Android first application String cannot be resolved or is not a field error -

i going through url -http://developer.android.com/training/basics/firstapp/creating-project.html , getting error extra_message cannot resolved or not field:

menifest

<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.myfirstapp" android:versioncode="1" android:versionname="1.0" >  <uses-sdk     android:minsdkversion="8"     android:targetsdkversion="17" />  <application     android:allowbackup="true"     android:icon="@drawable/ic_launcher"     android:label="@string/app_name"     android:theme="@style/apptheme" >     <activity         android:name="com.example.myfirstapp.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>     <activity         android:name="com.example.myfirstapp.displaymessageactivity"         android:label="@string/title_activity_display_message"         android:parentactivityname="com.example.myfirstapp.mainactivity" >         <meta-data             android:name="android.support.parent_activity"             android:value="com.example.myfirstapp.mainactivity" />     </activity> </application>  </manifest> 

mainactivity.java

package com.example.myfirstapp;  import android.os.bundle; import android.view.view; import android.app.activity; import android.view.menu; import android.content.intent; import android.widget.edittext;  public class mainactivity extends activity { public final static string extra_message = "com.example.myfirstapp.message";    @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_main); }   @override public boolean oncreateoptionsmenu(menu menu) {     // inflate menu; adds items action bar if present.     getmenuinflater().inflate(r.menu.main, menu);     return true; }   public void sendmessage(view view) {     intent intent = new intent(this, displaymessageactivity.class);     edittext edittext = (edittext) findviewbyid(r.id.edit_message);     string message = edittext.gettext().tostring();     intent.putextra(extra_message, message);     startactivity(intent);      } 

}

displaymessageactivity

package com.example.myfirstapp;  import android.os.bundle; import android.view.view; import android.app.activity; import android.view.menu; import android.content.intent; import android.widget.edittext; import android.widget.textview;  import android.app.activity; import android.view.menu; import android.view.menuitem; import android.support.v4.app.navutils; import android.annotation.targetapi; import android.os.build;  public class displaymessageactivity extends activity {   @override protected void oncreate(bundle savedinstancestate) {      super.oncreate(savedinstancestate);     intent intent = getintent();     string message = intent.getstringextra(mainactivity.extra_message);     setcontentview(r.layout.activity_display_message);     // create text view     textview textview = new textview(this);     textview.settextsize(40);     textview.settext(message);      // show button in action bar.     setupactionbar();  }  /**  * set {@link android.app.actionbar}, if api available.  */ @targetapi(build.version_codes.honeycomb) private void setupactionbar() {     if (build.version.sdk_int >= build.version_codes.honeycomb) {         getactionbar().setdisplayhomeasupenabled(true); } }  @override public boolean oncreateoptionsmenu(menu menu) {     // inflate menu; adds items action bar if present.     getmenuinflater().inflate(r.menu.display_message, menu); return true; }  @override public boolean onoptionsitemselected(menuitem item) {     switch (item.getitemid()) {      case android.r.id.home:         // id represents home or button. in case of         // activity, button shown. use navutils allow users         // navigate 1 level in application structure.         // more details, see navigation pattern on android design:         //          //         navutils.navigateupfromsametask(this);         return true;         }         return super.onoptionsitemselected(item);     }  } 

please guide.

include in main activity class

public final static string extra_message = "com.example.myfirstapp.message"; 

this worked me https://www.androidpit.com/forum/632071/error-cannot-find-symbol-variable-extra-message-in-android-studio