Data, Enum & Sealed Classes

Kotlin has special class kinds that remove boilerplate for common modeling needs.

Data classes

A data class auto-generates equals(), hashCode(), toString(), copy(), and destructuring components:

data class User(val name: String, val age: Int)

val a = User("Ada", 36)
val b = a.copy(age = 37)            // copy with one field changed
println(a)                         // User(name=Ada, age=36)
println(a == User("Ada", 36))      // true (value equality)

val (name, age) = a                // destructuring

Use data classes for values that are defined by their contents (DTOs, results, records).

Enum classes

A fixed set of named constants, which can carry data and methods:

enum class Direction(val degrees: Int) {
    NORTH(0), EAST(90), SOUTH(180), WEST(270);

    fun opposite(): Direction = when (this) {
        NORTH -> SOUTH
        SOUTH -> NORTH
        EAST -> WEST
        WEST -> EAST
    }
}

Direction.NORTH.degrees     // 0
Direction.valueOf("EAST")   // Direction.EAST
Direction.entries           // all values

Sealed classes and interfaces

A sealed type has a closed set of subtypes known at compile time — perfect for modeling “one of these cases”:

sealed interface Shape {
    data class Circle(val radius: Double) : Shape
    data class Rectangle(val width: Double, val height: Double) : Shape
}

fun area(shape: Shape): Double = when (shape) {
    is Shape.Circle -> Math.PI * shape.radius * shape.radius
    is Shape.Rectangle -> shape.width * shape.height
    // no `else` needed — the compiler knows all cases are covered
}

Because the compiler knows every subtype, a when over a sealed type is exhaustive without an else. Add a new subtype and the compiler flags every when you forgot to update — a huge safety win.

Inside each is branch, shape is smart-cast to that subtype, so you can access shape.radius directly without a manual cast.

Exercises

1. Model a Result sealed interface with Success(val data: String) and Failure(val error: String), then write a when that handles both.

2. Using the Shape type above, write area(shape: Shape): Double.

   Solution

   fun area(shape: Shape): Double = when (shape) {
       is Shape.Circle -> Math.PI * shape.radius * shape.radius
       is Shape.Rectangle -> shape.width * shape.height
   }
   

</details>

This solution is in examples/core/part3/ and is tested by CI.

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