Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

// Primary constructor with properties
class Person(val name: String, var age: Int) {

// Property with custom getter
    val isAdult: Boolean
        get() = age >= 18

// Property with backing field and custom setter
    var email: String = ""
        set(value) {
            require(value.contains('@')) { "Invalid email: $value" }
            field = value   // 'field' refers to the backing field
        }

// Init block — runs during construction
    init {
        require(name.isNotBlank()) { "Name must not be blank" }
        require(age >= 0)          { "Age must be non-negative" }
    }

// Secondary constructor
    constructor(name: String) : this(name, 0)

override fun toString() = "Person(name=$name, age=$age)"
}

// Sealed class hierarchy
sealed class Shape {
    abstract fun area(): Double
    class Circle(val radius: Double)            : Shape() { override fun area() = Math.PI * radius * radius }
    class Rectangle(val w: Double, val h: Double): Shape() { override fun area() = w * h }
}

fun main() {
    val alice = Person("Alice", 28)
    alice.email = "alice@example.com"
    println(alice)
    println("Adult: ${alice.isAdult}")

val shapes: List<Shape> = listOf(Shape.Circle(5.0), Shape.Rectangle(3.0, 4.0))
    shapes.forEach { s ->
        val desc = when (s) {
            is Shape.Circle    -> "Circle  area=${s.area()}"
            is Shape.Rectangle -> "Rect    area=${s.area()}"
        }
        println(desc)
    }
}