data class

// Defining a data class (the primary constructor must have at least one property)
data class Point(val x: Int, val y: Int)

// Creating an instance
val p = Point(3, 4)

// Auto-generated methods
p.toString()  // "Point(x=3, y=4)"
p.copy(x = 10)  // Point(x=10, y=4)
val (x, y) = p  // Destructuring declaration

// equals performs value comparison.
Point(1, 2) == Point(1, 2)  // true

// Defining data classes
data class User(
    val id: Int,
    val name: String,
    val email: String,
    val age: Int = 0  // Default values are supported.
)

data class Point(val x: Double, val y: Double) {
    // You can add methods to a data class.
    fun distanceTo(other: Point): Double {
        val dx = x - other.x
        val dy = y - other.y
        return Math.sqrt(dx * dx + dy * dy)
    }
}

fun main() {
    val user1 = User(1, "Alice", "alice@example.com", 30)
    val user2 = User(1, "Alice", "alice@example.com", 30)
    val user3 = User(2, "Bob", "bob@example.com")

    // toString() — useful for debugging
    println(user1)  // User(id=1, name=Alice, email=alice@example.com, age=30)

    // equals() — value comparison
    println(user1 == user2)  // true (same values)
    println(user1 == user3)  // false

    // copy() — change only some properties
    val updatedUser = user1.copy(age = 31, email = "alice_new@example.com")
    println(updatedUser)
    println(user1)  // The original is unchanged (immutable)

    // Destructuring declaration (componentN)
    val (id, name, email) = user1
    println("ID: $id, Name: $name, Email: $email")

    // Using data classes in a list
    val users = listOf(user1, user3, User(3, "Carol", "carol@example.com", 25))
    val sortedUsers = users.sortedBy { it.name }
    sortedUsers.forEach { println("${it.name} (age: ${it.age})") }

    // Calling a method on Point
    val p1 = Point(0.0, 0.0)
    val p2 = Point(3.0, 4.0)
    println("Distance: ${p1.distanceTo(p2)}")  // 5.0

    // Copying with a partial change
    val p3 = p1.copy(x = 1.0)
    println("Copy: $p3")  // Point(x=1.0, y=0.0)
}

data class is extremely useful in Kotlin for implementing value objects in DDD (Domain-Driven Design) and response models for APIs. Code that would take nearly 100 lines in Java can be written in a single line.

Note: The primary constructor of a data class must have at least one property. Also, data classes are not automatically open, so they cannot be used as base classes for inheritance (use a regular class if you need inheritance).

For the basic class structure, see class / Constructor. For inheritance with type restrictions, see sealed class / sealed interface.