list: List<out Any?>): Int = list.size
fun main() {
    val l = listOf(1, "two", true)
    println(sumStars(l))  // 3
    println(sumOut(l))    // 3
}

Bintang pada Parameter Fungsi

fun containerInfo(map: Map<*, *>): String {
    return "keys=${map.keys.size}, values=${map.values.size}"
}
fun main() {
    val m = mapOf(1 to "one", 2 to "two")
    println(containerInfo(m))
}

Bintang dengan MutableList

fun clearFirstElement(list: MutableList<*>) {
    if (list.isNotEmpty()) {
        list.removeAt(0)  // OK: remove by index doesn't need T
        // list.add("x") // Error: can't write unknown type
    }
}
fun main() {
    val m = mutableListOf(1, 2, 3)
    clearFirstElement(m)
    println(m) // [2, 3]
}

Kelas dengan Proyeksi Bintang

fun isListOfSomething(obj: Any): Boolean {
    return obj is List<*>  // OK with star
    // obj is List<String> // Warning: cannot check erased type
}
fun main() {
    println(isListOfSomething(listOf(1, 2)))  // true
    println(isListOfSomething("not a list"))  // false
}

Beberapa Parameter Bintang

fun describeMap(map: Map<*, *>) {
    map.forEach { (k, v) ->
        println("${k?.javaClass?.simpleName} -> ${v?.javaClass?.simpleName}")
    }
}
fun main() {
    describeMap(mapOf("key" to 42, 1 to true))
}

Kapan NOT Sebaiknya Digunakan

// Bad: loses type information
fun addToStar(list: MutableList<*>, item: Any?) {
    // list.add(item) // compile error
}
// Better: use bounded type parameter
fun <T : Any> addTyped(list: MutableList<T>, item: T) {
    list.add(item)
}
fun main() {
    val l = mutableListOf("a")
    addTyped(l, "b")
    println(l)
}

Bintang dengan Pemeriksaan Tipe dan Casting

fun handleResponse(response: Any) {
    if (response is List<*>) {
        val list = response as List<*>
        println("List with ${list.size} elements")
        val first = list.firstOrNull()
        if (first is String) println("First string: $first")
    }
}
fun main() {
    handleResponse(listOf("hello", "world"))
    handleResponse(42)
}

Praktik: Pengirim Peristiwa Generik

interface Handler<in T> { fun handle(event: T) }
class LogHandler : Handler<Any> {
    override fun handle(event: Any) = println("Event: $event")
}
fun dispatch(handler: Handler<*>, event: Any) {
    @Suppress("UNCHECKED_CAST")
    (handler as Handler<Any>).handle(event)
}
fun main() {
    dispatch(LogHandler(), "click")
    dispatch(LogHandler(), 42)
}

Bintang dalam Konteks reified

fun printClassName(klass: kotlin.reflect.KClass<*>) {
    println(klass.simpleName)
}
fun main() {
    printClassName(String::class)     // String
    printClassName(List::class)       // List
    printClassName(Int::class)        // Int
}

Pemeriksaan Singkat

Rekapitulasi