Java – NotSerializableException occurs when a FragmentActivity goes into the background in Android

I have 5 fragments in my activity, and one of the fragments remains active at once. Clicking the recyclerview project opens another fragment and puts the current fragment into the background stack.
The same code worked fine a few days ago, but now whenever I click the home button to put the app in the background, the app throws a NotSerializableException. I tried putting the initialization variable in onStart and then giving null in onStop, but that didn’t work.
fragment code:

public class PaperListFragment extends Fragment implements Serializable {

private static final String TAG = "PaperListFragment";
private static final String QUESTIONS_FRAGMENT_TAG = "questions_fragment";
private static final String ADD_PAPER_FRAGMENT_TAG = "add_paper_fragment";

private OnFragmentActiveListener mOnFragmentActiveListener;
private TextView mHeadingText;
private Bundle mOutState;
private FirebaseAuth mAuth;
private DatabaseReference mDatabaseReference;
private ProgressBar mProgressBar;
private OnItemClickListener mOnItemClickListener;
private FloatingActionButton mFab;
private RecyclerView mRecyclerViewPaper;
private ArrayList<Paper> mPaperList = new ArrayList<>();
private Subject mSubject = new Subject();
private Exam mExam = new Exam();

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

mProgressBar = (ProgressBar) rootView.findViewById(R.id.progressbar_news);
    mFab = (FloatingActionButton) rootView.findViewById(R.id.floatingActionButton);
    mProgressBar.setVisibility(View.VISIBLE);
    Log.d(TAG, "onCreateView: Fragment created");
    mAuth = FirebaseAuth.getInstance();
    mDatabaseReference = FirebaseDatabase.getInstance().getReference();
    if (mAuth.getCurrentUser() == null) {
        startActivity(new Intent(getActivity(), LoginActivity.class));
        getActivity().finish();
        return null;
    }

if (getArguments() != null) {
        mOnFragmentActiveListener = (OnFragmentActiveListener) getArguments().getSerializable(Keys.FRAGMENT_ACTIVE_LISTENER);
        mSubject = (Subject) getArguments().getSerializable(Keys.SUBJECT_KEY);
        mExam = (Exam) getArguments().getSerializable(Keys.EXAMS_KEY);
    }

mRecyclerViewPaper = (RecyclerView) rootView.findViewById(R.id.recycler_list);
    LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity()) {
        @Override
        public boolean canScrollVertically() {
            return false;
        }
    };
    mRecyclerViewPaper.setLayoutManager(layoutManager);
    Log.d(TAG, "onCreateView: Layout Manager Set.");

mFab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startAddPaperFragment();
        }
    });

mOnItemClickListener = new OnItemClickListener() {
        @Override
        public void onItemClicked(RecyclerView.ViewHolder holder, int position) {
            Log.d(TAG, "onItemClicked: Clicked item position is: "+ position);
            QuestionListFragment questionFragment = new QuestionListFragment();
            questionFragment.setRetainInstance(true);

startFragment(position, questionFragment, QUESTIONS_FRAGMENT_TAG);

}

@Override
        public void OnItemLongClicked(RecyclerView.ViewHolder holder, int position) {

}
    };

mHeadingText = (TextView) rootView.findViewById(R.id.heading_textview);
    mHeadingText.setText(mExam.getExam_name()+" > "+ mSubject.getSubject_name());

if (mOutState != null) {
        mPaperList = (ArrayList<Paper>) mOutState.getSerializable(Keys.PAPER_LIST_KEY);
        updateUI();
    } else {
        updateUIFromDatabase();
    }

return rootView;
}

private void startFragment(int position, Fragment fragment, String fragmentTag) {
    Paper paper = new Paper();
    if (mPaperList.size() > 0) {
        paper = mPaperList.get(position);
    }
    Bundle args = new Bundle();
    args.putSerializable(Keys.EXAMS_KEY, mExam);
    args.putSerializable(Keys.SUBJECT_KEY, mSubject);
    args.putSerializable(Keys.PAPER, paper);
    args.putSerializable(Keys.FRAGMENT_ACTIVE_LISTENER, mOnFragmentActiveListener);
    fragment.setArguments(args);
    FragmentTransaction fragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
    fragmentTransaction.setCustomAnimations(R.anim.slide_in_right, R.anim.slide_out_left, R.anim.slide_in_left, R.anim.slide_out_right);

fragmentTransaction.replace(R.id.questions_fragment_container, fragment, fragmentTag);
    fragmentTransaction.addToBackStack(fragmentTag);
    fragmentTransaction.commit();
}

private void startAddPaperFragment() {
    AddPaperFragment addPaperFragment = new AddPaperFragment();
    addPaperFragment.setRetainInstance(true);
    startFragment(0, addPaperFragment, ADD_PAPER_FRAGMENT_TAG);
}

