Android startActivityForResult() -

i started learning android programming have problem starting activity result(using onactivityresult(), setresult()).

first, there 3 activity - main, register, subactivity(but question regard 2 activity - main, register). registered of them 'androidmanifest.xml' this:

<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android"     package="com.wakwakwak.iwak.myapp" >     <uses-permission android:name="android.permission.vibrate"/>     <application         android:allowbackup="true"         android:icon="@mipmap/ic_launcher"         android:label="@string/app_name"         android:theme="@style/apptheme" >         <activity             android:name=".main"             android:label="@string/app_name" >             <intent-filter>                 <action android:name="android.intent.action.main" />                  <category android:name="android.intent.category.launcher" />             </intent-filter>         </activity>         <activity android:name=".subactivity"             android:label="here subworld!">          </activity>         <activity android:name=".register"             android:label="register">          </activity>     </application>  </manifest> 

as see, 3 activities registered.

this main activity.

and if touch 'reg' button, move 'register' activity.

if reg touched, register activity returns information main activity , main activity saves arraylist.

code below main activity(main.java).

package com.wakwakwak.iwak.myapp;  import android.app.activity; import android.content.context; import android.content.intent; import android.os.bundle; import android.os.vibrator; import android.view.view; import android.widget.edittext; import android.widget.toast;  import java.util.arraylist;   public class main extends activity {     private static arraylist<person> users;     @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);          vibrator vibrator = (vibrator)getsystemservice(context.vibrator_service);         vibrator.vibrate(300);     }      public void onclick(view v){         edittext edid = (edittext)findviewbyid(r.id.edit_id);         edittext edpw = (edittext)findviewbyid(r.id.edit_pw);         vibrator vibrator = (vibrator)getsystemservice(context.vibrator_service);         switch(v.getid()){             case r.id.btn_login:                 vibrator.vibrate(300);                 person person = new person(edid.gettext().tostring(),edpw.gettext().tostring());                  intent intent1 = new intent(this, subactivity.class);                 intent1.putextra("person",person);                 startactivity(intent1);                 break;             case r.id.btn_register:                 vibrator.vibrate(300);                 intent intent2 = new intent(this,register.class);                 startactivityforresult(intent2, 0);                 break;         }         }      @override     protected void onactivityresult(int requestcode, int resultcode, intent data) {         switch(requestcode)         {             case 0:                 person person = (person)data.getserializableextra("person");                 users.add(person);                  toast.maketext(this,person.tostring(),toast.length_short).show();                 break;         }     } } 

and code below register activity(register.java)

package com.wakwakwak.iwak.myapp;  import android.app.activity; import android.content.intent; import android.os.bundle; import android.view.view; import android.widget.edittext; import android.widget.toast;  public class register extends activity {     @override     protected void oncreate(bundle savedinstancestate){         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_register);     }      public void onclick(view view) {         edittext editid = (edittext)findviewbyid(r.id.edit_id);         edittext editpw = (edittext)findviewbyid(r.id.edit_pw);         edittext editpwagain = (edittext)findviewbyid(r.id.edit_pwagain);         switch(view.getid()){             case r.id.btn_register:                 if(!editpw.gettext().tostring().equals(editpwagain.gettext().tostring())){                     editpw.settext("");                     editpwagain.settext("");                     toast.maketext(this,"passwords not equal each other",toast.length_short).show();                 }                 else{                     person person = new person(editid.gettext().tostring(),editpw.gettext().tostring());                     intent intent2 = new intent();                     intent2.putextra("person",person);                     setresult(0,intent2);                     finish();                 }                 break;         }     } } 

(person class have member 'id' , 'pw'. seems there's no problem on person class)

it seems register activity works - shows error toast when each password different, when both of them same(when passing main activity), error occurs: 'myapp stopped' can't know problem!! please let me know wrong :(

when both of them same(when passing main activity), error occurs: 'myapp stopped'

probably getting npe exception users arraylist null when calling add method in onactivityresult. add following line in oncreate method:

users=new arraylist<person>();