android - Do fragments really need an empty constructor? -

i have fragment constructor multiple arguments, worked fine during testphase after 300 users downloaded app, have 1 occurence of exception:

android.support.v4.app.fragment$instantiationexception: unable instantiate fragment  make sure class name exists, public, , has empty constructor public 

i mean provide different constructor doesn't make sense since have call method set fragment.

i'm curious why happening sporadically , not , maybe im using fragmented viewpager wrong, because instantiate fragments myself , save them list inside activity. don't use fragmentmanager transaction stuff, since example fragmented viewpager not clear , in end worked fine.

yes do.

you shouldn't overriding constructor anyway. should have newinstance() static method defined , pass parameters via arguments (bundle)

for example:

public static final alertfragment newinstance(int title, string message) {     alertfragment f = new alertfragment();     bundle bdl = new bundle(2);     bdl.putint(extra_title, title);     bdl.putstring(extra_message, message);     f.setarguments(bdl);     return f; } 

and of course grabbing args way:

@override public void oncreate(bundle savedinstancestate) {     title = getarguments().getint(extra_title);     message = getarguments().getstring(extra_message);      //...     //etc     //... } 

then instantiate fragment manager so:

public oncreate(bundle savedinstancestate) {     if(savedinstancestate == null){         getsupportfragmentmanager()             .begintransaction()             .replace(r.id.content,alertfragment.newinstance(                 r.string.alert_title,                 "oh noes error occured!")             )             .commit();     } } 

this way if detached , re-attached object state can stored through arguments. bundles attached intents.

reason - reading

i thought explain why people wondering why.

if check: https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/app/fragment.java

you see instantiate(..) method in fragment class calls newinstance method.

in case of broken link, can see here:

http://docs.oracle.com/javase/6/docs/api/java/lang/class.html#newinstance() explains why, upon instantiation checks accessor public , that class loader allows access it.

it's pretty nasty method in all, allows fragmentmanger kill , recreate fragments states. (the android subsystem similar things activities).

example class

i asked alot calling newinstance,(do not confuse class method. whole class example should show usage.

/**  * created 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... }