android - Can't resolve R file -

every time call r says can'ts resolve r symbol ,, how fix ?

package com.mohammadmustafa.web_loginregister.registeractivity.loginandregistration; import android.app.activity; import android.content.intent; import android.os.bundle; import android.view.view; import android.widget.button; import android.widget.textview; import java.util.hashmap; import com.mohammadmustafa.web_loginregister.registeractivity.loginandregistration.helper.sessionmanager; import  com.mohammadmustafa.web_loginregister.registeractivity.loginandregistration.helper.sqlitehandler;  public class mainactivity extends activity {      private textview txtname;     private textview txtemail;     private button btnlogout;      private sqlitehandler db;     private sessionmanager session;      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main); 

all r here in red line :

        txtname = (textview) findviewbyid(r.id.name);         txtemail = (textview) findviewbyid(r.id.email);         btnlogout = (button) findviewbyid(r.id.btnlogout); 

r generated default in android contains reference of other variables check if added code in (res->layout).xml first save jump java code , use find view id (try select project->right click->android tools->fix project properties)