Do fragments really need an empty constructor

Question

I have a Fragment with a constructor that takes multiple arguments  My app worked fine during development  but in production my users sometimes see this crash   android support v4 app Fragment InstantiationException  Unable to instantiate fragment  make sure class name exists  is public  and has an empty constructor that is public   I could make an empty constructor as this error message suggests  but that doesn t make sense to me since then I would have to call a separate method to finish setting up the Fragment   I m curious as to why this crash only happens occasionally  Maybe I m using the ViewPager incorrectly  I instantiate all the Fragments myself and save them in a list inside the Activity  I don t use FragmentManager transactions  since the ViewPager examples I have seen did not require it and everything seemed to be working during development

User · Answer

Here is my simple solution   1 - Define your fragment  public class MyFragment extends Fragment        private String parameter       public MyFragment                public void setParameter String parameter            this parameter   parameter             2 - Create your new fragment and populate the parameter      myfragment   new MyFragment        myfragment setParameter  here the value of my parameter      3 - Enjoy it   Obviously you can change the type and the number of parameters  Quick and easy

User · Answer

Yes  as you can see the support-package instantiates the fragments too  when they get destroyed and re-opened   Your Fragment subclasses need a public empty constructor as this is what s being called by the framework

User · Answer

As noted by CommonsWare in this question https   stackoverflow com a 16064418 1319061  this error can also occur if you are creating an anonymous subclass of a Fragment  since anonymous classes cannot have constructors   Don t make anonymous subclasses of Fragment  -

User · Answer

Yes they do   You shouldn t really be overriding the constructor anyway  You should have a newInstance   static method defined and pass any parameters via arguments  bundle   For example   public static final MyFragment newInstance int title  String message        MyFragment f   new MyFragment        Bundle bdl   new Bundle 2       bdl putInt EXTRA TITLE  title       bdl putString EXTRA MESSAGE  message       f setArguments bdl       return f      And of course grabbing the args this way    Override public void onCreate Bundle savedInstanceState        title   getArguments   getInt EXTRA TITLE       message   getArguments   getString EXTRA MESSAGE                    etc               Then you would instantiate from your fragment manager like so    Override public void onCreate Bundle savedInstanceState        if  savedInstanceState    null           getSupportFragmentManager                beginTransaction                replace R id content  MyFragment newInstance                  R string alert title                   Oh no  an error occurred                               commit              This way if detached and re-attached the object state can be stored through the arguments  Much like bundles attached to Intents   Reason - Extra reading  I thought I would explain why for people wondering why   If you check  https   android googlesource com platform frameworks base   master core java android app Fragment java  You will see the instantiate     method in the Fragment class calls the newInstance method   public static Fragment instantiate Context context  String fname   Nullable Bundle args        try           Class lt   gt  clazz   sClassMap get fname           if  clazz    null                   Class not found in the cache  see if it s real  and try to add it             clazz   context getClassLoader   loadClass fname               if   Fragment class isAssignableFrom clazz                     throw new InstantiationException  Trying to instantiate a class     fname                             that is not a Fragment   new ClassCastException                               sClassMap put fname  clazz                     Fragment f    Fragment  clazz getConstructor   newInstance            if  args    null                args setClassLoader f getClass   getClassLoader                 f setArguments args                     return f        catch  ClassNotFoundException e            throw new InstantiationException  Unable to instantiate fragment     fname                      make sure class name exists  is public  and has an                      empty constructor that is public   e         catch  java lang InstantiationException e            throw new InstantiationException  Unable to instantiate fragment     fname                      make sure class name exists  is public  and has an                      empty constructor that is public   e         catch  IllegalAccessException e            throw new InstantiationException  Unable to instantiate fragment     fname                      make sure class name exists  is public  and has an                      empty constructor that is public   e         catch  NoSuchMethodException e            throw new InstantiationException  Unable to instantiate fragment     fname                      could not find Fragment constructor   e         catch  InvocationTargetException e            throw new InstantiationException  Unable to instantiate fragment     fname                      calling Fragment constructor caused an exception   e             http   docs oracle com javase 6 docs api java lang Class html newInstance   Explains why  upon instantiation it checks that the accessor is public and that that class loader allows access to it   It s a pretty nasty method all in all  but it allows the FragmentManger to kill and recreate Fragments with states   The Android subsystem does similar things with Activities    Example Class  I get asked a lot about calling newInstance  Do not confuse this with the class method   This whole class example should show the usage          Created by chris on 21 11 2013     public class StationInfoAccessibilityFragment extends BaseFragment implements JourneyProviderListener        public static final StationInfoAccessibilityFragment newInstance String crsCode            StationInfoAccessibilityFragment fragment   new StationInfoAccessibilityFragment             final Bundle args   new Bundle 1           args putString EXTRA CRS CODE  crsCode           fragment setArguments args            return fragment                Views     LinearLayout mLinearLayout                  Layout Inflater             private LayoutInflater mInflater                 Station Crs Code             private String mCrsCode        Override     public void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           mCrsCode   getArguments   getString EXTRA CRS CODE               Override     public View onCreateView LayoutInflater inflater  ViewGroup container  Bundle savedInstanceState            mInflater   inflater          return inflater inflate R layout fragment station accessibility  container  false               Override     public void onViewCreated View view  Bundle savedInstanceState            super onViewCreated view  savedInstanceState           mLinearLayout    LinearLayout view findViewBy R id station info accessibility linear             Do stuff             Override     public void onResume             super onResume            getActivity   getSupportActionBar   setTitle R string station info access mobility title                 Other methods etc

[android] Do fragments really need an empty constructor?

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