Adding a fragment in android on button click and adding data to it simultaneously -

i creating android app in have main activity has 2 fragments, fragup , fragdown. fragup has 2 edit text fields , button , fragdown has 2 text view fields.
initially main activity shows fragup , asks user input fields, when user clicks button onclick method triggered , fragdown created below fragup , shows 2 fields entered user.

summary-
fragments- fragup , fragdown
main activity- main activity
main activity has 2 linear layouts
1.ly1 fragup
2.ly2 fragdown

note- fragments added dynamically in main java file , not declared in main.xml file.

this main activity code

public class mainactivity extends appcompatactivity implements fragup.fraginterface { fragmenttransaction ft,ft1;     @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_second);          ft=getfragmentmanager().begintransaction();         fragup fu=new fragup();          ft.add(r.id.ly1,fu);          //fragment fragup added dynamically.      ft.commit();         }      public void set(string a,string b){           ft1=getfragmentmanager().begintransaction();         fragdown ob=new fragdown();         ft1.add(r.id.ly2,ob);         ft1.commit();         fragdown fd=(fragdown)getfragmentmanager().findfragmentbyid(r.id.ly2);         fd.seta(a, b);       } 

this fragup java code-

public class fragup extends fragment {     view v;     public interface fraginterface{         public void set(string a,string b);     }     fraginterface ob;      @override     public view oncreateview(layoutinflater inflater, viewgroup container,             bundle savedinstancestate) {     v=inflater.inflate(r.layout.fragup,container,false);     button b=(button)v.findviewbyid(r.id.button1);     b.setonclicklistener( new onclicklistener(){  public void onclick(view v){  store();    }});      return v;     }       public void onattach(activity activity){     super.onattach(activity);     try{         ob=(fraginterface)activity;     }     catch(classcastexception e){         throw new classcastexception();      }     }       public void store(){           main activity     edittext et1=(edittext)v.findviewbyid(r.id.edittext1);     edittext et2=(edittext)v.findviewbyid(r.id.edittext2);     ob.set(et1.gettext().tostring(),et2.gettext().tostring());   //interface implemented share data  } } 

this fragdown java code-

public class fragdown extends fragment { view v;     @override     public view oncreateview(layoutinflater inflater, viewgroup container,             bundle savedinstancestate) {         v=inflater.inflate(r.layout.fragdown,container,false);         return v;     }      public void seta(string a,string b){         textview t1=(textview)v.findviewbyid(r.id.textview1);         textview t2=(textview)v.findviewbyid(r.id.textview2);         t1.settext(a);         t2.settext(b);      } } 

when run app , press button app crashes , logcat says-

08-05 09:37:02.198: e/androidruntime(2866): java.lang.nullpointerexception: attempt invoke virtual method 'void com.example.fragment.fragdown.seta(java.lang.string, java.lang.string)' on null object reference 08-05 09:37:02.198: e/androidruntime(2866):     @ com.example.fragment.second.set(second.java:30) 08-05 09:37:02.198: e/androidruntime(2866):     @ com.example.fragment.fragup.store(fragup.java:52) 08-05 09:37:02.198: e/androidruntime(2866):     @ com.example.fragment.fragup$1.onclick(fragup.java:25) 

if can me great. spent hours debugging can't find went wrong. please help.

why don't pass values fragment arguments? check "layout" section on guide: http://developer.android.com/reference/android/app/fragment.html

here's excerpt guide you:

public static detailsfragment newinstance(int index) {     detailsfragment f = new detailsfragment();      // supply index input argument.     bundle args = new bundle();     args.putint("index", index);     f.setarguments(args);      return f; } 

then when setting fragment:

// prepare args bundle arguments = new bundle(); arguments.putstring(key, value);  // init fragment , pass args fragment = new detailsfragment(); fragment.setarguments(arguments);  // commit fragment manager, // inflating view getsupportfragmentmanager().begintransaction()     .add(r.id.your_id, fragment).commit(); 

you can access bundled arguments on fragment's oncreate method.

addressing code specifically, should:

• mainactivity.set, pass args fragment:

  public void set(string a,string b) {     ft1 = getfragmentmanager().begintransaction();     fragdown ob = new fragdown();      // add args. practice     // declare keys public static final     // variables, accessing them accordingly. here     // i'm putting string, should     // avoid that.     bundle args = new bundle();     args.putstring("key_a", a);     args.putstring("key_b", b);     ob.setarguments(args);      ft1.add(r.id.ly2, ob);     ft1.commit(); } 

• add fragdown.oncreate method:

  private string = ""; private string b = "";  @override public void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);      this.a = getarguments().getstring("key_a");     this.b = getarguments().getstring("key_b"); } 

• call seta on oncreateview:

  @override public view oncreateview(layoutinflater inflater, viewgroup container,     bundle savedinstancestate) {     v = inflater.inflate(r.layout.fragdown, container, false);     seta(this.a, this.b)     return v; } 

this due fragment's lifecycle: http://developer.android.com/reference/android/app/fragment.html#lifecycle

now, bit of advice:

1. if you're serious android development, go read links posted (from top bottom) until understand them. sorry, there's no way around them.
2. the "show me code" kind of questions considered rude , prone downvoted or hateful comments. stackoverflow working towards "be nice" campaign, , that's 1 of main reasons coped , followed up.
3. asking question in stackoverflow should last resource, after you've exhausted every other option , have gone through docs, tutorials, source code, github issues , pull requests, , on.
4. take advice grain of salt :)