Cannot resolve symbol UserAdapter -> Using a Custom Adapter for a Gridview Lesson

 package com.joshbgold.ribbit.ui;

 import android.app.AlertDialog;
 import android.os.Bundle;
 import android.support.v4.app.Fragment;
 import android.util.Log;
 import android.view.LayoutInflater;     
 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.ArrayAdapter;
 import android.widget.GridView;
 import android.widget.TextView;
 import com.joshbgold.ribbit.R;
 import com.joshbgold.ribbit.utils.ParseConstants;
 import com.parse.FindCallback;
 import com.parse.ParseException;
 import com.parse.ParseQuery;
 import com.parse.ParseRelation;
 import com.parse.ParseUser;
 import java.util.List;

public class FriendsFragment extends Fragment {

public static final String TAG = FriendsFragment.class.getSimpleName();

protected ParseRelation<ParseUser> mFriendsRelation;
protected ParseUser mCurrentUser;
protected List<ParseUser> mFriends;
protected GridView mGridView;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_friends,
            container, false);

    mGridView= (GridView)rootView.findViewById(R.id.friendsGrid);

    TextView emptyTextView = (TextView)rootView.findViewById(R.id.empty);
    mGridView.setEmptyView(emptyTextView);

    return rootView;
}

@Override
public void onResume() {
    super.onResume();

    mCurrentUser = ParseUser.getCurrentUser();
    mFriendsRelation = mCurrentUser.getRelation(ParseConstants.KEY_FRIENDS_RELATION);

    getActivity().setProgressBarIndeterminateVisibility(true);

    ParseQuery<ParseUser> query = mFriendsRelation.getQuery();
    query.addAscendingOrder(ParseConstants.KEY_USERNAME);
    query.findInBackground(new FindCallback<ParseUser>() {
        @Override
        public void done(List<ParseUser> friends, ParseException e) {
            getActivity().setProgressBarIndeterminateVisibility(false);

            if (e == null) {
                mFriends = friends;

                String[] usernames = new String[mFriends.size()];
                int i = 0;
                for (ParseUser friend : mFriends) {
                    usernames[i] = friend.getUsername();
                    i++;
                }

                if (mGridView.getAdapter()== null) {
                    UserAdapter adapter = new UserAdapter(getActivity(), mFriends);
                    mGridView.setAdapter(adapter);
                }
                else{
                    ((UserAdapter)mGridView.getAdapter()).refill(mFriends);
                }

            } else {

                Log.e(TAG, e.getMessage());
                AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
                builder.setMessage(e.getMessage())
                        .setTitle(R.string.error_title)
                        .setPositiveButton(android.R.string.ok, null);
                AlertDialog dialog = builder.create();
                dialog.show();
            }
        }
    });
}