選定的導航欄上不顯示片段？

Question

我無法使用我的片段，當選擇導航欄上的項目時它不會顯示，因爲它應該在選擇R.id.nav_gallery時進行選擇。我認爲onNavigationItemSelected（MenuItem項）的某處存在問題，但我無法對其進行本地化。有什麼想法嗎？

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; 
} 
} 

Answer 1

@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(); 
    } 
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
    drawer.closeDrawer(GravityCompat.START); 
    return true; 
} 
}