Как работает шаблон NavigationDrawer в AndroidStudio, где обрабатывать нажатия на навигационной панели?

public class MainActivity extends AppCompatActivity
        implements NavigationDrawerFragment.NavigationDrawerCallbacks {
 
    /**
     * Fragment managing the behaviors, interactions and presentation of the navigation drawer.
     */
    private NavigationDrawerFragment mNavigationDrawerFragment;
 
    /**
     * Used to store the last screen title. For use in {@link #restoreActionBar()}.
     */
    private CharSequence mTitle;
 
    public Document doc = null;
    public Elements elementsTitle = null;
    public Elements elementsAnnounce = null;
    public ArrayList<News> newsList = new ArrayList<>();
 
    private SimpleAdapter adapter;
    private ListView listViewNews;
    private ArrayList<Map<String, String>> data;
 
    final String ATTRIBUTE_TITLE = "title";
    final String ATTRIBUTE_ANNOUNCE = "announce";
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 
        mNavigationDrawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
        mTitle = getTitle();
 
        // Set up the drawer.
        mNavigationDrawerFragment.setUp(
                R.id.navigation_drawer,
                (DrawerLayout) findViewById(R.id.drawer_layout));
    }
 
    @Override
    public void onNavigationDrawerItemSelected(int position) {
        // update the main content by replacing fragments
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction()
                .replace(R.id.container, PlaceholderFragment.newInstance(position + 1))
                .commit();
    }
 
    public void onSectionAttached(int number) {
        switch (number) {
            case 1:
                mTitle = getString(R.string.title_section1);
                break;
            case 2:
                mTitle = getString(R.string.title_section2);
                break;
            case 3:
                mTitle = getString(R.string.title_section3);
                break;
            case 4:
                mTitle = getString(R.string.title_section4);
                break;
            case 5:
                mTitle = getString(R.string.title_section5);
                break;
            case 6:
                mTitle = getString(R.string.title_section6);
                break;
            case 7:
                mTitle = getString(R.string.title_section7);
                break;
            case 8:
                mTitle = getString(R.string.title_section8);
                break;
            case 9:
                mTitle = getString(R.string.title_section9);
                break;
            case 10:
                mTitle = getString(R.string.title_section10);
                break;
            case 11:
                mTitle = getString(R.string.title_section11);
                break;
        }
    }
 
    public void restoreActionBar() {
        ActionBar actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
        actionBar.setDisplayShowTitleEnabled(true);
        actionBar.setTitle(mTitle);
    }
 
 
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        if (!mNavigationDrawerFragment.isDrawerOpen()) {
            // Only show items in the action bar relevant to this screen
            // if the drawer is not showing. Otherwise, let the drawer
            // decide what to show in the action bar.
            getMenuInflater().inflate(R.menu.main, menu);
            restoreActionBar();
            return true;
        }
        return super.onCreateOptionsMenu(menu);
    }
 
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
 
        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }
 
        return super.onOptionsItemSelected(item);
    }
 
    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";
 
        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }
 
        public PlaceholderFragment() {
        }
 
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
 
        @Override
        public void onAttach(Activity activity) {
            super.onAttach(activity);
            ((MainActivity) activity).onSectionAttached(
                    getArguments().getInt(ARG_SECTION_NUMBER));
        }
    }
 
}

    @Override
    public void onNavigationDrawerItemSelected(int position) {
 
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
 
        if (position == 0) {
            Test1Fragment t1 = new Test1Fragment();
            ft.replace(R.id.container, t1, "TestFrag");
        } else if (position == 1) {
            Test2Fragment t2 = new Test2Fragment();
            ft.replace(R.id.container, t2, "TestFrag");
        }
        ft.commit();
 
    }