private void updateUIFromDatabase() {
    if (getArguments() != null){
        Exam exam = (Exam) getArguments().getSerializable(Keys.EXAMS_KEY);
        Subject subject = (Subject) getArguments().getSerializable(Keys.SUBJECT_KEY);
        DatabaseReference paperReference =
                mDatabaseReference
                .child(Keys.APP_DATA_KEY)
                .child(Keys.EXAM_PAPERS)
                .child(exam.getExam_name())
                .child(subject.getSubject_name());
        Query query = paperReference.orderByChild(Keys.TIME_ADDED);
        query.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mPaperList.clear();
                for (DataSnapshot paperChild : dataSnapshot.getChildren()) {
                    mPaperList.add(paperChild.getValue(Paper.class));
                }

updateUI();
            }

@Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

}
        });
    }
}

private void updateUI() {
    PaperRecyclerAdapter adapter = new PaperRecyclerAdapter(
            getActivity(),
            mRecyclerViewPaper,
            mPaperList,
            mOnItemClickListener
    );
    mRecyclerViewPaper.setAdapter(adapter);
    mProgressBar.setVisibility(View.GONE);
}

@Override
public void onResume() {
    super.onResume();
    if (getArguments()!=null){
        mOnFragmentActiveListener.onFragmentActive(
                this,
                "Topics"
        );
    }
}

@Override
public void onPause() {
    super.onPause();
    mOutState = new Bundle();
    mOutState.putSerializable(Keys.PAPER_LIST_KEY, mPaperList);
}

Exception:

2018-12-26 17:49:38.344 14834-14834/in.crazybytes.bankmaniaadmin E/AndroidRuntime: FATAL EXCEPTION: main
Process: in.crazybytes.bankmaniaadmin, PID: 14834
java.lang.RuntimeException: Parcelable encountered IOException writing serializable object (name = in.crazybytes.bankmaniaadmin.activities.QuestionsActivity)
    at android.os.Parcel.writeSerializable(Parcel.java:1526)
    at android.os.Parcel.writeValue(Parcel.java:1474)
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723)
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408)
    at android.os.Bundle.writeToParcel(Bundle.java:1133)
    at android.os.Parcel.writeBundle(Parcel.java:763)
    at android.support.v4.app.FragmentState.writeToParcel(FragmentState.java:124)
    at android.os.Parcel.writeTypedArray(Parcel.java:1306)
    at android.support.v4.app.FragmentManagerState.writeToParcel(FragmentManager.java:639)
    at android.os.Parcel.writeParcelable(Parcel.java:1495)
    at android.os.Parcel.writeValue(Parcel.java:1401)
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723)
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408)
    at android.os.Bundle.writeToParcel(Bundle.java:1133)
    at android.os.Parcel.writeBundle(Parcel.java:763)
    at android.app.ActivityManagerProxy.activityStopped(ActivityManagerNative.java:3697)
    at android.app.ActivityThread$StopInfo.run(ActivityThread.java:3768)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6123)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
 Caused by: java.io.NotSerializableException: com.google.firebase.auth.internal.zzj
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1224)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1584)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1549)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1472)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1218)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1584)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1549)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1472)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1218)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
    at android.os.Parcel.writeSerializable(Parcel.java:1521)
    at android.os.Parcel.writeValue(Parcel.java:1474) 
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723) 
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408) 
    at android.os.Bundle.writeToParcel(Bundle.java:1133) 
    at android.os.Parcel.writeBundle(Parcel.java:763) 
    at android.support.v4.app.FragmentState.writeToParcel(FragmentState.java:124) 
    at android.os.Parcel.writeTypedArray(Parcel.java:1306) 
    at android.support.v4.app.FragmentManagerState.writeToParcel(FragmentManager.java:639) 
    at android.os.Parcel.writeParcelable(Parcel.java:1495) 
    at android.os.Parcel.writeValue(Parcel.java:1401) 
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723) 
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408) 
    at android.os.Bundle.writeToParcel(Bundle.java:1133) 
    at android.os.Parcel.writeBundle(Parcel.java:763) 
    at android.app.ActivityManagerProxy.activityStopped(ActivityManagerNative.java:3697) 
    at android.app.ActivityThread$StopInfo.run(ActivityThread.java:3768) 
    at android.os.Handler.handleCallback(Handler.java:751) 
    at android.os.Handler.dispatchMessage(Handler.java:95) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6123) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)

Note: Strangely, one of the fragments has exactly the same code and is hosted in the same activity, but when that fragment is in the activity state and the application goes into the background, the interesting thing is that the application does not crash.

