java - Android app crashes upon replacing fragment: -

i followed tutorial here on how make fragment transactions whenever hit button transact fragment app crashes. code:

public class mainactivity extends activity {      public fragmenttransaction transaction = getfragmentmanager().begintransaction();     public fragment loginfragment = new loginfragment();     public fragment mainfragment = new mainfragment();      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);          setcontentview(r.layout.activity_main);          if (savedinstancestate == null) {              getfragmentmanager().begintransaction().add(r.id.container, loginfragment).commit();          } else {              getfragmentmanager().begintransaction().add(r.id.container, mainfragment).commit();          }     }      @override     public boolean oncreateoptionsmenu(menu menu) {          getmenuinflater().inflate(r.menu.main, menu);          return true;     }      public void login() {          transaction.replace(r.id.container, mainfragment);         transaction.addtobackstack(null);         transaction.commit();     }      public static class loginfragment extends fragment {          public loginfragment() {          }          @override         public view oncreateview(layoutinflater inflater, viewgroup container, bundle savedinstancestate) {              view rootview = inflater.inflate(r.layout.fragment_login, container, false);              return rootview;         }     }      public static class mainfragment extends fragment {          public mainfragment() {          }          @override         public view oncreateview(layoutinflater inflater, viewgroup container, bundle savedinstancestate) {              view rootview = inflater.inflate(r.layout.fragment_main, container, false);              return rootview;         }     } } 

xml:

<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"     tools:context="com.pointlight.kingdomcraft.mainactivity$loginfragment" >      <button         android:id="@+id/button_submit"         android:onclick="login" />  </relativelayout> 

so in conclusion submit button on login fragment replaces login fragment main fragment, or should instead crashes.

to narrow down, in xml set button android:onclick= "login" in turn calls login method inside activity class:

public void login() {              transaction.replace(r.id.container, mainfragment);             transaction.addtobackstack(null);             transaction.commit();         } 

what have done wrong, i've been working on fragments transactions 2 days no justice.

logcat:

?:??: w/?(?): --------- beginning of /dev/log/main

?:??: w/?(?): --------- beginning of /dev/log/system

edit:

if debug , hit submit button instead of crashing button gets highlighted blue , app freezes

the signature onclick handler android:onclick="login" wrong:

public void login() { 

should be

public void login(view v) { 

android:onclick attribute gets translated code looks handler method name using reflection. lookup process requires method signature i.e. parameter types. if signature doesn't match, method cannot found , exception thrown.