Data Classes

Data classes are classes whose primary purpose is to hold data (dumb POJOs). The benefit is that the compiler automatically generates

• 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

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

Data classes have to fulfill 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;

Properties Declared in the Class Body

Compiler only uses the properties defined inside the primary constructor for the automatically generated functions. To exclude a property from the generated implementations, declare it inside the class body:

data class Person(val name: String) {
    var age: Int = 0
}

In above example, only name used inside toString(), equals(), hashCode(), and copy() implementations, only one component function component1(). Two Person objects with different ages will be treated as equal.

Parameterless constructor

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

Copy()

Generated copy function looks like

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

Which allows us to copy certain properties and change certain properties like below

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

Data Classes and Destructuring Declarations

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