Как расширить несколько классов в адаптере?

Этого легко добиться с помощью библиотеки SectionedRecyclerViewAdapter.

Сначала создайте класс раздела, чтобы сгруппировать свои задачи:

class TaskSection extends StatelessSection {

    String title;
    List<Task> list;

    public TaskSection(String title, List<Task> list) {
        // call constructor with layout resources for this Section header, footer and items 
        super(R.layout.section_header, R.layout.section_item);

        this.title = title;
        this.list = list;
    }

    @Override
    public int getContentItemsTotal() {
        return list.size(); // number of items of this section
    }

    public int addTask(Task task) {
        return list.add(task;
    }

    public int removeTask(Task task) {
        return list.remove(task;
    }

    @Override
    public RecyclerView.ViewHolder getItemViewHolder(View view) {
        // return a custom instance of ViewHolder for the items of this section
        return new MyItemViewHolder(view);
    }

    @Override
    public void onBindItemViewHolder(RecyclerView.ViewHolder holder, int position) {
        MyItemViewHolder itemHolder = (MyItemViewHolder) holder;

        // bind your view here
        itemHolder.tvItem.setText(list.get(position).getTitle());
    }

    @Override
    public RecyclerView.ViewHolder getHeaderViewHolder(View view) {
        return new SimpleHeaderViewHolder(view);
    }

    @Override
    public void onBindHeaderViewHolder(RecyclerView.ViewHolder holder) {
        MyHeaderViewHolder headerHolder = (MyHeaderViewHolder) holder;

        // bind your header view here
        headerHolder.tvItem.setText(title);
    }
}

Затем вы настраиваете RecyclerView со своими разделами:

// Create an instance of SectionedRecyclerViewAdapter 
SectionedRecyclerViewAdapter sectionAdapter = new SectionedRecyclerViewAdapter();

// Create your sections with the list of data
TaskSection compSection = new TaskSection("Completed", compList);
TaskSection pendSection = new TaskSection("Pending", pendList);

// Add your Sections to the adapter
sectionAdapter.addSection(compSection);
sectionAdapter.addSection(pendSection);

// Set up your RecyclerView with the SectionedRecyclerViewAdapter
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recyclerview);
recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
recyclerView.setAdapter(sectionAdapter);

Теперь, если вы хотите отправить задачу из раздела «Ожидающие» в «Завершенные»:

pendSection.removeTask(task);
compSection.addTask(task);
sectionAdapter.notifyDataSetChanged();

В этом образце также размещена ссылка, которую вы разместили. Реализовать можно вот так.

public class MainAdapter extends SectionedRecyclerViewAdapter<MainAdapter.MainVH> {

        @Override
        public int getSectionCount() {
            return 20; // number of sections.
        }

        @Override
        public int getItemCount(int section) {
            return 8; // number of items in section (section index is parameter).
        }

        @Override
        public void onBindHeaderViewHolder(MainVH holder, int section) {
            // Setup header view.
        }

        @Override
        public void onBindViewHolder(MainVH holder, int section, int relativePosition, int absolutePosition) {
            // Setup non-header view.
            // 'section' is section index.
            // 'relativePosition' is index in this section.
            // 'absolutePosition' is index out of all non-header items.
            // See sample project for a visual of how these indices work.
        }

        @Override
        public MainVH onCreateViewHolder(ViewGroup parent, int viewType) {
            // Change inflated layout based on 'header'. 
            View v = LayoutInflater.from(parent.getContext())
                    .inflate(viewType == VIEW_TYPE_HEADER ? R.layout.header : R.layout.normal, parent, false);
            return new MainVH(v);
        }

        public static class MainVH extends RecyclerView.ViewHolder {

            public MainVH(View itemView) {
                super(itemView);
                // Setup view holder.
                // You'd want some views to be optional, e.g. for header vs. normal.
            }
        }
    }

Самый простой способ разделить представление ресайклера на разделы - использовать макет с заголовком и уже установленным элементом, а затем изменить видимость, если заголовок такой же.

Макет:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/tvHeader"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="start"
        android:padding="16dp"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:textColor="@color/primary"
        android:textStyle="bold"
        tools:text="A" />

    <TextView
        android:id="@+id/tvName"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/tvHeader"
        android:background="?android:attr/selectableItemBackground"
        android:padding="16dp"
        android:textAppearance="?android:attr/textAppearanceMedium"
        tools:text="Adam" />

</RelativeLayout>

Адаптер (Kotlin Edition 2018):

class ContactAdapter @Inject constructor() : RecyclerView.Adapter<ContactAdapter.ViewHolder>() {

    var onItemClick: ((Contact) -> Unit)? = null
    var contacts = emptyList<Contact>()

    override fun getItemCount(): Int {
        return contacts.size
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        return ViewHolder(LayoutInflater.from(parent.context).inflate(R.layout.item_contact, parent, false))
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val name = contacts[position].name
        holder.header.text = name.substring(0, 1)
        holder.name.text = name

        // if not first item, check if item above has the same header
        if (position > 0 && contacts[position - 1].name.substring(0, 1) == name.substring(0, 1)) {
            holder.headerTextView.visibility = View.GONE
        } else {
            holder.headerTextView.visibility = View.VISIBLE
        }
    }

    inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        val headerTextView: TextView = itemView.tvHeader
        val nameTextView: TextView = itemView.tvName

        init {
            itemView.setOnClickListener {
                onItemClick?.invoke(contacts[adapterPosition])
            }
        }
    }
}

Также может быть полезно: RecyclerView itemClickListener в Kotlin

Старая версия адаптера Java:

public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.DataViewHolder> {

    private List<Contact> mData;

    @Inject
    public RecyclerAdapter() {
        mData = new ArrayList<>();
    }

    public void setData(List<Contact> data) {
        mData = data;
    }

    public Contact getItem(int position){
        return mData.get(position);
    }

    @Override
    public DataViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_contact, parent, false);
        return new DataViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(final DataViewHolder holder, int position) {
        Contact contact = mData.get(position);
        holder.headerTextView.setText(contact.getName().substring(0, 1));
        holder.nameTextView.setText(contact.getName());

        // if not first item check if item above has the same header
        if (position > 0 && mData.get(position - 1).getName().substring(0, 1).equals(contact.getName().substring(0, 1))) {
            holder.headerTextView.setVisibility(View.GONE);
        } else {
            holder.headerTextView.setVisibility(View.VISIBLE);
        }
    }

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

    public class DataViewHolder extends RecyclerView.ViewHolder {

        @BindView(R.id.text_header)
        TextView headerTextView;
        @BindView(R.id.text_name)
        TextView nameTextView;

        public DataViewHolder(View itemView) {
            super(itemView);
            ButterKnife.bind(this, itemView);
        }
    }
}