Data Classes

Data classes are one of Kotlin's coolest features. They're classes made specifically to hold data, and Kotlin auto-generates a bunch of useful methods for you. Less boilerplate, more productivity.

The data class magic

Just slap data before class and Kotlin handles the rest.

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

fun main() {
  val user1 = User("Alice", 25)
  val user2 = User("Alice", 25)

  println(user1)
  println(user1 == user2)
}
    

toString() gives you a readable string. equals() compares by property values, not object identity. Without writing a single line of that logic.

Auto-generated methods

Here's what you get for free: toString(), equals(), hashCode(), copy(), and componentN() functions.

data class Book(val title: String, val author: String, val year: Int)

fun main() {
  val book = Book("1984", "George Orwell", 1949)
  val newerEdition = book.copy(year = 2024)

  println(book)
  println(newerEdition)

  val (title, author, year) = book
  println("$title by $author ($year)")
}
    

copy() creates a new object with some properties changed. Destructuring with componentN() lets you unpack properties into variables in one go.

Rules for data classes

• The primary constructor must have at least one parameter
• All primary constructor parameters must be val or var
• Data classes can't be abstract, open, sealed, or inner

Stick to these rules and the Kotlin compiler does all the heavy lifting for you.