# Data Classes

Kotlin reduces repetitive coding.

The class mechanism performs a fair amount of work for you. However, creating classes that primarily hold data still requires a significant amount of repetitive code. When you need a class that’s essentially a data holder, data classes simplify your code and perform common tasks.

You define a data class using the data keyword, which tells Kotlin to generate additional functionality. Each constructor parameter must be preceded by var or val:

// DataClasses/Simple.kt
package dataclasses
import atomictest.eq

data class Simple(
  val arg1: String,
  var arg2: Int
)

fun main() {
  val s1 = Simple("Hi", 29)
  val s2 = Simple("Hi", 29)
  s1 eq "Simple(arg1=Hi, arg2=29)"
  s1 eq s2
}

This example reveals two features of data classes:

  1. The String produced by s1 is different than what we usually see; it includes the parameter names and values of the data held by the object. data classes display objects in a nice, readable format without requiring any additional code.
  2. If you create two instances of the same data class containing identical data (equal values for properties), you probably also want those two instances to be equal. To achieve that behavior for a regular class, you must define a special function equals() to compare instances. In data classes, this function is automatically generated; it compares the values of all properties specified as constructor parameters.

Here’s an ordinary class Person and a data class Contact:

// DataClasses/DataClasses.kt
package dataclasses
import atomictest.*

class Person(val name: String)

data class Contact(
  val name: String,
  val number: String
)

fun main() {
  // These seem the same, but they're not:
  Person("Cleo") neq Person("Cleo")
  // A data class defines equality sensibly:
  Contact("Miffy", "1-234-567890") eq
  Contact("Miffy", "1-234-567890")
}
/* Sample output:
dataclasses.Person@54bedef2
Contact(name=Miffy, number=1-234-567890)
*/

Because the Person class is defined without the data keyword, two instances containing the same name are not equal. Fortunately, creating Contact as a data class produces a reasonable result.

Notice the difference between the display format of the data class, and Person, which just shows default object information.

Another useful function generated for every data class is copy(), which creates a new object containing the data from the current object. However, it also allows you to change selected values in the process:

// DataClasses/CopyDataClass.kt
package dataclasses
import atomictest.eq

data class DetailedContact(
  val name: String,
  val surname: String,
  val number: String,
  val address: String
)

fun main() {
  val contact = DetailedContact(
    "Miffy",
    "Miller",
    "1-234-567890",
    "1600 Amphitheatre Parkway")
  val newContact = contact.copy(
    number = "098-765-4321",
    address = "Brandschenkestrasse 110")
  newContact eq DetailedContact(
    "Miffy",
    "Miller",
    "098-765-4321",
    "Brandschenkestrasse 110")
}

The parameter names for copy() are identical to the constructor parameters. All arguments have default values that are equal to the current values, so you provide only the ones you want to replace.

# HashMap and HashSet

Creating a data class also generates an appropriate hash function so that objects can be used as keys in HashMaps and HashSets:

// DataClasses/HashCode.kt
package dataclasses
import atomictest.eq

data class Key(val name: String, val id: Int)

fun main() {
  val korvo: Key = Key("Korvo", 19)
  korvo.hashCode() eq -2041757108
  val map = HashMap<Key, String>()
  map[korvo] = "Alien"
  map[korvo] eq "Alien"
  val set = HashSet<Key>()
  set.add(korvo)
  set.contains(korvo) eq true
}

hashCode() is used in conjunction with equals() to rapidly look up a Key in a HashMap or a HashSet. Creating a correct hashCode() by hand is tricky and error-prone, so it is quite beneficial to have the data class do it for you. [Operator Overloading](javascript:void(0)) covers equals() and hashCode() in more detail.

Exercises and solutions can be found at www.AtomicKotlin.com.