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[Kotlin] Data Class

In Kotlin, these are called data classes and are marked with data which main purpose is to hold 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).

Rules​

To ensure consistency and meaningful behavior of the generated code, 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​

Only the property name will be used inside the toString(), equals(), hashCode(), and copy() implementations.

data class Person(val name: String) {
    var age: Int = 0
}
val person1 = Person("John")
println(person1.toString())   // Person(name=John)

Copying​

Use the copy() function to copy an object, allowing you to alter some of its properties while keeping the rest unchanged.

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

Destructuring declarations​

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

Reference​