NullPointerException : FragmentManager.beginTransaction()

Question

当我试图从第一个片段中启动一个片段时，我得到了这个错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'android.app.FragmentTransaction android.app.FragmentManager.beginTransaction()' on a null object reference

下面是我得到错误的方法：

    @Override
    public void onClick(View v) {
        Fragment fragment = new PropertyFragment();
        if (fragment != null) {
            FragmentManager fragmentManager = fragment.getFragmentManager();
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

            // Replace whatever is in the fragment_container view with this fragment,
            // and add the transaction to the back stack so the user can navigate back
            fragmentTransaction.replace(R.id.rent_viewer, fragment);  
            fragmentTransaction.addToBackStack(null);

            // Commit the transaction
            fragmentTransaction.commit();  
        }
    }

准确地说，下面的代码指令导致了错误：

fragmentManager.beginTransaction();

下面是类和嵌套类的样子：

public class RentFragment extends Fragment {    

    ...

    @SuppressWarnings("unused")
    private OnRentFragmentInteractionListener mListener;

    public RentFragment() {
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnRentFragmentInteractionListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnRentFragmentInteractionListener");
        }
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);         
    }

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

        myOnClickListener = new RentOnClickListener(getActivity());                     

        return rootView;
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {          
        super.onViewCreated(view, savedInstanceState);
    }   

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    public interface OnRentFragmentInteractionListener {
        public void onRentFragmentInteraction(Uri uri);
    }

    private static class RentOnClickListener implements View.OnClickListener {

        private final Context context;

        private RentOnClickListener(Context context) {
            this.context = context;
        }

        @Override
        public void onClick(View v) {   
            Fragment fragment = new PropertyFragment();    
            if (fragment != null) {    
                FragmentManager fragmentManager =  fragment.getFragmentManager();
                FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

                // Replace whatever is in the fragment_container view with this fragment,
                // and add the transaction to the back stack so the user can navigate back
                fragmentTransaction.replace(R.id.rent_viewer, fragment);  
                fragmentTransaction.addToBackStack(null);

                // Commit the transaction
                fragmentTransaction.commit();  
            }   
        }        
    }       
}

Answer 1

FragmentManager将为null，直到它附加到下面的代码中的activity.So use为止，如果它是一个嵌套的片段，请使用this.getChildFragmentManager()作为片段类，否则使用getActivity().getFragmentManager()或getActivity().getSupportFragmentManager()。

Answer 2

声明您的FragmentTransaction并按以下方式初始化它

FragmentTransaction fm;

@Override
public void onAttach(@NonNull Context context) {
    super.onAttach(context);
    fm = getFragmentManager();
}

然后，在您的代码中，您可以调用这个

Fragment fragment = new PropertyFragment();
FragmentTransaction transaction = fm.beginTransaction();
transaction.add(R.id.rent_viewer, fragment).addToBackStack(null).commit();;

Answer 3

在基类中使用Customize并使用它。

public void replaceFragment(Fragment newFragment, Bundle bundle, boolean isAddToBackStack) {

        String tag = newFragment.getClass().getSimpleName();
        if (bundle != null) {
            newFragment.setArguments(bundle);
        }

        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();

        if (getCurrentFragment() != null) {
            ft.hide(getCurrentFragment());
        }
        ft.add(R.id.frameContainer, newFragment, tag);
        newFragment.setRetainInstance(true);
        if (isAddToBackStack) {
            ft.addToBackStack(tag);
        }
        try {
            ft.commitAllowingStateLoss();
        } catch (Exception ex) {
            ex.printStackTrace();
//            ft.commitAllowingStateLoss();
        }
    }