Kotlin: Type mismatch (RecyclerView.Adapter<RecyclerView.ViewHolder>)

Issue

I'm pretty new using Kotlin, I tried to port my project from java to kotlin. Basically, I have a helper class that try to init recyclerview layout, adapter and so on.

In java: Adapter

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder>
{
  ...
  public static class ViewHolder extends RecyclerView.ViewHolder
  {
    ...
  }
  ...
}

my helper function:

public void init(RecyclerView recyclerView, boolean horizontal, boolean divider, RecyclerView.Adapter adapter, Context context)

When I created the adapter and call the function, it works fine.

But in Kotlin: Adapter

class MyAdapter(...) : RecyclerView.Adapter<MyAdapter.ViewHolder>() {
  ...
  inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
    ...
  }
  ...
}

helper function

fun init(recyclerView : RecyclerView, horizontal: Boolean, divider: Boolean, adapter: RecyclerView.Adapter<RecyclerView.ViewHolder>, context: Context) {
  ...
}

when I created the adapter and call the function

val myAdapter = MyAdapter(...)
helper.init(listView, false, false, myAdapter, this)

I got mismatch error where it required: RecyclerView.Adapter<RecyclerView.ViewHolder> and found: MyAdapter

Inheritance wise, it should work right? or did I miss something?

Thanks!

Solution

Not quite, the correct declaration you require is:

class MyAdapter() : RecyclerView.Adapter<RecyclerView.ViewHolder>() 

and you then need to implement methods such as this signature:

override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int)
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder

etc.

onCreateViewHolder must return a ViewHolder or a subclass of it, e.g.

return ItemViewHolder(view)

where:-

inner class ItemViewHolder(val mView: View) : RecyclerView.ViewHolder(mView)
{
  val mFieldView: TextView = mView.field
  ...
}

and you can use this knowledge within the bind function, e.g.

override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int)
{
  val itemHolder = holder as ItemViewHolder
  val item = dataModel[position]
  itemHolder.mFieldView.text = item.field
}

Answered By - Chris Shaw