Android shows error "Cannot resolve symbol R" -

before mark question duplicate, please know have tried this , this.

i getting following error in messages:

information:gradle tasks [:app:generatedebugsources, :app:generatedebugandroidtestsources] :app:prebuild up-to-date :app:predebugbuild up-to-date :app:checkdebugmanifest :app:prereleasebuild up-to-date :app:preparecomandroidsupportappcompatv72311library up-to-date :app:preparecomandroidsupportdesign2311library up-to-date :app:preparecomandroidsupportrecyclerviewv72311library up-to-date :app:preparecomandroidsupportsupportv42311library up-to-date :app:preparedebugdependencies :app:compiledebugaidl up-to-date :app:compiledebugrenderscript up-to-date :app:generatedebugbuildconfig up-to-date :app:generatedebugassets up-to-date :app:mergedebugassets up-to-date :app:generatedebugresvalues up-to-date :app:generatedebugresources up-to-date :app:mergedebugresources up-to-date :app:processdebugmanifest up-to-date :app:processdebugresources d:\androidstudioproject\bluealert\app\src\main\res\layout\content_connected.xml error:(15, 21) no resource found matches given name (at 'id' value '@id/screenimageview'). error:execution failed task ':app:processdebugresources'. > com.android.ide.common.process.processexception: org.gradle.process.internal.execexception: process 'command 'd:\android\androidsdk\build-tools\23.0.2\aapt.exe'' finished non-zero exit value 1 information:build failed information:total time: 5.214 secs information:2 errors information:0 warnings information:see complete output in console 

