Complete solution to implement a Recyclerview with sticky group headers. The ItemDecorations class is already given in this question https://stackoverflow.com/questions/32949971/how-can-i-make-sticky-headers-in-recyclerview-without-external-lib

Directory.java

public class Directory {
    String name;
    String branch;
    boolean isHeader;
	
	public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getBranch() {
        return branch;
    }

    public void setBranch(String branch) {
        this.branch = branch;
    }

    public boolean isHeader() {
        return isHeader;
    }

    public void setHeader(boolean header) {
        isHeader = header;
    }
}

HeaderItemDecoration.java

public class HeaderItemDecoration extends RecyclerView.ItemDecoration {

    private StickyHeaderInterface mListener;
    private int mStickyHeaderHeight;

    public HeaderItemDecoration(RecyclerView recyclerView, @NonNull StickyHeaderInterface listener) {
        mListener = listener;

        // On Sticky Header Click
        recyclerView.addOnItemTouchListener(new RecyclerView.OnItemTouchListener() {
            public boolean onInterceptTouchEvent(RecyclerView recyclerView, MotionEvent motionEvent) {
                if (motionEvent.getY() <= mStickyHeaderHeight) {
                    // Handle the clicks on the header here ...
                    return true;
                }
                return false;
            }

            public void onTouchEvent(RecyclerView recyclerView, MotionEvent motionEvent) {

            }

            public void onRequestDisallowInterceptTouchEvent(boolean disallowIntercept) {

            }
        });
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

        View topChild = parent.getChildAt(0);
        if (isNull(topChild)) {
            return;
        }

        int topChildPosition = parent.getChildAdapterPosition(topChild);
        if (topChildPosition == RecyclerView.NO_POSITION) {
            return;
        }

        View currentHeader = getHeaderViewForItem(topChildPosition, parent);
        fixLayoutSize(parent, currentHeader);
        int contactPoint = currentHeader.getBottom();
        View childInContact = getChildInContact(parent, contactPoint);
        if (isNull(childInContact)) {
            return;
        }

        if (mListener.isHeader(parent.getChildAdapterPosition(childInContact))) {
            moveHeader(c, currentHeader, childInContact);
            return;
        }

        drawHeader(c, currentHeader);
    }

    private View getHeaderViewForItem(int itemPosition, RecyclerView parent) {
        int headerPosition = mListener.getHeaderPositionForItem(itemPosition);
        int layoutResId = mListener.getHeaderLayout(headerPosition);
        View header = LayoutInflater.from(parent.getContext()).inflate(layoutResId, parent, false);
        mListener.bindHeaderData(header, headerPosition);
        return header;
    }

    private void drawHeader(Canvas c, View header) {
        c.save();
        c.translate(0, 0);
        header.draw(c);
        c.restore();
    }

    private void moveHeader(Canvas c, View currentHeader, View nextHeader) {
        c.save();
        c.translate(0, nextHeader.getTop() - currentHeader.getHeight());
        currentHeader.draw(c);
        c.restore();
    }

    private View getChildInContact(RecyclerView parent, int contactPoint) {
        View childInContact = null;
        for (int i = 0; i < parent.getChildCount(); i++) {
            View child = parent.getChildAt(i);
            if (child.getBottom() > contactPoint) {
                if (child.getTop() <= contactPoint) {
                    // This child overlaps the contactPoint
                    childInContact = child;
                    break;
                }
            }
        }
        return childInContact;
    }

    /**
     * Properly measures and layouts the top sticky header.
     * @param parent ViewGroup: RecyclerView in this case.
     */
    private void fixLayoutSize(ViewGroup parent, View view) {

        // Specs for parent (RecyclerView)
        int widthSpec = View.MeasureSpec.makeMeasureSpec(parent.getWidth(), View.MeasureSpec.EXACTLY);
        int heightSpec = View.MeasureSpec.makeMeasureSpec(parent.getHeight(), View.MeasureSpec.UNSPECIFIED);

        // Specs for children (headers)
        int childWidthSpec = ViewGroup.getChildMeasureSpec(widthSpec, parent.getPaddingLeft() + parent.getPaddingRight(), view.getLayoutParams().width);
        int childHeightSpec = ViewGroup.getChildMeasureSpec(heightSpec, parent.getPaddingTop() + parent.getPaddingBottom(), view.getLayoutParams().height);

        view.measure(childWidthSpec, childHeightSpec);

        view.layout(0, 0, view.getMeasuredWidth(), mStickyHeaderHeight = view.getMeasuredHeight());
    }

    private boolean isNull(View view) {
        return (view == null);
    }

    public interface StickyHeaderInterface {

        /**
         * This method gets called by {@link HeaderItemDecoration} to fetch the position of the header item in the adapter
         * that is used for (represents) item at specified position.
         * @param itemPosition int. Adapter's position of the item for which to do the search of the position of the header item.
         * @return int. Position of the header item in the adapter.
         */
        int getHeaderPositionForItem(int itemPosition);

