Object Declarations and Companion Objects

// Singleton via object declaration
object AppConfig {
    var debug = false
    val version = "2.0"
    fun log(msg: String) { if (debug) println("[DEBUG] $msg") }
}

// Class with companion object (static-like factory)
class Database private constructor(val url: String) {

    companion object {
        private var instance: Database? = null

        fun getInstance(url: String): Database {
            return instance ?: Database(url).also { instance = it }
        }

        // Companion can implement an interface
        fun create(url: String) = Database(url)
    }

    fun query(sql: String) = println("Querying [$url]: $sql")
}

// Object expression — anonymous singleton implementing an interface
interface ClickListener {
    fun onClick(id: Int)
}

fun setListener(listener: ClickListener) = listener.onClick(42)

fun main() {
    AppConfig.debug = true
    AppConfig.log("App started v${AppConfig.version}")

    val db = Database.getInstance("jdbc:sqlite:app.db")
    db.query("SELECT * FROM users")

    // Second call returns same instance
    val db2 = Database.getInstance("jdbc:sqlite:app.db")
    println("Same instance: ${db === db2}")

    setListener(object : ClickListener {
        override fun onClick(id: Int) = println("Clicked: $id")
    })
}