sqlite - Android Cant execute Update Query -

i little new in so, trying execute update query change value of field on basis of selection made in radio group. giving me nullpointerexception , havent been able figure out cause yet. code:

genderfragment:

package com.example.yipeedo;   import android.database.sqlite.sqlitedatabase; import android.os.bundle; import android.support.v4.app.fragment; import android.util.log; import android.view.layoutinflater; import android.view.view; import android.view.view.onclicklistener; import android.view.viewgroup; import android.widget.button; import android.widget.radiobutton; import android.widget.radiogroup;  public class genderfragment extends fragment  { int mcurrentpage; button next; radiogroup gendergroup; int x; string choice; radiobutton gender; view v; public sqlitedatabase db; databasehelper dbh; @override public void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);      /** getting arguments bundle object */     bundle data = getarguments();      /** getting integer data of key current_page bundle */     mcurrentpage = data.getint("current_page", 0);  }  @override public view oncreateview(layoutinflater inflater, viewgroup container, bundle savedinstancestate) {     v = inflater.inflate(r.layout.gender_layout, container,false);     next=(button)v.findviewbyid(r.id.next);     gendergroup=(radiogroup)v.findviewbyid(r.id.gendergroup);      next.setonclicklistener(new onclicklistener()     {          @override         public void onclick(view arg0) {             // todo auto-generated method stub             updatedatabase();             ((preferencesactivity)getactivity()).setcurrentitem(1, true);          }      });     return v; }  public void updatedatabase() {     int selectedid=gendergroup.getcheckedradiobuttonid();     gender=(radiobutton)v.findviewbyid(selectedid);     string selection=gender.gettext().tostring();     log.e("add gender database","gender:"+selection);     try     {      //dbh.createdatabase();     //dbh.opendatabase();     dbh.updategender(selection);     }     catch(exception e)     {         e.printstacktrace();     } }   } 

databasehelper:

package com.example.yipeedo;  import java.io.file; import java.io.fileoutputstream; import java.io.ioexception; import java.io.inputstream; import java.io.outputstream;  import android.content.context; import android.database.sqlite.sqlitedatabase; import android.database.sqlite.sqliteopenhelper;   public class databasehelper extends sqliteopenhelper{   private static string db_path = "/data/data/com.example.yipeedo/databases/";  private static string db_name = "yipeedodb1.txt";  private sqlitedatabase mydatabase;   private final context mycontext; inputstream is;   public databasehelper(context context) {      super(context, db_name, null, 1);     this.mycontext = context; }     public void write(inputstream is) {     try {         outputstream out = new fileoutputstream(new file(db_path));         int read = 0;         byte[] bytes = new byte[1024];          while ((read = is.read(bytes)) != -1) {             out.write(bytes, 0, read);         }         is.close();         out.flush();         out.close();         system.err.println(out + "\n");     } catch (ioexception e) {         e.printstacktrace();     } }   @override public void oncreate(sqlitedatabase db)  {     try      {         is=mycontext.getassets().open("yipeedodb1.txt");         write(is);         mydatabase.openorcreatedatabase(db_path+db_name, null);     }      catch (ioexception e)      {         // todo auto-generated catch block         e.printstacktrace();     } }  @override public void onupgrade(sqlitedatabase db, int oldversion, int newversion)  {  }  public void updategender(string x) {      mydatabase.execsql("update userdata set gender = " + x + " name 'ram';");  }    } 

logcat:

08-18 22:34:46.088: w/system.err(17086): java.lang.nullpointerexception 08-18 22:34:46.108: w/system.err(17086):    @     com.example.yipeedo.genderfragment.updatedatabase(genderfragment.java:71) 08-18 22:34:46.108: w/system.err(17086):    @ com.example.yipeedo.genderfragment$1.onclick(genderfragment.java:51) 08-18 22:34:46.108: w/system.err(17086):    @ android.view.view.performclick(view.java:4432) 08-18 22:34:46.108: w/system.err(17086):    @ android.view.view$performclick.run(view.java:18338) 08-18 22:34:46.108: w/system.err(17086):    @ android.os.handler.handlecallback(handler.java:725) 08-18 22:34:46.108: w/system.err(17086):    @ android.os.handler.dispatchmessage(handler.java:92) 08-18 22:34:46.108: w/system.err(17086):    @ android.os.looper.loop(looper.java:137) 08-18 22:34:46.108: w/system.err(17086):    @ android.app.activitythread.main(activitythread.java:5283) 08-18 22:34:46.108: w/system.err(17086):    @ java.lang.reflect.method.invokenative(native method) 08-18 22:34:46.108: w/system.err(17086):    @ java.lang.reflect.method.invoke(method.java:511) 08-18 22:34:46.108: w/system.err(17086):    @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:1102) 08-18 22:34:46.108: w/system.err(17086):    @ com.android.internal.os.zygoteinit.main(zygoteinit.java:869) 08-18 22:34:46.108: w/system.err(17086):    @ dalvik.system.nativestart.main(native method) 

the problem variable dbh, null.

on oncreate method add:

@override public void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);      /** getting arguments bundle object */     bundle data = getarguments();      /** getting integer data of key current_page bundle */     mcurrentpage = data.getint("current_page", 0);      dbh = new databasehelper(getactivity()); } 

and modify:

public void updategender(string x) {     if(mydatabase == null)         mydatabase = getwritabledatabase();      mydatabase.execsql("update userdata set gender = " + x + " name 'ram';");  } 

and need copy database asset internal storage before call oncreate method of dbopenhelper, please reconsider class implementation. here example of implementation what best practices sqlite on android?.