About RecyclerView.ViewHolder and RecyclerView.Adapter

Question

1. The RecyclerView.ViewHolder class has a field that is public final View itemView. It says that the onBindViewHolder method should update the contents of the itemView to reflect the item at the given position . Doesn’t the final modifier indicate that the value of this field cannot change ?

2. The code below is from the textbook :

   public class ViewHolder extends RecyclerView.ViewHolder {
      ...
         @Override
         public int getItemCount() {
             ...
         }
         @Override
         public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int    viewType) {
            ...
         }
         @Override
         public void onBindViewHolder(ViewHolder viewHolder, int position) {
            ...
         }
   }
   

   Why do these methods here can override the methods in the RecyclerView.Adapter class which is derived from the RecyclerView.ViewHolder class ?

https://developer.android.com/reference/android/support/v7/widget/RecyclerView.Adapter.html

https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ViewHolder.html

Could someone explain it ?

Thank you.

Answer 1

Doesn’t the final modifier indicate that the value of this field cannot change ?

The final modifier on a View indicate that you can only initiate the view once (by creating a new View(context) or inflate a view from an xml file). But you can still modify the view property. (i.e. your view contains a TextView, you can set the text)

For your second question, the text book is not very precise about how to implement the adapter with a view holder. Here is a simple implementation of an adapter with a custom view holder.

public class Adapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder>{

    private List<String> titles;

    public Adapter(List<String> titles) {
        this.titles = titles;
    }

    @Override
    public MyViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
        return new MyViewHolder(LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.item_view, viewGroup, false));
    }

    @Override
    public void onBindViewHolder(MyViewHolder myViewHolder, int i) {
        String title = titles.get(i);
        myViewHolder.title.setText(title);
    }

    @Override
    public int getItemCount() {
        return titles.size();
    }

    public static class MyViewHolder extends RecyclerView.ViewHolder {

        TextView title;

        public MyViewHolder(View itemView) {
            super(itemView);

            title = (TextView) itemView.findViewById(R.id.title_TV);
        }
    }

}

and the xml file for it:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <TextView
        android:id="@+id/title_tv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</RelativeLayout>

You can see that if you extend RecyclerView.Adapter, you will have to override these 3 methods.

Hope this will help you to understand more the RecyclerView.