DragSelectRecyclerView list = (DragSelectRecyclerView) findViewById(R.id.list);
list.setLayoutManager(new GridLayoutManager(this, 3));
list.setAdapter(adapter);

public class MainAdapter extends DragSelectRecyclerViewAdapter<MainAdapter.MainViewHolder> {

    public interface ClickListener {
        void onClick(int index);

        void onLongClick(int index);
    }

    private final ClickListener mCallback;

    // Constructor takes click listener callback
    protected MainAdapter(ClickListener callback) {
        super();
        mCallback = callback;
    }

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

    @Override
    public void onBindViewHolder(MainViewHolder holder, int position) {
        super.onBindViewHolder(holder, position); // this line is important!
    
        // Sets position + 1 to a label view
        holder.label.setText(String.format("%d", position + 1));

        if (isIndexSelected(position)) {
            // Item is selected, change it somehow 
        } else {
            // Item is not selected, reset it to a non-selected state
        }
    }

    @Override
    protected boolean isIndexSelectable(int index) {
        // This method is OPTIONAL, returning false will prevent the item at the specified index from being selected.
        // Both initial selection, and drag selection.
        return true;
    }

    @Override
    public int getItemCount() {
        return 60;
    }

    public class MainViewHolder extends RecyclerView.ViewHolder
            implements View.OnClickListener, View.OnLongClickListener{

        public final TextView label;

        public MainViewHolder(View itemView) {
            super(itemView);
            this.label = (TextView) itemView.findViewById(R.id.label);
            this.itemView.setOnClickListener(this);
            this.itemView.setOnLongClickListener(this);
        }
        
        @Override
        public void onClick(View v) {
            // Forwards to the adapter's constructor callback
            if (mCallback != null) mCallback.onClick(getAdapterPosition());
        }
    
        @Override
        public boolean onLongClick(View v) {
            // Forwards to the adapter's constructor callback
            if (mCallback != null) mCallback.onLongClick(getAdapterPosition());
            return true;
        }
    }
}

public class MainActivity extends AppCompatActivity implements
        MainAdapter.ClickListener, DragSelectRecyclerViewAdapter.SelectionListener {

    private DragSelectRecyclerView mList;
    private MainAdapter mAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Setup adapter and callbacks
        mAdapter = new MainAdapter(this);
        // Receives selection updates, recommended to set before restoreInstanceState() so initial reselection is received
        mAdapter.setSelectionListener(this);
        // Restore selected indices after Activity recreation
        mAdapter.restoreInstanceState(savedInstanceState);

        // Setup the RecyclerView
        mList = (DragSelectRecyclerView) findViewById(R.id.list);
        mList.setLayoutManager(new GridLayoutManager(this, getResources().getInteger(R.integer.grid_width)));
        mList.setAdapter(mAdapter);
    }

    @Override
    public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
        super.onSaveInstanceState(outState, outPersistentState);
        // Save selected indices to be restored after recreation
        mAdapter.saveInstanceState(outState);
    }

    @Override
    public void onClick(int index) {
        // Single click will select or deselect an item
        mAdapter.toggleSelected(index);
    }

    @Override
    public void onLongClick(int index) {
        // Long click initializes drag selection, and selects the initial item
        mList.setDragSelectActive(true, index);
    }

    @Override
    public void onDragSelectionChanged(int count) {
        // TODO Selection was changed, updating an indicator, e.g. a Toolbar or contextual action bar
    }
}

// Clears all selected indices
adapter.clearSelected();

// Sets an index as selected (true) or unselected (false);
adapter.setSelected(index, true);

// If an index is selected, unselect it. Otherwise select it. Returns new selection state.
boolean selectedNow = adapter.toggleSelected(index);

// Gets the number of selected indices
int count = adapter.getSelectedCount();

// Gets all selected indices
Integer[] selectedItems = adapter.getSelectedIndices();

// Checks if an index is selected, useful in adapter subclass
boolean selected = adapter.isIndexSelected(index);

// Sets a listener that's notified of selection changes, used in the section above
adapter.setSelectionListener(listener);

// Used in section above, saves selected indices to Bundle
adapter.saveInstanceState(outState);

// Used in section above, restores selected indices from Bundle
adapter.restoreInstanceState(inState);