Классы в Kotlin: data class, sealed class и companion object

Обычный класс

class Person(val name: String, var age: Int) {
    val isAdult: Boolean get() = age >= 18
    
    fun greet() = println("Привет, я $name, мне $age лет")
    
    init {
        require(age >= 0) { "Возраст не может быть отрицательным" }
    }
}

val person = Person("Саша", 25)
person.greet()
println(person.isAdult)  // true

data class

data class Student(val name: String, val grade: Int, val subject: String = "Математика")

val s1 = Student("Саша", 5)
val s2 = Student("Саша", 5)
val s3 = s1.copy(grade = 4)

println(s1)          // Student(name=Саша, grade=5, subject=Математика)
println(s1 == s2)    // true!
println(s1 === s2)   // false
println(s3)          // Student(name=Саша, grade=4, subject=Математика)

val (name, grade) = s1  // Деструктуризация

sealed class

sealed class NetworkResult {
    data class Success(val data: T) : NetworkResult()
    data class Error(val message: String, val code: Int) : NetworkResult()
    object Loading : NetworkResult()
}

fun handle(result: NetworkResult) {
    when (result) {
        is NetworkResult.Success -> println("Данные: ${result.data}")
        is NetworkResult.Error -> println("Ошибка ${result.code}: ${result.message}")
        NetworkResult.Loading -> println("Загрузка...")
        // else не нужен!
    }
}

handle(NetworkResult.Loading)
handle(NetworkResult.Success("Пользователи загружены"))
handle(NetworkResult.Error("Нет сети", 503))

object — синглтон

object AppConfig {
    val appName = "МоёПриложение"
    val version = "1.0.0"
    var isDebug = false
    
    fun getFullVersion() = "$appName v$version"
}

println(AppConfig.appName)
println(AppConfig.getFullVersion())

companion object

class DatabaseHelper private constructor(val name: String) {
    companion object {
        private var instance: DatabaseHelper? = null
        
        fun getInstance(name: String): DatabaseHelper =
            instance ?: DatabaseHelper(name).also { instance = it }
        
        fun isValidName(name: String) = name.isNotBlank()
    }
}

val db1 = DatabaseHelper.getInstance("mydb")
val db2 = DatabaseHelper.getInstance("mydb")
println(db1 === db2)  // true!
println(DatabaseHelper.isValidName("test"))  // true

Interface и abstract

interface Drawable {
    fun draw()
    fun getColor(): String = "Black"  // Default!
}

abstract class Shape(val color: String) : Drawable {
    abstract fun area(): Double
}

class Circle(val radius: Double, color: String) : Shape(color) {
    override fun draw() = println("Круг радиуса $radius")
    override fun area() = Math.PI * radius * radius
}

val c = Circle(5.0, "Red")
c.draw()
println("%.2f".format(c.area()))

Итоги урока