Manifest File (Cont.)
android:theme="@style/AppTheme"
It is a reference to a style resource, which can be found in the file res/values/styles.xml. This attribute points to the empty style AppTheme as follows: 
  <resources>
   <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- Customize your theme here. -->
   </style>
  </resources>
<activity>
An activity represents a single screen with a user interface. This element declares an activity that implements part of the application’s visual user interface. One of the two activities points to the class: 
     com.example.wenchen.helloworld.MainActivity
The other activity points to the class: 
     com.example.wenchen.helloworld.NextActivity
<intent-filter>
Intents are asynchronous messages which allow the application to request functionality from other services or activities. For example, an application could ask via an intent for a contact application. This element specifies the types of intents that an activity can respond to.

android:name="android.intent.action.MAIN"
For this class, an intent filter is registered which defines that this activity is started once the application starts.

android:name="android.intent.category.LAUNCHER"
The category definition defines that this application is added to the application directory on the Android device.
HelloWorld/app/src/main/AndroidManifest.xml 
<?xml version="1.0" encoding="utf-8"?>
<manifest
  xmlns:android  = "http://schemas.android.com/apk/res/android"
  package        = "com.example.wenchen.helloworld" >
 <application
  android:icon   = "@mipmap/ic_launcher"
  android:label  = "@string/app_name"
  android:theme  = "@style/AppTheme" >
  <activity
   android:name  = ".MainActivity"
   android:label = "@string/home_page" >
   <intent-filter>
    <action android:name   = "android.intent.action.MAIN" />
    <category android:name = "android.intent.category.LAUNCHER" />
   </intent-filter>
  </activity>
  <activity android:name  = ".NextActivity"
            android:label = "@string/next_page" />
 </application>
</manifest>