Extend data class in Kotlin

Question

Data classes seem to be the replacement to the old-fashioned POJOs in Java. It is quite expectable that these classes would allow for inheritance, but I can see no convenient way to extend a data class. What I need is something like this:

open data class Resource (var id: Long = 0, var location: String = "")
data class Book (var isbn: String) : Resource()

The code above fails because of clash of component1() methods. Leaving data annotation in only one of classes does not do the work, too.

Perhaps there is another idiom to extend data classes?

UPD: I might annotate only child child class, but data annotation only handles properties declared in the constructor. That is, I would have to declare all parent's properties open and override them, which is ugly:

open class Resource (open var id: Long = 0, open var location: String = "")
data class Book (
    override var id: Long = 0,
    override var location: String = "",
    var isbn: String
) : Resource()

Answer 1

The truth is: data classes do not play too well with inheritance. We are considering prohibiting or severely restricting inheritance of data classes. For example, it's known that there's no way to implement equals() correctly in a hierarchy on non-abstract classes.

So, all I can offer: don't use inheritance with data classes.

Answer 2

Declare properties in super-class outside of constructor as abstract, and override them in sub-class.

abstract class Resource {
    abstract var id: Long
    abstract var location: String
}

data class Book (
    override var id: Long = 0,
    override var location: String = "",
    var isbn: String
) : Resource()

Answer 3

Above solution using abstract class actually generates corresponding class and let the data class extends from it.

If you don't prefer abstract class, how about using an interface?

Interface in Kotlin can have properties as shown in this this article..

interface History {
    val date: LocalDateTime
    val name: String
    val value: Int
}

data class FixedHistory(override val date: LocalDateTime,
                        override val name: String,
                        override val value: Int,
                        val fixedEvent: String) : History

I was curious how Kotlin compile this. Here's equivalent Java code (generated using the Intellij [Kotlin bytecode] feature):

public interface History {
   @NotNull
   LocalDateTime getDate();

   @NotNull
   String getName();

   int getValue();
}

public final class FixedHistory implements History {
   @NotNull
   private final LocalDateTime date;
   @NotNull
   private final String name;
   private int value;
   @NotNull
   private final String fixedEvent;

   // Boring getters/setters as usual..
   // copy(), toString(), equals(), hashCode(), ...
}

As you can see, it works exactly like a normal data class!

Answer 4

Kotlin Traits can help.

interface IBase {
    val prop:String
}

interface IDerived : IBase {
    val derived_prop:String
}

data classes

data class Base(override val prop:String) : IBase

data class Derived(override val derived_prop:String,
                   private val base:IBase) :  IDerived, IBase by base

sample usage

val b = Base("base")
val d = Derived("derived", b)

print(d.prop) //prints "base", accessing base class property
print(d.derived_prop) //prints "derived"

This approach can also be a workaround for inheritance issues with @Parcelize

@Parcelize 
data class Base(override val prop:Any) : IBase, Parcelable

@Parcelize // works fine
data class Derived(override val derived_prop:Any,
                   private val base:IBase) : IBase by base, IDerived, Parcelable

Answer 5

You can inherit a data class from a non-data class.

Base class

open class BaseEntity (

@ColumnInfo(name = "name") var name: String? = null,
@ColumnInfo(name = "description") var description: String? = null,
// ...
)

child class

@Entity(tableName = "items", indices = [Index(value = ["item_id"])])
data class CustomEntity(

    @PrimaryKey
    @ColumnInfo(name = "id") var id: Long? = null,
    @ColumnInfo(name = "item_id") var itemId: Long = 0,
    @ColumnInfo(name = "item_color") var color: Int? = null

) : BaseEntity()

It worked.

Answer 6

@Željko Trogrlić answer is correct. But we have to repeat the same fields as in an abstract class.

Also if we have abstract subclasses inside the abstract class, then in a data class we cannot extend fields from these abstract subclasses. We should first create data subclass and then define fields.

abstract class AbstractClass {
    abstract val code: Int
    abstract val url: String?
    abstract val errors: Errors?

    abstract class Errors {
        abstract val messages: List<String>?
    }
}



data class History(
    val data: String?,

    override val code: Int,
    override val url: String?,
    // Do not extend from AbstractClass.Errors here, but Kotlin allows it.
    override val errors: Errors?
) : AbstractClass() {

    // Extend a data class here, then you can use it for 'errors' field.
    data class Errors(
        override val messages: List<String>?
    ) : AbstractClass.Errors()
}

Answer 7

How I did it.

open class ParentClass {
  var var1 = false
  var var2: String? = null
}

data class ChildClass(
  var var3: Long
) : ParentClass()

It's working fine.

