android - findFragmentById for a viewpager, of a tab, returns the same fragment regardless of the fragment displayed -

i have tab 2 fragments. regardless of tab selected,

getsupportfragmentmanager().findfragmentbyid(r.id.container) 

returns id of fragment in second tab every time.

i experimented , added tab, repeating 1 of fragments. strange behaviour. clicking button on first fragment changes second, clicking on third changes second , clicking second changes third. swiping between tabs resets first , third fragment.

any idea how fix or work around this?

activity:

public class mainactivity extends appcompatactivity {      //the {@link android.support.v4.view.pageradapter} provide fragments each of sections.      private sectionspageradapter msectionspageradapter;     public android.support.v4.app.fragment test;      //the {@link viewpager} host section contents.      private viewpager mviewpager;     private activitymainbinding binding;      public interface interface {         public void interfacemethod(int number);     }      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         binding = databindingutil.setcontentview(this, r.layout.activity_main);          // create adapter return fragment each of 3 primary sections of activity.         msectionspageradapter = new sectionspageradapter(getsupportfragmentmanager());          // set viewpager sections adapter.         mviewpager = (viewpager) findviewbyid(r.id.container);         mviewpager.setadapter(msectionspageradapter);          tablayout tablayout = (tablayout) findviewbyid(r.id.tabs);         tablayout.setupwithviewpager(mviewpager);          //test button1 click behaviour         binding.button1.setonclicklistener(new view.onclicklistener() {             @override             public void onclick(view view) {                 interface interfaceuse = (interface)(getsupportfragmentmanager().findfragmentbyid(r.id.container));                 interfaceuse.interfacemethod(1);             }         });     }      protected void buttonclick(int a, int b){         int position = binding.tabs.getselectedtabposition();      }      //a {@link fragmentpageradapter} returns fragment corresponding 1 of sections/tabs/pages.      public class sectionspageradapter extends fragmentpageradapter {           public sectionspageradapter(fragmentmanager fm) {             super(fm);         }          @override         public android.support.v4.app.fragment getitem(int index)         {             switch (index)             {                 case 0:                     return new unitfragment();                 case 1:                     return new rmfragment();                 case 2:                     return new unitfragment();             }              return null;         }          @override         public int getcount() {             // show 2 total pages.             return 3;         }          @override         public charsequence getpagetitle(int position) {             switch (position) {                 case 0:                     return "unit converter";                 case 1:                     return "one rep max";                 case 2:                     return "dum";             }             return null;         }     } } 

unitfragment:

public class unitfragment extends fragment implements mainactivity.interface{      private textview textview1;      public void interfacemethod(int position) {         textview1.settext(integer.tostring(position));     }      @override     public void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);     }      @override     public view oncreateview(layoutinflater inflater, viewgroup container,                              bundle savedinstancestate) {         // inflate layout fragment         final view rootview = inflater.inflate(r.layout.unit_fragment, container, false);          //r.id         textview1 = (textview) rootview.findviewbyid(r.id.textview);          return rootview;     } } 

---

fix:

interface interfaceuse =(interface) getsupportfragmentmanager().findfragmentbytag("android:switcher:" + r.id.container + ":" + mviewpager.getcurrentitem()); 

try using code in button click,

binding.button1.setonclicklistener(new view.onclicklistener() {     @override     public void onclick(view view) {        interface interfaceuse = getsupportfragmentmanager().findfragmentbytag("android:switcher:" + r.id.container + ":" + mviewpager.getcurrentitem());        interfaceuse.interfacemethod(1);     } });