        /**
         * This method gets called by {@link HeaderItemDecoration} to get layout resource id for the header item at specified adapter's position.
         * @param headerPosition int. Position of the header item in the adapter.
         * @return int. Layout resource id.
         */
        int getHeaderLayout(int headerPosition);

        /**
         * This method gets called by {@link HeaderItemDecoration} to setup the header View.
         * @param header View. Header to set the data on.
         * @param headerPosition int. Position of the header item in the adapter.
         */
        void bindHeaderData(View header, int headerPosition);

        /**
         * This method gets called by {@link HeaderItemDecoration} to verify whether the item represents a header.
         * @param itemPosition int.
         * @return true, if item at the specified adapter's position represents a header.
         */
        boolean isHeader(int itemPosition);
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private final String TAG = MainActivity.this.getClass().getSimpleName();

	  RecyclerView mRecyclerView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        try {
            setContentView(R.layout.main_activity);
            mRecyclerView = (RecyclerView) rootView.findViewById(R.id.recycler_view);

			mRecyclerView.setHasFixedSize(true);
			final LinearLayoutManager mLayoutManager = new LinearLayoutManager(getActivity());
			mLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
			mRecyclerView.setLayoutManager(mLayoutManager);

			MyRecyclerAdapter mMyRecyclerAdapter = new MyRecyclerAdapter(getActivity(), yourDataArrayList);
			mRecyclerView.setAdapter(mMyRecyclerAdapter);

			mRecyclerView.addItemDecoration(new HeaderItemDecoration(mRecyclerView, mMyRecyclerAdapter));
        }
        catch (Exception ex) {
            Log.e(TAG, "onCreate: " + ex.toString());
        }
    }
}

MyRecyclerAdapter.java

public class MyRecyclerAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder>
        implements HeaderItemDecoration.StickyHeaderInterface {

    private List<Directory> mList;
    private Context mContext;

    private final int VIEW_HEADER = 1;
    private final int VIEW_DETAIL = 0;

    public MyRecyclerAdapter(Context context, List<Directory> mList) {
        this.mList = mList;
        this.mContext = context;
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {
        if (viewType == VIEW_HEADER) {
            View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.row_header, null);
            return new HeaderViewHolder(v);
        } else {
            View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.row_detail, null);
            return new DetailViewHolder(v);
        }
    }

    @Override
    public void onBindViewHolder(@NonNull final RecyclerView.ViewHolder holder, final int position) {
        if (holder instanceof HeaderViewHolder) {
            ((HeaderViewHolder) holder).bind(getItem(position));
        } else {
            ((DetailViewHolder) holder).bind(getItem(position));
        }
    }

    public class HeaderViewHolder extends RecyclerView.ViewHolder {
        TextView tvName;

        HeaderViewHolder(@NonNull View itemView) {
            super(itemView);
            tvName = (TextView) itemView.findViewById(R.id.tv_name);
        }

        void bind(@NonNull Directory model) {
            tvName.setText(model.getSubCategory());
        }
    }

    public class DetailViewHolder extends RecyclerView.ViewHolder {
        TextView tvName;
        TextView tvBranch;

        DetailViewHolder(@NonNull View view) {
            super(view);
            tvName = (TextView) view.findViewById(R.id.tv_name);
            tvBranch = (TextView) view.findViewById(R.id.tv_branch);
        }

        void bind(@NonNull Directory child) {
            tvName.setText(child.getName());
            tvBranch.setText(child.getBranch());
        }
    }

    @Override
    public int getItemCount() {
        return (null != mList ? mList.size() : 0);
    }

    public Directory getItem(int position) {
        return mList.get(position);
    }

    @Override
    public int getHeaderPositionForItem(int itemPosition) {
        int headerPosition = 0;
        do {
            if (this.isHeader(itemPosition)) {
                headerPosition = itemPosition;
                break;
            }
            itemPosition -= 1;
        } while (itemPosition >= 0);
        return headerPosition;
    }

    @Override
    public int getHeaderLayout(int headerPosition) {
        return R.layout.row_header;
    }

    @Override
    public void bindHeaderData(View header, int headerPosition) {
        Directory child = mList.get(headerPosition);

        TextView tvName = header.findViewById(R.id.tv_name);
        tvName.setText(child.getSubCategory());
    }

    @Override
    public boolean isHeader(int itemPosition) {
        return mList.get(itemPosition).isHeader();
    }

    @Override
    public int getItemViewType(int position) {
        return (mList.get(position).isHeader()) ? VIEW_HEADER : VIEW_DETAIL;
    }
}

If there are thousands of Directory in database, how to handle it? so I can't load all in one time. If doesn't have "Head", I can use Page + Room to handle it. any suggestion?

If you have thousands of records then the you need to use the pagination to fetch limited records one at a time and append it to the recyclerview. In order to implement sticky group header in recyclerview you need to write a logic to group all of your data and mark true for "Directory" "isHeader" parameter when you found a header item in your list.

@kasunn25, thanks a lot. I want to use PagedListAdapter + Room to do pagination, can you provide some idea?

header does not match parent

I'm sorry, but what is getSubCategory() ??