Is it possible to have properties other than those generated automatically in a data class?

Question

Kotlin allows you to simplify the creation of what in Java we call POJO:

data class Person(val firstName: String, val lastName: String)

With this we get a class with the getter for all variables declared in the constructor and methods equals(), toString(), copy(), among others.

My question is whether it is possible to add other properties. For example in the class Person add/declare the property fullName: String.

Answer 1

Yes, it is possible. One way to do this is through of extensions.

One can create both functions and properties in this way.

To declare an extension, you need to prefix its name with the type being extended.

To keyword this is valid within extension functions and is used to refer to the "receiver" object (which is extended).

Extension properties do not support the use of backing Fields.

See a full example

fun main(args: Array<String>) {
    val per = Person("Linq", "Bueno")
    println(per.obterFullName())
    println(per.fullName)
}

fun Person.obterFullName() = "${this.firstName} ${this.lastName}"
// ^ Isto é uma função de extensão

val Person.fullName: String
  get() = "${this.firstName} ${this.lastName}"
// ^ Isto é uma propriedade de extensão    

data class Person(val firstName: String, val lastName: String)

See working here.

Answer 2

It is possible to create property extensions, and in the example quoted it should work because I believe that in the background it is not a property, to tell the truth or I don’t really like the name, because I think that property implies having a state field linked, which is not very the case. Then you can create a method getter and even a Setter depending on what you want to do and the usage syntax will be equal to that of a field, but it cannot have an associated state field, so the answer is yes to the behavior and not to the state.

You can add behaviors and even extra states to your own data class also, but they will not be part of the basic class structure, for example a property that is not in the primary constructor does not enter the equals(), or toString().

fun main(args: Array<String>) {
    val pessoa = Person("João", "Silva")
    pessoa.age = 47
    println(pessoa.fullName)
    println(pessoa.Firula())
    println(pessoa.firula)
    val pessoa2 = pessoa.copy()
    println(pessoa2.age)
    pessoa2.age = 40
    println(pessoa2.age)
    println(pessoa == pessoa2)
    println(pessoa2)
}

data class Person(val firstName: String, val lastName: String) {
    fun Firula(): String = "O nome dele é " + firstName + " " + lastName
    val firula: String
        get() = "O nome dele é " + firstName + " " + lastName
    var age: Int = 0
    override fun toString() = firstName + " " + lastName
}

val Person.fullName: String
    get() = firstName + " " + lastName

Behold working in the ideone. And in the Kotlin Play. Also put on the Github for future reference.

Of course it is possible to extend with functions also that only has behavior.

Just note that the extension occurs statically, has nothing of inheritance in this extension. So they’re just facilitators, it doesn’t change the class structure, whether it’s a data class or a normal class.

Answer 3

Complementing the excellent responses, you can create immutable properties based on existing ones.

Either at initialization/declaration:

data class Person(val firstName: String, val lastName: String) {
    val fullName = "$firstName $lastName"
}

If it makes more sense for organization (of course if the boot is small we don’t need to use the init):

data class Person(val firstName: String, val lastName: String) {
    val fullName: String

    init {
        fullName = "$firstName $lastName"
    }
}

Or via Lazy instantiation:

data class Person(val firstName: String, val lastName: String) {
    val fullName: String by lazy { "$firstName $lastName" }
}