Most Frequently used implicit Intents

1. opening a browser :

Intent intent=new Intent(Intent.ACTION_VIEW,Uri.pars("https://www.google.com"));
satrtActivity(intent);

2. Call when button click :

Intent callIntent = new Intent(Intent.ACTION_CALL);
callIntent.setData(Uri.parse("tel:"+txtphn.getText().toString().trim()));
callIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(callIntent);

3. External Camera :

Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
intent.putExtra(android.provider.MediaStore.EXTRA_OUTPUT, Constants.mImageCaptureUri);
startActivityForResult(intent, PICK_FROM_CAMERA);

4. Pick from Gallary :

Intent i = new Intent(Intent.ACTION_PICK, android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
startActivityForResult(i, RESULT_LOAD_IMAGE);

The Differences between Service and IntentService in Android

Differences

• Service class uses the application's main thread, while IntentService creates a worker thread and uses that thread to run the service.
• IntentService creates a queue that passes one intent at a time to onHandleIntent(). Thus, implementing a multi-thread should be made by extending Service class directly.
• Service class needs a manual stop using stopSelf(). Meanwhile, IntentService automatically stops itself when there is no intent in queue. 
• IntentService implements onBind() that returns null. This means that the IntentService can not be bound by default.
• IntentService implements onStartCommand() that sends Intent to queue and to onHandleIntent().

Implicit Intent and Explicit Intents

1.Implicit Intents:

• This is the main power of android design, we just declare and intent and leave it to the platform to find the activity that can respond to the intent. Here we do not declare the target component and hence is typically used for activating components of other applications seamlessly.
• Implicit Intents have not specified a component; Instead, they must include enough information for the system to determine which of the available components is best to run for that intent.

Ex: Intent webIntent=new Intent(Intent.ACTION_VIEW,Uri.pars("http://www.google.com"));

startActivity(webIntent);

Ex: Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);

intent.putExtra(android.provider.MediaStore.EXTRA_OUTPUT, Constants.mImageCaptureUri);

startActivity(intent );

2.Explicit Intents:

In an Explicit Intents, we actually specify the activity that is required to respond to the intent.In othere words we explicitly designate the target component.This is typically used for application internal messages

Ex: Intent launchExplicitIntent=new Intent(this,Game.class);

startActivity(launchExplicitIntent);

Things That Cannot Change

Sometimes a developer will make a change to an application that has surprising results when installed as and update to a previous version - shortcuts break,widgets disappear, or it can't even be installed at all. 

There are certain parts in application that are immutable once you publish it, and you can avoid surprises by understanding them.

1. Your package name and certificate.

• Once you publish your application under its manifest package name, this is the unique identity of the application forever.
• The Signing certificate represents the author of the application.If you change the certificate an application signed with, it is now a different application because if comes with new author.

The exact behavior the user sees when installing an application that has changed in one of these two ways is different:

• If the Manifest package name has changed, the new application will be installed alongside the old application, so thy both co-exist on the user device at the same time.
• If the signing certificate changes, trying to install the new application on to the device will fail until the old version the uninstalled.

2. Your AndroidManifest.xml is a public API:

• A major function of the AndroidManifest.xml is essentially to declare a public API from your application for use by other applications and the Android system.
• Every component you declare in the manifest that is not private ( android:exported="true" - public, android:exported="false" - private) should be treated as a public API .
• We think android:name attribute of your activity,service and receiver components as pointing to your private code implementing your application, but it is also the official unique public name for that component.

Change the component name inside of an application can have negative consequences for your users.

• If the name of the main activity of your application is changed - short cuts will no longer work.
• If the name of a service implementing a live wallpaper changes - then the users who enabled your Live wallpaper will have there wallpaper reverted to system default.
• If the name of a receiver implementing a Device Admin changes - It will be disabled, this will effect all the other kinds of receiver, such as app widgets. 

These behaviors are an outcome of how the Intent system is used on Android.