空指针异常: FragmentManager.beginTransaction()

Question

我在试图从第一个 Fragment 启动另一个 Fragment 时遇到了这个错误：

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

这是我遇到错误的方法：

    @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();  
        }
    }

具体而言，以下代码指令导致了错误：

fragmentManager.beginTransaction();

以下是该类及其嵌套类的样子：

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();  
            }   
        }        
    }       
}

- hyunikel

---

尝试使用getSupportFragmentManager()。 - Rajen Raiyarela

@RajenRaiyarela：我尝试了一下，但它显示“getSupportFragmentManager()方法对于Fragment类型未定义”。可能是因为我的类扩展了Fragment而不是Activity。 - hyunikel

请问您能否发布您完整的活动代码？ - Rajen Raiyarela

@RajenRaiyarela：我在那里编辑了代码。 - hyunikel

Answer 1

FragmentManager 在被附加到活动之前将为null。因此，请使用下面的代码， 如果它是一个嵌套碎片，请在您的碎片类中使用 this.getChildFragmentManager()，否则使用 getActivity().getFragmentManager() 或 getActivity().getSupportFragmentManager()。

- Krish

---

onClick方法位于嵌套的静态监听器类中。我无法使用this来引用Fragment类。请问我该如何访问它？ - hyunikel

谢谢@Krish，我所要做的就是从嵌套的监听器类中删除static关键字，并在容器类中添加一个getInstance方法。因此，我的指令变成了：FragmentManager fragmentManager = getRentFragment().getActivity().getFragmentManager(); - hyunikel

Answer 2

声明 FragmentTransaction 并按如下方式进行初始化。

FragmentTransaction fm;

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

然后在你的代码中，你可以调用这个。

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

- SilverC0de

Answer 3

使用基类中的Customize方法并使用它。

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();
        }
    }

- Alok Singh

Answer 4

您需要在代码中设置活动的上下文，如下所示：HomeActivity.this.getSupportFragmentManager();

private Fragment fragment;
private FragmentManager fragmentManager;
fragmentManager=HomeActivity.this.getSupportFragmentManager();
 transaction = fragmentManager.beginTransaction();
 transaction.add(R.id.main_container,  new ProfileFragment()).commit();

- Pramila Rawat

---

你需要在代码中设置活动的上下文，如下所示："activityname.this.getSupportFragmentManager();" - Pramila Rawat

Answer 5

     FragmentManager fragmentManager = 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();  

- Asthme

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阅读问题，你回答的第二行正在崩溃。 - BlueLeaf