Java vs Kotlin generics

Question

I have the following Java class I'm trying to convert to Kotlin that uses generics.

abstract class MvpViewHolder, M, V : View?>(itemView: View) : RecyclerView.ViewHolder(itemView) {

public abstract class MvpViewHolder<P extends BasePresenter> extends RecyclerView.ViewHolder {
    protected P presenter;

    public MvpViewHolder(View itemView) {
        super(itemView);
    }

    public void bindPresenter(P presenter) {
        this.presenter = presenter;
        presenter.bindView(this);
    }

    public void unbindPresenter() {
        presenter = null;
    }
}

Here is my Kotlin attempt

abstract class MvpViewHolder<P : BasePresenter>(itemView: View) : RecyclerView.ViewHolder(itemView) {

    protected var presenter: P? = null

    fun bindPresenter(presenter: P?): Unit {
        this.presenter = presenter
        presenter?.bindView(this)

    }

    fun unbindPresenter(): Unit {
        this.presenter = null
    }
}

I'm particularly running into a problem with the generics. It turns out that in Kotlin it's simply not enough to do MvpViewHolder<P : BasePresenter> as Kotlin requires that we pass in the 2 type arguments for BasePresenter (whose implementation I put below)

However, if I need to pass in the type arguments for BasePresenter then my method signature would then look like this

`abstract class MvpViewHolder<P : BasePresenter<*, *>>(itemView: View) : RecyclerView.ViewHolder(itemView) {`

This does not help me however, because in presenter.bindView(this) I get a type error of Required: Nothing, Found: MvpViewHolder

I could also get more specific and pass in MvpViewHolder<P: BasePresenter<M, V>, M, V> but then that would mean that wherever I call MvpViewHolder, I then also have to include 2 extra type parameters. Not only will that be tedious to deal with now having to maintain, but it just makes me sad.

How can I either get rid of the error when I use BasePresenter<,> or avoid having to pass in 3 type parameters into my MvpViewHolder class, just so I can define P as a BasePresenter

abstract class BasePresenter<M, V> {
    var model: M? = null

    var view: WeakReference<V>? = null

    fun setM(model: M?): Unit {
        this.model = model

        if (setupDone()) {
            updateView()
        }
    }

    fun bindView(view: V) {
        this.view = WeakReference(view)
    }

    fun unbindView() {
        this.view = null
    }

    abstract fun updateView()

    fun view(): V? {
        return if (view == null) null else view?.get()
    }

    fun setupDone(): Boolean {
        return view() != null && model != null
    }

}

Sangeet Suresh · Accepted Answer · 2017-11-03 06:45:41Z

1

Change abstract class to the following code

 abstract class MvpViewHolder<P :BasePresenter<P,MvpViewHolder<P>>>(itemView: View) : RecyclerView.ViewHolder(itemView) {
        protected var presenter: P? = null
        fun bindPresenter(presenter: P) {
            this.presenter = presenter
            presenter.bindView(this)
        }

        fun unbindPresenter() {
            presenter = null
        }
    }

answered Nov 3, 2017 at 6:45

Sangeet Suresh

2,5451 gold badge20 silver badges20 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Rafa Over a year ago

what would happen to a class that extends MvpViewHolder? How would I pass in a parameter to that? open class VenuesViewHolder(itemView: View) : MvpViewHolder<VenuePresenter>(itemView), VenueView here is an example. VenuePresenter, which extends BasePresenter throws an error that it's not within its bounds

Rafa Over a year ago

I can turn this into another question if you wish

Sangeet Suresh Over a year ago

open class VenuesViewHolder<M,P:BasePresenter<P, MvpViewHolder<P>>>(itemView: View) : MvpViewHolder<P>(itemView) Your presenter will look like below

class VenuePresenter<P:BasePresenter<P, MvpViewHolder<P>>,M:MvpViewHolder<P>> :BasePresenter<P,M>(){     override fun updateView() {      }

Rafa Over a year ago

What is the pattern here? Is there any resource that you're using?

Sangeet Suresh Over a year ago

I am just referring Java generics and kotlin generics. Error can be corrected by just seeing the error message from ide itself

|

Collectives™ on Stack Overflow

Java vs Kotlin generics

1 Answer 1

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related