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Data Classes

We frequently create classes whose main purpose is to hold data. In such a class some standard functionality and utility functions are often mechanically derivable from the data. In Kotlin, this is called a data class and is marked as data:

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

The compiler automatically derives the following members from all properties declared in the primary constructor:

  • equals()/hashCode() pair;
  • toString() of the form "User(name=John, age=42)";
  • componentN() functions corresponding to the properties in their order of declaration;
  • copy() function (see below).

To ensure consistency and meaningful behavior of the generated code, data classes have to fulfil the following requirements:

  • The primary constructor needs to have at least one parameter;
  • All primary constructor parameters need to be marked as val or var;
  • Data classes cannot be abstract, open, sealed or inner;
  • (before 1.1) Data classes may only implement interfaces.

Additionally, the members generation follows these rules with regard to the members inheritance:

  • If there are explicit implementations of equals(), hashCode() or toString() in the data class body or final implementations in a superclass, then these functions are not generated, and the existing implementations are used;
  • If a supertype has the componentN() functions that are open and return compatible types, the corresponding functions are generated for the data class and override those of the supertype. If the functions of the supertype cannot be overridden due to incompatible signatures or being final, an error is reported;
  • Providing explicit implementations for the componentN() and copy() functions is not allowed.

Since 1.1, data classes may extend other classes (see Sealed classes for examples).

On the JVM, if the generated class needs to have a parameterless constructor, default values for all properties have to be specified (see Constructors).

data class User(val name: String = "", val age: Int = 0)

Copying

It's often the case that we need to copy an object altering some of its properties, but keeping the rest unchanged. This is what copy() function is generated for. For the User class above, its implementation would be as follows:

fun copy(name: String = this.name, age: Int = this.age) = User(name, age)     

This allows us to write:

val jack = User(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2)

Data Classes and Destructuring Declarations

Component functions generated for data classes enable their use in destructuring declarations:

val jane = User("Jane", 35) 
val (name, age) = jane
println("$name, $age years of age") // prints "Jane, 35 years of age"

Standard Data Classes

The standard library provides Pair and Triple. In most cases, though, named data classes are a better design choice, because they make the code more readable by providing meaningful names for properties.