'无法解析符号R'android studio错误

任务执行失败：app:mergeDebugResources。什么是这个错误，以及如何解决它？'无法解析符号R'android studio错误

<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" /> 
<uses-permission android:name="android.permission.INTERNET" /> 

<application 
    android:allowBackup="true" 
    android:icon="@mipmap/ic_launcher" 
    android:label="@string/app_name" 
    android:supportsRtl="true" 
    android:theme="@style/AppTheme"> 
    <activity android:name=".MainActivity"> 
     <intent-filter> 
      <action android:name="android.intent.action.MAIN" /> 

      <category android:name="android.intent.category.LAUNCHER" /> 
     </intent-filter> 
    </activity> 
    <!-- ATTENTION: This was auto-generated to add Google Play services to your project for 
     App Indexing. See https://g.co/AppIndexing/AndroidStudio for more information. --> 
    <meta-data 
     android:name="com.google.android.gms.version" 
     android:value="@integer/google_play_services_version" /> 
</application>

这是我的XML以上...以下是onCreate()其中setContentView(R.layout.activity_main) 是造成问题：

无法解决标记R

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE); 

    provider = locationManager.getBestProvider(new Criteria(), false); 

    // ATTENTION: This was auto-generated to implement the App Indexing API. 
    // See https://g.co/AppIndexing/AndroidStudio for more information. 
    client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build(); 
}

来源

2016-02-08 Adam Goldberg

'Clean-Rebuild-Restart'＆import'import PackageName.R' –

请确保所有资源都作为manifest中的相同包.mainactivity是否带有红色符号？ –

当你建立你的项目没有警告或错误？也尝试关闭并再次打开AS，有时适用于我 –