How to let a data class implements Interface / extends Superclass properties in Kotlin?

Question

I have several data classes which include a var id: Int? field. I want to express this in an interface or superclass, and have data classes extending that and setting this id when they are constructed. However, if I try this:

interface B {
  var id: Int?
}

data class A(var id: Int) : B(id)

It complains I'm overriding the id field, which I am haha..

Q: How can I let the data class A in this case to take an id when it's constructed, and set that id declared in the interface or superclass?

Answer 1

Indeed, you needn't an abstract class yet. you can just override the interface properties, for example:

interface B {
    val id: Int?
}

//           v--- override the interface property by `override` keyword
data class A(override var id: Int) : B

An interface has no constructors so you can't call the constructor by super(..) keyword , but you can using an abstract class instead. Howerver, a data class can't declare any parameters on its primary constructor, so it will overwritten the field of the superclass, for example:

//               v--- makes it can be override with `open` keyword
abstract class B(open val id: Int?)

//           v--- override the super property by `override` keyword
data class A(override var id: Int) : B(id) 
//                                   ^
// the field `id` in the class B is never used by A

// pass the parameter `id` to the super constructor
//                            v
class NormalClass(id: Int): B(id)