CODEWITHSUNDEEP
androidinheritancekotlin
Paul Okeke
Paul Okeke

Reputation: 1414

How does kotlin inheritance work? and how can "*", "in" and "out" be used

I have the below 

open class Model

class WorkOrder : Model()

//An interface

interface ViewInterface<T : Model> {
    fun notifyDataSuccessful(model: T?, models:ArrayList<T>?)
}

class WorkOrderSystemImpl(val viewInterface: ViewInterface<Model>) {

    fun doSomething() {
        val workOrders: ArrayList<WorkOrder> = ArrayList()
        //the below line complains of type mismatch
        viewInterface.notifyDataSuccessful(workOrders)
    }

}

It complains of type-mismatch which is quite strange to me, because WorkOrder is a sub-type of Model and i'd expect it to resolve to same type.

Upvotes: 1

Views: 74

Answers (1)

Hong Duan
Hong Duan

Reputation: 4294

It's about the Generics's invariant & covariant, see Kotlin docs here.

In short, you can just remember:

Consumer in, Producer out!

which the Consumer & Producer is determined from the List's view, that means you should think about the role of you List, is it aConsumer or Producer? In your case, the models:ArrayList<T>? is a Producer, because it will be used by the implementation of ViewInterface, so you should define the ViewInterface like this:

interface ViewInterface<T: Model> {
    fun notifyDataSuccessful(model: T?, models: ArrayList<out T>?)
}

Upvotes: 1

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