Classes & Objects

class User(val name: String, var age: Int, val country: String = "IN")

fun main() {
    val ada = User("Ada", 36)
    val linus = User(name = "Linus", age = 28, country = "FI")
    ada.age += 1
    println("${ada.name} ${ada.age} ${ada.country}")
    println("${linus.name} ${linus.age} ${linus.country}")
}

class Rect(val width: Int, val height: Int) {
    val area = width * height

    init {
        require(width > 0 && height > 0) { "sides must be positive" }
        println("init: ${width}x$height, area $area")
    }

    constructor(side: Int) : this(side, side) {
        println("secondary: a square")
    }
}

fun main() {
    Rect(3, 4)
    Rect(5)
    val bad = runCatching { Rect(0, 2) }
    println(bad.exceptionOrNull()?.message)
}

class Temperature(celsius: Double) {
    var celsius: Double = celsius
        set(value) {
            require(value >= -273.15) { "below absolute zero: $value" }
            field = value
        }

    val fahrenheit: Double
        get() = celsius * 9 / 5 + 32      // computed, no backing field

    var label: String = ""
        set(value) { field = value.trim().lowercase() }
}

fun main() {
    val t = Temperature(20.0)
    println(t.fahrenheit)
    t.celsius = 100.0
    println(t.fahrenheit)
    t.label = "  Boiling "
    println("[${t.label}]")
    println(runCatching { t.celsius = -300.0 }.exceptionOrNull()?.message)
}

class Counter {
    var count = 0
        private set

    fun increment() { count++ }
}

class Screen {
    lateinit var title: String
    fun isReady() = this::title.isInitialized
}

fun main() {
    val c = Counter()
    repeat(3) { c.increment() }
    println(c.count)
    // c.count = 10   // error: the setter is private

    val s = Screen()
    println(s.isReady())
    s.title = "Home"
    println("${s.isReady()} ${s.title}")
}

class BankAccount(val owner: String) {
    var balance = 0
        private set
    private val history = mutableListOf<String>()

    fun deposit(amount: Int) {
        require(amount > 0) { "amount must be positive" }
        balance += amount
        log("+$amount")
    }

    fun withdraw(amount: Int): Boolean {
        if (amount > balance) return false
        balance -= amount
        log("-$amount")
        return true
    }

    fun statement(): List<String> = history.toList()
    private fun log(entry: String) { history += entry }
}

fun main() {
    val acc = BankAccount("Ada")
    acc.deposit(100)
    println(acc.withdraw(250))
    println(acc.withdraw(30))
    println("${acc.owner}: ${acc.balance} ${acc.statement()}")
}

class Money private constructor(val paise: Long) {
    companion object {
        const val PAISE_PER_RUPEE = 100
        val ZERO = Money(0)
        fun rupees(r: Long) = Money(r * PAISE_PER_RUPEE)
        fun parse(text: String): Money? =
            text.removePrefix("Rs ").toLongOrNull()?.let { rupees(it) }
    }
    operator fun plus(other: Money) = Money(paise + other.paise)
    override fun toString() = "Rs ${paise / PAISE_PER_RUPEE}.${(paise % PAISE_PER_RUPEE).toString().padStart(2, '0')}"
}

object IdGenerator {
    private var last = 0
    fun next() = "ID-${++last}"
}

fun main() {
    println(Money.rupees(5) + Money.parse("Rs 20")!!)
    println(Money.ZERO)
    println(Money.parse("twenty"))
    println(IdGenerator.next() + " " + IdGenerator.next())
}

fun main() {
    val byLength = object : Comparator<String> {
        override fun compare(a: String, b: String) = a.length - b.length
    }
    println(listOf("kiwi", "fig", "banana").sortedWith(byLength))

    val point = object {
        val x = 3
        val y = 4
    }
    println(point.x * point.x + point.y * point.y)
}

class Playlist(val name: String) {
    private val songs = mutableListOf<String>()

    class Builder {                     // nested: no outer instance
        private val items = mutableListOf<String>()
        fun add(s: String) = apply { items += s }
        fun build(name: String) = Playlist(name).also { it.songs += items }
    }

    inner class Cursor {                // inner: bound to one Playlist
        private var i = 0
        fun next(): String? = songs.getOrNull(i++)?.let { "$it (from ${[email protected]})" }
    }
}

fun main() {
    val p = Playlist.Builder().add("Intro").add("Outro").build("Focus")
    val cursor = p.Cursor()             // needs an outer instance
    println(cursor.next())
    println(cursor.next())
    println(cursor.next())
}

class Isbn(val code: String) {
    override fun equals(other: Any?): Boolean =
        other is Isbn && other.code == code

    override fun hashCode(): Int = code.hashCode()

    override fun toString() = "Isbn($code)"
}

class Tag(val name: String)       // no overrides

fun main() {
    val a = Isbn("978-0")
    val b = Isbn("978-0")
    println(a == b)
    println(a === b)
    println(setOf(a, b).size)
    println(a)

    println(Tag("x") == Tag("x"))
    println(setOf(Tag("x"), Tag("x")).size)
}