object 선언: Kotlin 싱글턴

object Logger {
    fun log(msg: String) = println("[LOG] $msg")
}
fun main() {
    Logger.log("App started")
    Logger.log("User signed in")
}

object Counter {
    var count = 0
    fun increment() { count++ }
}
fun main() {
    Counter.increment()
    Counter.increment()
    Counter.increment()
    println(Counter.count) // 3
}

object Heavy {
    init { println("Initializing Heavy...") }
    fun work() = "done"
}
fun main() {
    println("Before access")
    println(Heavy.work()) // triggers init now
    println(Heavy.work()) // no init again
}

interface Greeter { fun greet(name: String): String }
object EnglishGreeter : Greeter {
    override fun greet(name: String) = "Hello, $name!"
}
fun main() {
    println(EnglishGreeter.greet("Ada"))
}

open class Animal(val sound: String) {
    fun makeSound() = "The animal says $sound"
}
object Cat : Animal("Meow")
fun main() {
    println(Cat.makeSound())
}

object AppConfig {
    val apiUrl = "https://api.example.com"
    val timeout = 5000
    val version = "1.0.0"
}
fun main() {
    println("v${AppConfig.version} -> ${AppConfig.apiUrl}")
}

object StringUtils {
    fun reverse(s: String) = s.reversed()
    fun capitalize(s: String) = s.replaceFirstChar { it.uppercase() }
}
fun main() {
    println(StringUtils.reverse("Kotlin"))
    println(StringUtils.capitalize("hello"))
}

object MySingleton
fun main() {
    val a = MySingleton
    val b = MySingleton
    println(a === b) // true
}

class Service { fun ping() = "pong (class)" }
object SingletonService { fun ping() = "pong (object)" }
fun main() {
    val s = Service() // instance per call
    println(s.ping())
    println(SingletonService.ping()) // no instantiation
}

object SharedCounter {
    @Volatile var count = 0
    @Synchronized fun inc() { count++ }
}
fun main() {
    repeat(1000) { SharedCounter.inc() }
    println(SharedCounter.count) // 1000
}