Kotlin Generics

Kotlin_Generics_problem.kt

open class A {

}

abstract class Adapter<T : A> {
    abstract fun create(): T
    abstract fun itemCount(): Int
    open fun bind(a: T) {
    }
}

class AdapterImpl : Adapter<B>() {
    override fun create(): B = B()
    override fun bind(a: B) {
    }

    override fun itemCount(): Int = 2
}

class B : A() {

}

class View {
    var adapter: Adapter<A> by Delegates.notNull() //You must use <A>, <out A>, <in A> doesn work, because it excludes A, and bind method becomes unavailable
    fun use() {
        var a = adapter.create();
        adapter.bind(a)
    }
}

class Main{
    fun run(){
        var view= View()
        var adapter = AdapterImpl()
        view.adapter=adapter // Will not compile, since you can't make Adapter Raw type
    }
}

Kotlin_Generics_solution.kt

open class A {

}

abstract class Adapter<T : A> {
    abstract fun create(): T
    abstract fun itemCount(): Int
    open fun bind(a: T) {
    }
}


class AdapterImpl : Adapter<A>() { //Use A
    override fun create(): B = B() //Use B since B:A
    override fun bind(a: A) { // Use A
    }

    override fun itemCount(): Int = 2
}

class B : A() {

}

class View {
    var adapter: Adapter<A> by Delegates.notNull()
    fun use() {
        var a = adapter.create();
        adapter.bind(a)
    }
}

class Main {
    fun run() {
        var view = View()
        var adapter = AdapterImpl()
        view.adapter = adapter
    }
}