Fragment doesn't show up on activity : I can't get to work my fragment, it just won't show up when item on navigation bar is selected as it should do when selecting R.id.nav_gallery. I think there is a problem somewhere on onNavigationItemSelected(MenuItem item), but I couldn't localize it. Any thoughts?
 
    public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
      (...)

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
    }

  



    public static class PlaceholderFragment extends Fragment{

        private static final String ARG_SECTION_NUMBER = "section_number";

        public PlaceholderFragment() {
        }

        public static PlaceholderFragment newInstance() {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, 0);
            fragment.setArguments(args);
            Log.d("frag", "6");
            return fragment;
        }


        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {

            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }

    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        int id = item.getItemId();

        FragmentManager fragmentManager = getFragmentManager();
        PlaceholderFragment fragment;

        if (id == R.id.nav_camera) {
            // Handle the camera action
        } else if (id == R.id.nav_gallery) {
            fragment = new PlaceholderFragment();
            fragmentManager.beginTransaction()
                    .replace(R.id.container, fragment)
                    .commit();
            PlaceholderFragment.newInstance();
      }
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
    }