**Exam Model Class:
Packages are available at .crazybytes.bankmaniaadmin.models;

import java.io.Serializable;

public class Exam implements Serializable {
private String mExam_name;
private String mExam_key;
private Long mTime_added;
private int mNum_subjects;
private int mNum_questions;

public Exam(String exam_name, String exam_key, Long time_added, int num_subjects, int num_questions) {
    mExam_name = exam_name;
    mExam_key = exam_key;
    mTime_added = time_added;
    mNum_subjects = num_subjects;
    mNum_questions = num_questions;
}

public Exam() {
}

public String getExam_name() {
    return mExam_name;
}

public void setExam_name(String exam_name) {
    mExam_name = exam_name;
}

public String getExam_key() {
    return mExam_key;
}

public void setExam_key(String exam_key) {
    mExam_key = exam_key;
}

public Long getTime_added() {
    return mTime_added;
}

public void setTime_added(Long time_added) {
    mTime_added = time_added;
}

public int getNum_subjects() {
    return mNum_subjects;
}

public void setNum_subjects(int num_subjects) {
    mNum_subjects = num_subjects;
}

public int getNum_questions() {
    return mNum_questions;
}

public void setNum_questions(int num_questions) {
    mNum_questions = num_questions;
}

Paper model class

package in.crazybytes.bankmaniaadmin.models;

import java.io.Serializable;

public class Paper implements Serializable {
private String mPaper_name;
private String mPaper_key;
private Long mTime_added;
private int mNum_questions;

public Paper(String paper_name, String paper_key, Long time_added, int num_questions) {
    mPaper_name = paper_name;
    mPaper_key = paper_key;
    mTime_added = time_added;
    mNum_questions = num_questions;
}

public Paper() {
}

public String getPaper_key() {
    return mPaper_key;
}

public void setPaper_key(String paper_key) {
    mPaper_key = paper_key;
}

public Long getTime_added() {
    return mTime_added;
}

public void setTime_added(Long time_added) {
    mTime_added = time_added;
}

public int getNum_questions() {
    return mNum_questions;
}

public void setNum_questions(int num_questions) {
    mNum_questions = num_questions;
}

public String getPaper_name() {
    return mPaper_name;
}

public void setPaper_name(String paper_name) {
    mPaper_name = paper_name;
}

Topic model class:

package in.crazybytes.bankmaniaadmin.models;

import java.io.Serializable;

public class Subject implements Serializable {
private String mSubject_name;
private String mSubject_key;
private Long mTime_added;
private int mNum_papers;
private int mNum_questions;

public Subject(String subject_name, String subject_key, Long time_added, int num_papers, int num_questions) {
    mSubject_name = subject_name;
    mSubject_key = subject_key;
    mTime_added = time_added;
    mNum_papers = num_papers;
    mNum_questions = num_questions;
}

public Subject() {
}

public String getSubject_name() {
    return mSubject_name;
}

public void setSubject_name(String subject_name) {
    mSubject_name = subject_name;
}

public String getSubject_key() {
    return mSubject_key;
}

public void setSubject_key(String subject_key) {
    mSubject_key = subject_key;
}

public Long getTime_added() {
    return mTime_added;
}

public void setTime_added(Long time_added) {
    mTime_added = time_added;
}

public int getNum_papers() {
    return mNum_papers;
}

public void setNum_papers(int num_papers) {
    mNum_papers = num_papers;
}

public int getNum_questions() {
    return mNum_questions;
}

public void setNum_questions(int num_questions) {
    mNum_questions = num_questions;
}

QuestionsActivity somehow goes into fragment save state, even if you don’t want this to happen. While serializing the QuestionsActivity, another non-serializable object was encountered. That’s why you’ll see TextView and others trying to serialize, because all instance variables of a QuestionsActivity are serialized by default.

My best guess as to why this is happening is due to this line :

args.putSerializable(Keys.FRAGMENT_ACTIVE_LISTENER, mOnFragmentActiveListener);

But it’s hard to determine without looking at where OnFragmentActiveListener is defined. I assume that QuestionsActivity implements OnFragmentActiveListener, or that QuestionsActivity defines OnFragmentActiveListener as an inner class. Either way, if you put OnFragmentActiveListener in a fragment parameter, you will encounter an exception because you are also indirectly storing the entire QuestionsActivity as a fragment parameter. When the fragment stops, all fragment parameters become part of the fragment save state. That’s why it’s wrong.

I recommend not passing OnFragmentActiveListener as a fragment parameter. If the OnFragmentActiveListener is from an activity, simply use getActivity() to get a reference to the activity and then a reference to the listener.

I

also noticed that PaperListFragment implements Serializable, and I assume you did the same with QuestionsActivity. You might do this to avoid compilation errors. But this results in a run-time error because not all instance variables on both classes are serializable. Therefore, to avoid more runtime issues, I recommend never making activities or fragments serializable, as these classes are inherently unserializable due to their members.