java - Android app dev, custom numberpicker for api level 8 -

i trying create custom number picker applications require minimum of api level 8, far got code, simple don't know how fix error get.

the code far this:

package com.example.symbol_temp;

import android.os.bundle; import android.support.v7.app.actionbaractivity; import android.view.view; import android.widget.button; import android.widget.textview;  public class mainactivity extends actionbaractivity {      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);          integer counter=0;         button add,sub;         final textview display;          add = (button) findviewbyid(r.id.plus);         sub = (button) findviewbyid(r.id.minus);         display = (textview) findviewbyid(r.id.showtemperature);          add.setonclicklistener(new view.onclicklistener() {              public void onclick(view v) {             counter++;             display.settext( "" + counter);             }         });          sub.setonclicklistener(new view.onclicklistener() {              public void onclick(view v) {             counter--;             display.settext( "" + counter);             }         });          }  } 

and xml this:

<relativelayout xmlns:android="http://schemas.android.com/apk/res/android"     xmlns:tools="http://schemas.android.com/tools"     android:layout_width="match_parent"     android:layout_height="match_parent"     android:paddingbottom="@dimen/activity_vertical_margin"     android:paddingleft="@dimen/activity_horizontal_margin"     android:paddingright="@dimen/activity_horizontal_margin"     android:paddingtop="@dimen/activity_vertical_margin"     tools:context="com.example.symbol_temp.mainactivity$placeholderfragment" >      <textview         android:id="@+id/showtemperature"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_centerhorizontal="true"         android:layout_centervertical="true"         android:textappearance="?android:attr/textappearancelarge" />      <button         android:id="@+id/minus"         style="?android:attr/buttonstylesmall"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_above="@+id/showtemperature"         android:layout_centerhorizontal="true"         android:text="+"          android:onclick="add"/>      <button         android:id="@+id/plus"         style="?android:attr/buttonstylesmall"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_alignleft="@+id/minus"         android:layout_below="@+id/showtemperature"         android:text="-" />  </relativelayout> 

getting in logcat:

04-14 13:31:59.222: e/androidruntime(359): fatal exception: main 04-14 13:31:59.222: e/androidruntime(359): java.lang.runtimeexception: unable start activity componentinfo{com.example.symbol_temp/com.example.symbol_temp.mainactivity}: java.lang.nullpointerexception 04-14 13:31:59.222: e/androidruntime(359):  @ android.app.activitythread.performlaunchactivity(activitythread.java:2663) 04-14 13:31:59.222: e/androidruntime(359):  @ android.app.activitythread.handlelaunchactivity(activitythread.java:2679) 04-14 13:31:59.222: e/androidruntime(359):  @ android.app.activitythread.access$2300(activitythread.java:125) 04-14 13:31:59.222: e/androidruntime(359):  @ android.app.activitythread$h.handlemessage(activitythread.java:2033) 04-14 13:31:59.222: e/androidruntime(359):  @ android.os.handler.dispatchmessage(handler.java:99) 04-14 13:31:59.222: e/androidruntime(359):  @ android.os.looper.loop(looper.java:123) 04-14 13:31:59.222: e/androidruntime(359):  @ android.app.activitythread.main(activitythread.java:4627) 04-14 13:31:59.222: e/androidruntime(359):  @ java.lang.reflect.method.invokenative(native method) 04-14 13:31:59.222: e/androidruntime(359):  @ java.lang.reflect.method.invoke(method.java:521) 04-14 13:31:59.222: e/androidruntime(359):  @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:868) 04-14 13:31:59.222: e/androidruntime(359):  @ com.android.internal.os.zygoteinit.main(zygoteinit.java:626) 04-14 13:31:59.222: e/androidruntime(359):  @ dalvik.system.nativestart.main(native method) 04-14 13:31:59.222: e/androidruntime(359): caused by: java.lang.nullpointerexception 04-14 13:31:59.222: e/androidruntime(359):  @ com.example.symbol_temp.mainactivity.oncreate(mainactivity.java:24) 04-14 13:31:59.222: e/androidruntime(359):  @ android.app.instrumentation.callactivityoncreate(instrumentation.java:1047) 04-14 13:31:59.222: e/androidruntime(359):  @ android.app.activitythread.performlaunchactivity(activitythread.java:2627) 04-14 13:31:59.222: e/androidruntime(359):  ... 11 more 

i think have changed few things, line number 24 :

add.setonclicklistener(new view.onclicklistener() { 

thank your help.

from comment and

tools:context="com.example.symbol_temp.mainactivity$placeholderfragment" 

i assume views belong fragment_main.xml.

you inflate wrong layout. , looking view in wrong layout.

change this

setcontentview(r.layout.activity_main); 

to

setcontentview(r.layout.fragment_main); 

also change this

integer counter=0; 

to

int counter; // primitive data type @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_main);      counter =0;