Data Classes

Data classes make it easy to create classes that are used to store values. Such classes are automatically provided with methods for copying, getting a string representation, and using instances in collections. You can override these methods with your own implementations in the class declaration.

data class User(val name: String, val id: Int) {           // 1
    override fun equals(other: Any?) =
        other is User && other.id == this.id               // 2
}
fun main() {
    val user = User("Alex", 1)
    println(user)                                          // 3

    val secondUser = User("Alex", 1)
    val thirdUser = User("Max", 2)

    println("user == secondUser: ${user == secondUser}")   // 4
    println("user == thirdUser: ${user == thirdUser}")

    // hashCode() function
    println(user.hashCode())                               // 5
    println(secondUser.hashCode())
    println(thirdUser.hashCode())

    // copy() function
    println(user.copy())                                   // 6
    println(user === user.copy())                          // 7
    println(user.copy("Max"))                              // 8
    println(user.copy(id = 3))                             // 9

    println("name = ${user.component1()}")                 // 10
    println("id = ${user.component2()}")
}

1. Defines a data class with the data modifier.
2. Override the default equals method by declaring users equal if they have the same id.
3. Method toString is auto-generated, which makes println output look nice.
4. Our custom equals considers two instances equal if their ids are equal.
5. Data class instances with exactly matching attributes have the same hashCode.
6. Auto-generated copy function makes it easy to create a new instance.
7. copy creates a new instance, so the object and its copy have distinct references.
8. When copying, you can change values of certain properties. copy accepts arguments in the same order as the class constructor.
9. Use copy with named arguments to change the value despite of the properties order.
10. Auto-generated componentN functions let you get the values of properties in the order of declaration.