帮我看下android这个异常到底怎么回事？

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android_demo/com.example.android_demo.MainActivity}: android.content.ActivityNotFoundException: Unable to find explicit activity class {com.example.android_demo/com.example.android_demo.ArtistisActivity}; have you declared this activity in your AndroidManifest.xml? 

我已经在mainifest.xml加了

        <activity android:name=".AlbumsActivity"  android:label="@string/app_name"></activity> 
         <activity android:name=".ArtistsActivity" android:label="@string/app_name"></activity> 
         <activity android:name=".SongsActivity"  android:label="@string/app_name"></activity> 

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

但它还是包异常。球解释。