Android OnBackpressed in Fragment Null Exception

Question

Hello I'm trying to override back button to back to home page it gives me this error when press back from BasicInfoFragment then press back again in home fragment (MainFragment) it gives

java.lang.NullPointerException: Attempt to invoke virtual method 'android.support.v4.app.FragmentManager android.support.v4.app.FragmentActivity.getSupportFragmentManager()' on a null object reference

then the application crushs

java.lang.NullPointerException: Attempt to invoke virtual method 'android.support.v4.app.FragmentManager android.support.v4.app.FragmentActivity.getSupportFragmentManager()' on a null object reference at com.teqneia.blooddonation.BasicInfoFragment.doBack(BasicInfoFragment.java:245) at com.teqneia.blooddonation.MainActivity.onBackPressed(MainActivity.java:258) at android.app.Activity.onKeyUp(Activity.java:2530) at android.view.KeyEvent.dispatch(KeyEvent.java:2726) at android.app.Activity.dispatchKeyEvent(Activity.java:2798) at android.support.v7.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:543) at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:53) at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.dispatchKeyEvent(AppCompatDelegateImplBase.java:315) at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:53) at com.android.internal.policy.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2368) at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:4333) at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4295) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3836) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3889) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3855) at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3981) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3863) at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4038) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3836) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3889) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3855) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3863) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3836) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3889) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3855) at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4014) at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:4175) at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2378) at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1999) at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:1990) at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:2355) at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141) at android.os.MessageQueue.nativePollOnce(Native Method) at android.os.MessageQueue.next(MessageQueue.java:330) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:5546) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:794) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:684)

BasicInfoFragment java

public class BasicInfoFragment extends Fragment implements MainActivity.OnBackPressedListener, View.OnClickListener {
.
.

public BasicInfoFragment() {
}


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

    initialize(view);

    // Inflate the layout for this fragment
    return view;
}

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

void initialize(View view) {
    //VIEW OLD DATA AND ADD LISTENERS
    name = (EditText) view.findViewById(R.id.name_edit);

    email = (EditText) view.findViewById(R.id.email_edit);
    mobile = (EditText) view.findViewById(R.id.mobile_edit);
    LastDonation = (EditText) view.findViewById(R.id.date_edit);
    LastDonation.setInputType(InputType.TYPE_NULL);
    LastDonation.requestFocus();
    LastDonation.setOnClickListener(this);

    //TAKE DATA FROM SERVER
    //name.setText("");
    //email.setText();
    //mob.setText();
    //LastDonation.setText();

    b = (Button) view.findViewById(R.id.save_edit);
    b.setOnClickListener(this);
    //get Radio Groups

    radioBgGroup = (RadioGroup) view.findViewById(R.id.bg_edit);
    radioRhGroup = (RadioGroup) view.findViewById(R.id.rh_edit);

    if (mSharedGetter.getBloodType().equals("A")) {

}





@Override
public void doBack() {
    Fragment fragment = null;
    fragment = new MainFragment();
    FragmentManager fm = getActivity().getSupportFragmentManager();
    FragmentTransaction transaction = fm.beginTransaction();
    transaction.replace(R.id.fragment_container, fragment);
    transaction.commit();
}

MainActivity

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    .
            .
            .
            .
            .
            .
    protected OnBackPressedListener onBackPressedListener;


    public interface OnBackPressedListener {
        void doBack();
    }

    public void setOnBackPressedListener(OnBackPressedListener onBackPressedListener) {
        this.onBackPressedListener = onBackPressedListener;
    }

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


        //Initially Load MainFFragment xml
        setContentView(R.layout.activity_main);
        Fragment fragment = new MainFragment();
        //Apply fragment
        FragmentTransaction fragmentTransaction =
                getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.fragment_container, fragment);
        fragmentTransaction.commit();


        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);


        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);