this mainactivity.java

package vertex2016.mvjce.edu.bluealert;  import android.bluetooth.bluetoothadapter; import android.bluetooth.bluetoothdevice; import android.content.broadcastreceiver; import android.content.context; import android.content.intent; import android.content.intentfilter; import android.content.pm.activityinfo; import android.os.bundle; import android.os.handler; import android.support.design.widget.floatingactionbutton; import android.support.design.widget.snackbar; import android.support.v7.app.appcompatactivity; import android.support.v7.widget.toolbar; import android.view.gravity; import android.view.view; import android.view.menu; import android.view.menuitem; import android.widget.button; import android.widget.listview; import android.widget.textview; import android.widget.toast; import android.widget.*;  import java.util.uuid;  import static java.lang.thread.sleep;  public class mainactivity extends appcompatactivity {      //to default bluetooth adapter on android device     public bluetoothadapter ba = bluetoothadapter.getdefaultadapter();      //a request code identify activity executed     private int req_code = 1;      private boolean on = false;      //the search button on main screen     private button searchbutton;      //the view lists nearby bluetooth devices found     private listview listbtdevices;      //display welcome text     private textview btdesc;      //store found bluetooth devices & pass them on listview     private arrayadapter btarrayadapter;      //a variable points actual bluetooth on device     private bluetoothdevice btd;      //uuid specify services can provide       //intent filter detect discovery of nearby bluetooth devices     intentfilter filter = new intentfilter(bluetoothdevice.action_found);       @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_main);         toolbar toolbar = (toolbar) findviewbyid(r.id.toolbar);         setsupportactionbar(toolbar);          //lock rotation of screen         setrequestedorientation(activityinfo.screen_orientation_locked);          searchbutton = (button) findviewbyid(r.id.searchbutton);         listbtdevices = (listview) findviewbyid(r.id.listbtdevices);          //initially listview hidden, after search button has been pressed,         // listview visible         listbtdevices.setvisibility(view.gone);         btdesc = (textview) findviewbyid(r.id.btdesc);           searchbutton.setonclicklistener(new view.onclicklistener() {             @override             public void onclick(view v) {                  if (!on) {                     connect();                  } else if (on) {                     stopdiscovery();                     on = false;                     searchbutton.settext("search");                 }              }         });      }      @override     public boolean oncreateoptionsmenu(menu menu) {         // inflate menu; adds items action bar if present.         getmenuinflater().inflate(r.menu.menu_main, menu);         return true;     }      @override     public boolean onoptionsitemselected(menuitem item) {         // handle action bar item clicks here. action bar         // automatically handle clicks on home/up button, long         // specify parent activity in androidmanifest.xml.         int id = item.getitemid();          //noinspection simplifiableifstatement         if (id == r.id.action_settings) {             return true;         }          return super.onoptionsitemselected(item);     }       //a method checks if targeted device supports bluetoth or not     //in case does, execute searchbtdevices method search     public void connect() {         //registering intentfilter         this.registerreceiver(receiver, filter);          //if device doesn't have bluetooth, bluetooth adapter ba returns null         if (ba == null)             toast.maketext(mainactivity.this, "system doesn't support bluetooth", toast.length_short).show();              //in case device has bluetooth, bluetooth isn't enabled             //enables bluetooth on device             //startactivityforresult() takes in intent & request code identify intent         else if (!ba.isenabled()) {             intent enablebt = new intent(bluetoothadapter.action_request_enable);             startactivityforresult(enablebt, req_code);         }         //in case bluetooth enabled on device, start discovery         else {             searchbtdevices();         }     }       @override     protected void onactivityresult(int requestcode, int resultcode, intent data) {         if (resultcode != result_canceled) {             toast.maketext(mainactivity.this, "turned on!", toast.length_short).show();             searchbtdevices();         } else             toast.maketext(mainactivity.this, "failed enable bluetooth", toast.length_long).show();     }       public void searchbtdevices() {         //as search starts, welcome screen textview disappears & listview appears         btdesc.setvisibility(view.gone);         listbtdevices.setvisibility(view.visible);           btarrayadapter = new arrayadapter(this, android.r.layout.simple_list_item_1);           //in case discovery fails begin         if (!ba.startdiscovery())             toast.maketext(mainactivity.this, "failed start discovery", toast.length_short).show();          else {             toast.maketext(mainactivity.this, "discovery started", toast.length_short).show();             on = true;             searchbutton.settext("stop discovery");          }          listbtdevices.setadapter(btarrayadapter);           //setting onitemclick selecting bluetooth device connect         listbtdevices.setonitemclicklistener(clicklistener);      }       private final broadcastreceiver receiver = new broadcastreceiver() {         @override         public void onreceive(context context, intent intent) {             string action = intent.getaction();              if (bluetoothdevice.action_found.equals(action)) {                 btd = intent.getparcelableextra(bluetoothdevice.extra_device); //get device details                  btarrayadapter.add(btd.getname() + "\t\t" + btd.getaddress());             }          }     };      private void stopdiscovery() {         ba.canceldiscovery();         toast.maketext(mainactivity.this, "discovery stopped", toast.length_short).show();         this.unregisterreceiver(receiver);     }      @override     protected void onresume() {         super.onresume();         ba.canceldiscovery();         ba.startdiscovery();     }      @override     protected void onrestart() {         super.onrestart();         btarrayadapter.clear();         toast.maketext(mainactivity.this, "discovery resumed", toast.length_short).show();     }      public final adapterview.onitemclicklistener clicklistener = new adapterview.onitemclicklistener() {         @override         public void onitemclick(adapterview<?> parent, view view, int position, long id)         {             intent connectedbt = new intent(mainactivity.this, connected.class);             connectedbt.putextra("bluetooth device", btd);             startactivity(connectedbt);         }     }; } 

and content_connected.xml

<?xml version="1.0" encoding="utf-8"?> <relativelayout xmlns:android="http://schemas.android.com/apk/res/android"     xmlns:app="http://schemas.android.com/apk/res-auto"     xmlns:tools="http://schemas.android.com/tools"     android:layout_width="match_parent"     android:layout_height="match_parent"     app:layout_behavior="@string/appbar_scrolling_view_behavior"     tools:context="vertex2016.mvjce.edu.bluealert.connected"     tools:showin="@layout/activity_connected">       <imageview         android:layout_width="match_parent"         android:layout_height="match_parent"         android:id="@id/screenimageview"         android:layout_alignparenttop="true"         android:layout_alignparentleft="true"         android:layout_alignparentstart="true" /> </relativelayout> 

i tried cleaning project, rebuilding & syncing gradle. none of worked...

please help. thank time!!

you've missed "+", try @+id/screenimageview