Android Building Blocks explains about the various building blocks of Android. Declaring an Activity in AndroidManifest.xml file: After creating an activity, declare it in AndroidManifest.xml file. Ex: [code lang="xml"] <manifest xmlns:android=http://schemas.android.com/apk/res/android <application> <activity android:name="yourpackagename.MainActivity"> <intent-filter> <action android:name="android.intent.action.MAIN"/> <category android:name="android.intent.category.LAUNCHER"/> </intent-filter> </activity> <activity android:name="yourpackagename.NextActivity"> <intent-filter> <action android:name="android.intent.action.NEXTACTIVITY"/> <category android:name="android.intent.category.DEFAULT"/> </intent-filter> </activity> </application> </manifest> [/code] Starting an Activity: In the above mentioned example, since MainActivity is a LAUNCHER activity (presented to the user when launching the application for the first time) it does not require to be started  by a code. But if it is required switch from one activity to another activity you need to start an activity using Intents. You can start an activity in 2 ways: • startActivity():Will start the activity you want to start without worrying about getting  any result from new child activity started by startActivity and returning to parent activity. Ex: [java] Intent i=new Intent (MainActivity.this, NextActivity.class); startActivity(i); [/java] • startAcitvityForResult(): Will start another activity from your activity and  expects to get some data from the newly started child activity by startAcitvityForResult() and returns the same to the parent activity. Ex: [java] int k=1; Intent i=new Intent (MainActivity.this, NextActivity.class); startActivityForResult(i,k); [/java] Example: [java] public class MainActivity extends Activity { @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // The activity is being created. } @Override protected void onRestart() { super.onRestart(); // The activity is been stopped and restarted } @Override protected void onStart() { super.onStart(); // The activity becomes visible } @Override protected void onResume() { super.onResume(); // The activity will start interacting with the user. } @Override protected void onPause() { super.onPause(); // another activity is about to resume. } @Override protected void onStop() { super.onStop(); //activity is no longer visible to the user, because another activity has been resumed and is covering this one. } @Override protected void onDestroy() { super.onDestroy(); // The activity is about to be destroyed. } } [/java] Service: A Service is an application component that performs long-running operations in the background and does not provide a user interface. Service class is a subclass of ContextThemeWrapper. This is added in API Level 1. Ann application component can start a service and will continue to run in the background even if the user switches to a different application. Additionally, a component can bind to a service to interact with the service and can also perform inter process communication (IPC). For example, a service might handle play music in the background or it can interact with a content provider. Service Lifecycle: Declaring Service in AndroidManifest.xml file: After creating an activity, the activity has to be declared in AndroidManifest.xml file. Ex: [code lang="xml"] <manifest xmlns:android=http://schemas.android.com/apk/res/android <application> <activity android:name="yourpackagename.MainActivity"> <intent-filter> <action android:name="android.intent.action.MAIN"/> <category android:name="android.intent.category.LAUNCHER"/> </intent-filter> </activity> <service android:name="yourpackagename.MainService" /> </application> </manifest> [/code] Example: MainActivity.java: [java] public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } // Start the service public void startService(View view) { startService(new Intent(this, MainService.class)); } // Stop the service public void stopService(View view) { stopService(new Intent(this, MainService.class)); } } [/java] activity_main.xml: [code lang="xml"] <?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:layout_gravity="center" android:background="#FFFFFF" tools:context=".MainActivity" > <TextView android:id="@+id/textView1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignParentTop="true" android:layout_centerHorizontal="true" android:layout_marginTop="20dp" android:textColor="#000000" android:text="Android-Service Example" /> <Button android:id="@+id/button1" android:layout_width="150dp" android:layout_height="30dp" android:layout_below="@+id/textView1" android:layout_centerHorizontal="true" android:layout_marginTop="115dp" android:background="#009ACD" android:onClick="startService" android:text="Start Service" /> <Button android:id="@+id/button2" android:layout_width="150dp" android:layout_height="30dp" android:layout_alignRight="@+id/button1" android:layout_below="@+id/button1" android:background="#009ACD" android:layout_marginTop="76dp" android:onClick="stopService" android:text="Stop Service" /> </RelativeLayout> [/code] MainService.java: [java] public class MainService extends Service { int onStart; IBinder onBind; boolean