android instant Error : Unable to instantiate activity ComponentInfo -

i want in this, can't find out problem.

<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android"     package="com.alnassre.prayingobserver"     android:versioncode="2"     android:versionname="1.0" >       <uses-sdk         android:minsdkversion="9"         android:targetsdkversion="9" />     <uses-feature android:name="android.hardware.sensor.light" />      <application         android:icon="@drawable/ic_launcher"         android:label="@string/app_name"         android:theme="@style/apptheme" >         <activity             android:name="com.alnassre.prayingobserver.mainactivity"             android:label="@string/app_name" >             <intent-filter>                 <action android:name="android.intent.action.main" />                  <category android:name="android.intent.category.launcher" />             </intent-filter>         </activity>     </application>  </manifest> 

and

package com.alnassre.prayingobserver;   import android.hardware.sensor; import android.hardware.sensorevent; import android.hardware.sensoreventlistener; import android.hardware.sensormanager; import android.os.bundle; import android.app.activity; import android.util.log; import android.view.menu; import android.view.view; import android.widget.button; import android.widget.textview; import android.widget.toast;  public class mainactivity extends activity implements sensoreventlistener {      private sensormanager m_sensor_manager;     private  sensor m_accelerometer;       final button button = (button) findviewbyid(r.id.btn1);     final textview txt_rkoa = (textview) findviewbyid(r.id.textview1);     final textview txt_sjod = (textview) findviewbyid(r.id.textview2);       @override     protected void oncreate(bundle savedinstancestate)     {         super.oncreate(savedinstancestate);          m_sensor_manager = (sensormanager) getsystemservice(sensor_service);         m_accelerometer = m_sensor_manager.getdefaultsensor(sensor.type_light);         setcontentview(r.layout.activity_main);             button.setonclicklistener(new view.onclicklistener() {             public void onclick(view v) {                  int tkoa = integer.parseint(txt_rkoa.gettext().tostring());                 tkoa++;                 txt_rkoa.settext(string.valueof(tkoa));              }         });      }      @override     public boolean oncreateoptionsmenu(menu menu)     {         // inflate menu; adds items action bar if present.         getmenuinflater().inflate(r.menu.activity_main, menu);         return true;     }      protected void onresume()     {         super.onresume();         m_sensor_manager.registerlistener(this, m_accelerometer, sensormanager.sensor_delay_normal);     }      public void onsensorchanged(sensorevent event)     {         _("onsensorchanged");         _("" + event.values[0]);         // toast.maketext(this, "onsensorchanged "+ event.values[0], toast.length_short).show();         log.i("onsensorchanged ", event.values[0]+"");     }      public void onaccuracychanged(sensor sensor, int accuracy)     {         toast.maketext(this, "onaccuracychanged", toast.length_short).show();      }      private void _(string msg)     {         log.d("chovanec", msg);     }  } 

error statement:

05-19 22:13:24.664: e/androidruntime(12565): java.lang.runtimeexception: unable instantiate activity componentinfo{com.alnassre.prayingobserver/com.alnassre.prayingobserver.mainactivity}: java.lang.nullpointerexception 

your error at:

final button button = (button) findviewbyid(r.id.btn1); final textview txt_rkoa = (textview) findviewbyid(r.id.textview1); final textview txt_sjod = (textview) findviewbyid(r.id.textview2); 

you trying find views before oncreate called set content setcontentview(r.layout.activity_main);

should more like:

@override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);      m_sensor_manager = (sensormanager) getsystemservice(sensor_service);     m_accelerometer = m_sensor_manager.getdefaultsensor(sensor.type_light);     setcontentview(r.layout.activity_main);      button = (button) findviewbyid(r.id.btn1);     txt_rkoa = (textview) findviewbyid(r.id.textview1);     txt_sjod = (textview) findviewbyid(r.id.textview2);      button.setonclicklistener(new view.onclicklistener() {         public void onclick(view v) {              int tkoa = integer.parseint(txt_rkoa.gettext().tostring());             tkoa++;             txt_rkoa.settext(string.valueof(tkoa));          }     }); } 

also in future, past entire trace stack tell failing @ (nullpointerexception)