        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        View header = navigationView.getHeaderView(0);
        TVnav_name = (TextView) header.findViewById(R.id.nav_name);
        navigationView.setNavigationItemSelectedListener(this);
        TVnav_name.setText(mSharedGetter.getUserName());
    }


    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else if (onBackPressedListener != null)
            onBackPressedListener.doBack();
        else
            super.onBackPressed();
    }

    @Override
    protected void onDestroy() {
        onBackPressedListener = null;
        super.onDestroy();
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();
        Fragment fragment = null;


        if (id == R.id.nav_edit_info) {

            fragment = new BasicInfoFragment();
            if (fragment != null) {
                //Apply fragment
                FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.fragment_container, fragment);
                fragmentTransaction.commit();
            }


        } else if (id == R.id.nav_home) {
            fragment = new MainFragment();

            if (fragment != null) {
                //Apply fragment
                FragmentTransaction fragmentTransaction =
                        getSupportFragmentManager().beginTransaction();
                fragmentTransaction.replace(R.id.fragment_container, fragment, "MY_FRAGMENT");
                fragmentTransaction.commit();
            }
        }


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


}

MainFragment

public class MainFragment extends Fragment{



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

        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_main, container, false);





        return view;
    }





        //DIALOG BUILDER
        new AlertDialog.Builder(getActivity())
                .setTitle("DONATION REQUEST")
                .setView(dialogView)
                .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                    }
                })
                .setPositiveButton("Request now", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        //Send the request

                        Dialog f = (Dialog) dialog;

                    }
                }).show();


    }


}

Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Shubham Shukla, Feb 14 '17 at 19:12

petey · Answer 1 · 2017-02-14T19:40:00.917

1

in doBack method getActivity() is returning null. It is also not needed as you can call getFragmentManager() from a Fragment. Here's how to do just that, with fewer lines.

@Override
public void doBack() {
    Fragment fragment = new MainFragment();
    getFragmentManager().beginTransaction()
            .replace(R.id.fragment_container, fragment)
            .commit();
}

edited Feb 14 '17 at 19:40

answered Feb 14 '17 at 19:17

petey

16,914
6
65
97

the method `getSupportFragmentManager()` is for `Activity`, `FragmentActivity` is actually an `Activity` provide by the support library. I think you are referring to `getChildFragmentManager()` which you can use in a `Fragment` – leobelizquierdo Feb 14 '17 at 19:27
2

`BasicInfoFragment extends Fragment` so you don't have `getSupportFragmentManager()` here. – leobelizquierdo Feb 14 '17 at 19:35
ahh, I corrected the link : fixed to use Fragment#getFragmentManager() – petey Feb 14 '17 at 19:40

Hanan Rofe Haim · Answer 2 · 2017-02-14T22:32:46.443

in your BasicInfoFragment you have

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

MainFragment should have the same line, otherwise pressing back twice as you described will invoke BasicInfoFragment doBack() twice.

Since you detach it from its activity after the first back click you get a null value from getActivity() here when the second back arrives:

FragmentManager fm = getActivity().getSupportFragmentManager();

score 0 · Answer 3 · answered Feb 14 '17 at 19:56

0

Always call like this.

//In Activity
FragmentManager fm = getSupportFragmentManager();

//In Fragment 
FragmentManager fm = getActivity().getFragmentManager();

//For Nested Fragments 
FragmentManager fm = getChildFragmentManager();

answered Feb 14 '17 at 19:56

albeee

1,452
1
12
20

score 0 · Answer 4 · answered Feb 15 '17 at 16:13

I think it would be better if you describe what you are trying to solve. I believe you may be having an error in the concept of how fragments work and the management of the back stack.

I believe the problem is your back event is being triggered but this is popping the fragment out but the reference is still there.

Take a look at: https://aarcoraci.wordpress.com/2017/02/13/android-tutorial-drawer-and-fragment-navigation-made-easyier/

Android OnBackpressed in Fragment Null Exception

4 Answers4