title: NotSerializableException when FragmentActivity goes to background in Android
body: <p>I have 5 fragments inside my activity where one fragment stays active at one time. Clicking on a recyclerview item opens another fragment and puts current fragment in the backstack.<br>
    The same code was working fine some days ago, but now the app is throwing <strong>NotSerializableException</strong> whenever I click the home button to put the app in background. I have tried putting the initializing the variables inside onStart and then giving the null value in onStop but that didn't work.<br>
    <strong>Fragment Code:</strong></p>
  
  <pre><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&lt;Paper&gt; mPaperList = new ArrayList&lt;&gt;();
  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()+" &gt; "+ mSubject.getSubject_name());
  
      if (mOutState != null) {
          mPaperList = (ArrayList&lt;Paper&gt;) 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() &gt; 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);
  }
  </code></pre>
  
  <p>}</p>
  
  <p><strong>Exception:</strong></p>
  
  <pre><code>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)
  </code></pre>
  
  <p><strong>Note:</strong> The weird thing is that one of fragment has the exact same code and is hosted inside the same activity, but when that fragment is active and app goes to background, interestingly the app is not crashing. </p>
  
  <p>**Exam Model Class: 
      package in.crazybytes.bankmaniaadmin.models;</p>
  
  <pre><code>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;
  }
  </code></pre>
  
  <p>}</p>
  
  <p><strong>Paper Model Class</strong></p>
  
  <pre><code>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;
  }
  </code></pre>
  
  <p>}</p>
  
  <p><strong>Subject Model Class:</strong></p>
  
  <pre><code>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;
  }
  </code></pre>
  
  <p>}</p>