android - Null pointer exception on pressing favorites button in single item view activity -

i have listview opens new single itemview activity when clucked on each list item . inside singleitemview activity have favorites button adds list item opened singleitemview activity favorites activity using sharedpreferences.. pass listitem clicked use jackson library convert respective listitem json string , use putextra via intent singleitemview activity . convert json string listitem object in singleitemview , use add favorites

but when click favorite button in singleitemview app crashes , after reopening app listutem added favorites activity

here code

onitemclicklistener of list activity

    @override public void onitemclick(adapterview<?> parent, view view, int position,                         long id) {                              objectmapper mapper = new objectmapper();                             product pro = productlistadapter.getitem(position);     try     {         string jsoninstring = mapper.writevalueasstring(pro);         intent intent = new intent(activity.getapplicationcontext(), singleitemview.class);         intent.putextra("selected item", jsoninstring);          startactivity(intent);     }     catch (jsonprocessingexception e)     {//something went wrong           }     } 

singleitemview.java

public class singleitemview extends activity  { productlistadapter padaptr; sharedpreference sharedpreference;  list<product> products = null;  @override protected void oncreate(bundle savedinstancestate) {     // todo: implement method     super.oncreate(savedinstancestate);     setcontentview(r.layout.singleitem);     sharedpreference = new sharedpreference();     padaptr = new productlistadapter(singleitemview.this, products);        button btn = (button) findviewbyid(r.id.singleitembutton1);     btn.setonclicklistener(new onclicklistener(){         @override         public void onclick(view v){              bundle extras = getintent().getextras();             string jsonobj = extras.getstring("selected item");             objectmapper mapper = new objectmapper();              try             {                 product pro = mapper.readvalue(jsonobj, product.class);                         //the fav image present on evry list item                 imageview button = (imageview) findviewbyid(r.id.imgbtn_favorite);                 if (checkfavoriteitem(pro)) {                      sharedpreference.removefavorite(singleitemview.this, pro);                     button.settag("no");                     button.setimageresource(r.drawable.heart_grey);                     toast.maketext(singleitemview.this,                                    singleitemview.this.getresources().getstring(r.string.remove_favr),                                    toast.length_short).show();                 } else {                     sharedpreference.addfavorite(singleitemview.this, pro);                     toast.maketext(singleitemview.this,                                    singleitemview.this.getresources().getstring(r.string.add_favr),                                    toast.length_short).show();                      button.settag("yes");                     button.setimageresource(r.drawable.heart_red);                 }             }             catch (ioexception e)             {};              }                private boolean checkfavoriteitem(product checkproduct) {                 boolean check = false;                 list<product> favorites = sharedpreference.getfavorites(getapplicationcontext());                 if (favorites != null) {                     (product product : favorites) {                         if (product.equals(checkproduct)) {                             check = true;                             break;                         }                     }                 }                 return check;             }     });     }   } 

the line pointing nullpointer eception log cat is

                        button.settag("yes"); 

singleitem.xml

<linearlayout     xmlns:android="http://schemas.android.com/apk/res/android"     android:layout_width="fill_parent"     android:layout_height="fill_parent"     android:orientation="vertical"     android:background="#a25550"     android:gravity="center">      <button         android:layout_height="wrap_content"         android:text="addcto fav"         android:layout_width="wrap_content"         android:id="@+id/singleitembutton1"/>      <textview         android:layout_height="wrap_content"         android:text="large text"         android:textappearance="?android:attr/textappearancelarge"         android:layout_width="wrap_content"         android:id="@+id/singleitemtextview1"/>  </linearlayout> 

list item xml

<?xml version="1.0" encoding="utf-8"?>  <relativelayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="wrap_content" android:background="@color/product_list_item_bg" android:descendantfocusability="blocksdescendants" >  <relativelayout     android:id="@+id/pdt_layout_item"     android:layout_width="fill_parent"     android:layout_height="wrap_content" >      <textview         android:id="@+id/txt_pdt_name"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:padding="6dp" />      <textview         android:id="@+id/txt_pdt_price"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_below="@+id/txt_pdt_name"         android:padding="6dp" />      <textview         android:id="@+id/txt_pdt_desc"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_below="@+id/txt_pdt_price"         android:padding="6dp" />      <imageview         android:id="@+id/imgbtn_favorite"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:layout_alignbottom="@+id/txt_pdt_desc"         android:layout_alignparentright="true"         android:layout_marginright="3dp"         android:background="@null"         android:contentdescription="@string/favorites" /> </relativelayout>  <view     android:layout_width="match_parent"     android:layout_height="1dp"     android:layout_below="@+id/pdt_layout_item"     android:background="@color/view_divider_color" />    </relativelayout> 

my logcat

01-27 17:29:19.777 14852 14852 e   androidruntime fatal exception: main 01-27 17:29:19.777 14852 14852 e   androidruntime java.lang.nullpointerexception 01-27 17:29:19.777 14852 14852 e   androidruntime @ com.mycompany.myapp.singleitemview$100000000.onclick(singleitemview.java:62) 01-27 17:29:19.777 14852 14852 e   androidruntime @ android.view.view.performclick(view.java:4452) 01-27 17:29:19.777 14852 14852 e   androidruntime @ android.widget.button.performclick(button.java:148) 01-27 17:29:19.777 14852 14852 e   androidruntime @ android.view.view$performclick.run(view.java:18428) 01-27 17:29:19.777 14852 14852 e   androidruntime @ android.os.handler.handlecallback(handler.java:725) 01-27 17:29:19.777 14852 14852 e   androidruntime @ android.os.handler.dispatchmessage(handler.java:92) 01-27 17:29:19.777 14852 14852 e   androidruntime @ android.os.looper.loop(looper.java:176) 01-27 17:29:19.777 14852 14852 e   androidruntime @ android.app.activitythread.main(activitythread.java:5365) 01-27 17:29:19.777 14852 14852 e   androidruntime @ java.lang.reflect.method.invokenative(native method) 01-27 17:29:19.777 14852 14852 e   androidruntime @ java.lang.reflect.method.invoke(method.java:511) 01-27 17:29:19.777 14852 14852 e   androidruntime @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:1102) 01-27 17:29:19.777 14852 14852 e   androidruntime @ com.android.internal.os.zygoteinit.main(zygoteinit.java:869) 01-27 17:29:19.777 14852 14852 e   androidruntime @ dalvik.system.nativestart.main(native method) 01-27 17:29:20.316 17044 17044 d   androidruntime calling main entry com.android.commands.am.am 

reason: reason getting nullpointerexception trying access id imgbtn_favorite in singlelistitem.xml not exist. you can access variable in xml associated activity class.

solution: noticed using variable set tag , change imageresource. can't activity. can either

1. store global value in sharedpreferences , show changed value when listview opens again.

2. or, if do not need open new activity, can access view in listview activity itself.

edit:

3. you can use startactivityforresult send values listview activity and then make changes in variable. can refer this link example.