Android : Error in updating the main content by replacing fragments

In my sample project, I implemented the code using activity, running fine.

After then I implemented that code in my main App which is using Fragment.

For that I extends Fragment, made a default constructor, made onCreateView & inside that inflated the layout, then changed getApplicationContext() to getActivity(). Thats how I managed to remove errors in my main app.

An in my MainActivity this is how we update the main content by replacing fragments by calling constructor & assigning it to fragments:

 /**
 * Diplaying fragment view for selected nav drawer list item
 * */
private void displayView(int position) {
    // update the main content by replacing fragments
    Fragment fragment = null;
    switch (position) {
    case 0:
        fragment = new IntroductionFragment();
        break;
    case 1:
        fragment = new PrefaceFragment();
        break;
    case 2:
        fragment = new PreambleFragment();
        break;
    case 3:
        fragment = new ContentsFragment();//ERROR here: because I modified this class
        break;

    case 4:
        fragment = new SchedulesFragment();
        break;
    case 5:
        fragment = new AppendixFragment();
        break;
    case 6:
        fragment = new AmendmentFragment();
        break;
    case 7:
        fragment = new PhotoFragment();
        break;
    default:
        break;
    }

    if (fragment != null) {
        FragmentManager fragmentManager = getFragmentManager();
        fragmentManager.beginTransaction()
                .replace(R.id.frame_container, fragment).commit();

        // update selected item and title, then close the drawer
        mDrawerList.setItemChecked(position, true);
        mDrawerList.setSelection(position);
        setTitle(navMenuTitles[position]);
        mDrawerLayout.closeDrawer(mDrawerList);
    } else {
        // error in creating fragment
        Log.e("MainActivity", "Error in creating fragment");
    }
}

That line shows me Type mismatch: cannot convert from ContentsFragment to Fragment

I hope the problem is clear. Please tell me how can I avoid this error...

This is ContentsFragment.java

    public class ContentsFragment extends Fragment//  implements OnTouchListener
{   
    public ContentsFragment() {
        // TODO Auto-generated constructor stub
    }

    public static ArrayList<GS> q = new ArrayList<GS>();
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
         DBAdapter db = DBAdapter.getDBAdapter(getActivity());

         if (!db.checkDatabase())   
                db.createDatabase(getActivity());

            db.openDatabase();

            q = db.getData();

//      setContentView(R.layout.sample_activity);
    }
     @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
            // Inflate the layout for this fragment
          View v =inflater.inflate(R.layout.sample_activity, container, false);


        final FloatingGroupExpandableListView list = (FloatingGroupExpandableListView) v.findViewById(R.id.sample_activity_list);

     //   final LayoutInflater inflater = getLayoutInflater();



        // Even though the child divider has already been set on the layout file, we have to set it again here
        // This prevents a bug where the background turns to the color of the child divider when the list is expanded
        list.setChildDivider(new ColorDrawable(Color.BLACK));

        final ArticlesAdapter adapter = new ArticlesAdapter(getActivity());
        final WrapperExpandableListAdapter wrapperAdapter = new WrapperExpandableListAdapter(adapter);
        list.setAdapter(wrapperAdapter);

        for(int i = 0; i < wrapperAdapter.getGroupCount(); i++) {
            list.collapseGroup(i);//expandGroup(i);
        }

        list.setOnScrollFloatingGroupListener(new FloatingGroupExpandableListView.OnScrollFloatingGroupListener() {

            @Override
            public void onScrollFloatingGroupListener(View floatingGroupView, int scrollY) {
                float interpolation = - scrollY / (float) floatingGroupView.getHeight();

                // Changing from RGB(162,201,85) to RGB(255,255,255)
                final int greenToWhiteRed = (int) (0 + 3 * interpolation);
                final int greenToWhiteGreen = (int) (0 + 6* interpolation);
                final int greenToWhiteBlue = (int) ( 0+ 20 * interpolation);
                final int greenToWhiteColor = Color.argb(200, greenToWhiteRed, greenToWhiteGreen, greenToWhiteBlue);

                // Changing from RGB(255,255,255) to RGB(0,0,0)
                final int whiteToBlackRed = (int) (255 - 1 * interpolation);
                final int whiteToBlackGreen = (int) (255 - 1 * interpolation);
                final int whiteToBlackBlue = (int) (255 - 1 * interpolation);
                final int whiteToBlackColor = Color.argb(255, whiteToBlackRed, whiteToBlackGreen, whiteToBlackBlue);


                final View background = floatingGroupView.findViewById(R.id.sample_activity_list_group_item_background);
                background.setBackgroundColor(greenToWhiteColor);

                final TextView text = (TextView) floatingGroupView.findViewById(R.id.sample_activity_list_group_item_text);
                text.setTextColor(whiteToBlackColor);

                final ImageView expanded = (ImageView) floatingGroupView.findViewById(R.id.sample_activity_list_group_expanded_image);
                final Drawable expandedDrawable = expanded.getDrawable().mutate();
                expandedDrawable.setColorFilter(whiteToBlackColor, PorterDuff.Mode.SRC_ATOP);
            }
        });
        return v;
    }
}