onRebind; @Override public void onCreate() { // The service is being created } @Override public int onStartCommand(Intent intent, int flags, int startId) { // The service is starting, due to a call to startService() return onStart; } @Override public IBinder onBind(Intent intent) { // A client is binding to the service with bindService() return onBind; } @Override public boolean onUnbind(Intent intent) { // All clients have unbound with unbindService() return onRebind; } @Override public void onRebind(Intent intent) { //a client is rebinding to the service with bindService() after onUnbind() has //already been called. } @Override public void onDestroy() { // The service is no longer used and is being destroyed } } [/java] Output: Content Providers: Content provider creates an abstraction layer between its repository of data and external applications that are using the data. Android applications have content providers to provide data to other applications. Customized content providers can also be created to get data from database / sdcard / media etc. Content Provider is added in API Level 1. Every application has its own id data directory and  protected memory area. Applications have to register themselves as a provider of data. Other applications can request Android to read/write the data through a fixed API. To share data use Content Providers. Content Providers use standard insert(), query(), update(), delete() methods to access application data. App 2 stores its data in its own database. App 1 communicates with the provider to access App 2's data. Broadcast Receiver: Android system initiates many system-wide broadcast announcements such as "battery is low broadcasts". Android apps can also broadcast custom announcements. A broadcast is a way to send a message within an application or to a different application. A BroadcastReceiver is a component that listens and responds to the initiated broadcasts. First and foremost is to  register a broadcast receiver in AndroidManifest.xml file. BroadcastReceiver is a subclass of the Object and is introduced in API Level 1. Creating the Broadcast Receiver: A broadcast receiver class(MainReceiver) should extend BroadcastReceiver class and should override the onReceive() which receives  each message as an intent object. [java] public class MyBroadCastReceiver extends BroadcastReceiver{ @Override public void onReceive(Context context, Intent intent) { // TODO Auto-generated method stub Toast.makeText(context, "Custom Event Received"+" "+"Message is="+intent.getStringExtra("Message"), Toast.LENGTH_SHORT).show(); } } [/java] Registering Broadcast Receiver: Register receiver by declaring in AndroidManifest.xml or by java code. [java] @Override protected void onResume() { super.onResume(); registerReceiver(new MyBroadCastReceiver(), new IntentFilter("com.spl.CUSTOM_EVENT")); } [/java] OR [code lang="xml"] <receiver android:name="MyBroadCastReceiver"> <intent-filter> <action android:name="com.spl.CUSTOM_EVENT"> </action> </intent-filter> </receiver> [/code] We can also register the receiver for a system generated event [code lang="xml"] <receiver android:name="MyReceiver"> <intent-filter> <action android:name="android.intent.action.BOOT_COMPLETED"> </action> </intent-filter> </receiver> [/code] The following example explains how to create a custom BroadcastReceiver. Create MainActvity.java under src/<your packagename> MainActvity.java: [java] public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); //find view by Id Button send=(Button)findViewById(R.id.button1); //on click send send.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { // TODO Auto-generated method stub Intent i=new Intent(); i.setAction("com.spl.CUSTOM_EVENT"); i.putExtra("Message", "Hi,How are you?"); sendBroadcast(i); } }); } } [/java] Create activity.xml under res/layout folder activity.xml: [code lang="xml"] <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:layout_gravity="center" android:gravity="center" > <Button android:id="@+id/button1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Send BroadCast" android:background="#009ACD" /> </RelativeLayout> [/code] Create a BroadCastReceiver class MyBroadCastReceiver.java MyBroadCastReceiver.java: [java] public class MyBroadCastReceiver extends BroadcastReceiver{ @Override public void onReceive(Context context, Intent intent) { // TODO Auto-generated method stub Toast.makeText(context, "Custom Event Received"+" "+"Message is="+intent.getStringExtra("Message"), Toast.LENGTH_SHORT).show(); } } [/java] AndroidManifest.xml: [code lang="xml"] <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.spl.myproject" android:versionCode="1" android:versionName="1.0" > <uses-sdk android:minSdkVersion="8" android:targetSdkVersion="16" /> <application android:allowBackup="true" android:icon="@drawable/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" > <activity android:name="com.spl.myproject.MainActivity" android:label="@string/app_name" > <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <receiver android:name="MyBroadCastReceiver"> <intent-filter> <action android:name="com.spl.CUSTOM_EVENT"> </action> </intent-filter> </receiver> </application> </manifest> [/code] Output: