Classes

// 1) Standard class with primary ctor + property
class User(val name: String, var email: String) {
    val createdAt = System.currentTimeMillis()

    init {
        require(email.contains('@')) { "invalid email" }
    }

    fun verify() = println("\$name verified")
}

val u = User("Ada", "ada@example.com")
u.verify()

// 2) data class — equals, hashCode, toString, copy() generated
data class Money(val amount: Double, val currency: String)

val price = Money(9.99, "AUD")
val bumped = price.copy(amount = 11.99)    // copy with override
println(price == Money(9.99, "AUD"))      // true — value equality

// 3) sealed class — closed hierarchy, exhaustive when
sealed class Result<out T> {
    data class Ok<T>(val value: T) : Result<T>()
    data class Err(val message: String) : Result<Nothing>()
    object  Loading : Result<Nothing>()
}

fun render(r: Result<String>) = when (r) {                 // exhaustive
    is Result.Ok      -> "got \${r.value}"
    is Result.Err     -> "failed: \${r.message}"
    Result.Loading -> "…"
}

// 4) object — singleton
object Logger {
    fun info(msg: String) = println("[info] \$msg")
}
Logger.info("hello")

// 5) companion object — static-like members on a class
class User2 private constructor(val id: Int) {
    companion object {
        fun create(id: Int): User2 = User2(id)
        const val MAX = 1_000
    }
}
User2.create(1)

// 6) by — delegation
interface Repo { fun load(id: Int): String }
class InMemoryRepo : Repo { override fun load(id: Int) = "in-mem(\$id)" }

class Service(repo: Repo) : Repo by repo            // delegates all Repo methods

val svc = Service(InMemoryRepo())
println(svc.load(7))

// 7) Inheritance — open / override
open class Animal { open fun sound() = "?" }
class Cat : Animal() {
    override fun sound() = "meow"
}

// 8) Abstract
abstract class Shape {
    abstract val area: Double
    override fun toString() = "\${this::class.simpleName}(area=\$area)"
}
class Rect(val w: Double, val h: Double) : Shape() {
    override val area: Double get() = w * h
}

// 9) Properties — backing fields + getters / setters
class Counter {
    var count: Int = 0
        private set                              // read-only outside
    fun bump() { count++ }
}

class User(val name: String, var age: Int) {
    fun greet() = "hi, $name"
}
val u = User("Ada", 36)
println(u.greet())