Answer 8

You can inherit a data class from a non-data class. Inheriting a data class from another data class is not allowed because there is no way to make compiler-generated data class methods work consistently and intuitively in case of inheritance.

Answer 9

As usual, when inheritance becomes problematic, the solution is composition. See Prefer composition over inheritance?.

If you just want to "extend" your class with a few additional fields, you can use composition along with some additional getters for convenience:

data class Book(
  val id: Long,
  val isbn: String,
  val author: String,
)

data class StoredBook(
  val book: Book,
  val version: Long,
  val createdAt: ZonedDateTime,
  val updatedAt: ZonedDateTime,
) {
  // proxy fields for convenience
  val id get() = book.id
  val isbn get() = book.isbn
  val author get() = book.author
}

This delegates Book properties to the book instance, so that a StoredBook can be used just like a Book in most cases, but you can still have some type-safety around whether you are dealing with an intermediate Book state or a persisted StoredBook.

To take it a step further, you could create a StoredResource interface for any stored database entry:

interface StoredResource {
  val id: Long
  val version: Long
  val createdAt: ZonedDateTime
  val updatedAt: ZonedDateTime
}

data class Book(
  val id: Long,
  val isbn: String,
  val author: String,
)

data class StoredBook(
  val book: Book,
  override val version: Long,
  override val createdAt: ZonedDateTime,
  override val updatedAt: ZonedDateTime,
) : StoredResource {
  override val id get() = book.id
  val isbn get() = book.isbn
  val author get() = book.author
}

Answer 10

While implementing equals() correctly in a hierarchy is indeed quite a pickle, it would still be nice to support inheriting other methods, for example: toString().

To be a bit more concrete, let's assume we have the following construct (obviously, it doesn't work because toString() is not inherited, but wouldn't it be nice if it would?):

abstract class ResourceId(open val basePath: BasePath, open val id: Id) {

    // non of the subtypes inherit this... unfortunately...
    override fun toString(): String = "/${basePath.value}/${id.value}"
}

data class UserResourceId(override val id: UserId) : ResourceId(UserBasePath, id)

data class LocationResourceId(override val id: LocationId) : ResourceId(LocationBasePath, id)

Assuming our User and Location entities return their appropriate resource IDs (UserResourceId and LocationResourceId respectively), calling toString() on any ResourceId could result in quite a nice little representation that is generally valid for all subtypes: /users/4587, /locations/23, etc. Unfortunately, because non of the subtypes inherited to overridden toString() method from the abstract base ResourceId, calling toString() actually results in a less pretty representation: <UserResourceId(id=UserId(value=4587))>, <LocationResourceId(id=LocationId(value=23))>

There are other ways to model the above, but those ways either force us to use non-data-classes (missing out on a lot of the benefits of data classes), or we end up copying/repeating the toString() implementation in all our data classes (no inheritance).

Answer 11

Another way to implement inheritance is to use class with abstract values

sealed class Parent {
    
    abstract val someVal: String
    
    data class Child1(override val someVal: String) : Parent()
    
    data class Child2(override val someVal: String) : Parent()
}

Answer 12

The data class I wanted to inherit from turned out to have no behavior that shouldn't be encapsulated in an interface. With a private data class for "common" implementors of the interface, all objects can have the benefits of data while it feels like UnitVector extends V.

interface Vector {
    companion object {
        fun build(x : Float ...) : Vector = V(x ...)
        private data class V(override val x : Float ...) : Vector
    }
    val x : Float
    //functions, y etc. 
} 
data class UnitVector(override var x : Float ...) : Vector {
    init {
        //special behavior
    } 
}

Answer 13

data class User(val id: Long, var name: String)

fun main() {
    val user1 = User(id:1, name:"Kart")
    val name = user1.name
    println(name)
    user1.name = "Michel"
    val user2 = User(id:1, name:"Michel")
    println(user1 == user2)
    println(user1)
    val updateUser = user1.copy(name = "DK DK")
    println(updateUser)
    println(updateUser.component1())
    println(updateUser.component2())
    val (id, name) = updateUser
    println("$id,$name")
}

//here is the output below

Check the image why it shows error id:1 (compiler says that use = instead of double dot where I insert the value)

Answer 14

I found the best way for having an option to use inheritance in DTO is to make data classes in java with Lombok plugin.

Dont forget to place lombok.equalsAndHashCode.callSuper to true in annotation