For launching an application, we will need to create an intent that explicitly defines the activity that we wish to start. For example:

Code

        Intent intent = new Intent(this, MyTestActivity.class);

        startActivity(intent);

Intents are created to define an action that we want to perform and the launches the appropriate activity from another application.

Code

        Intent intent = new Intent(Intent.ACTION_SEND);

        intent.putExtra(Intent.EXTRA_EMAIL, recipientArray);

        startActivity(intent);

Each application gets its own Linux user ID and virtual machine which means that the application code runs in isolation from other applications.

Android Application Architecture has the following components:

> Services - like Network Operation

> Intent - To perform inter-communication between activities or services

> Resource Externalization - such as strings and graphics

> Notification signaling users - light, sound, icon, notification, dialog etc.

> Content Providers - They share data between applications

- AIDL is the abbreviation for Android Interface Definition Language. 

- It handles the interface requirements between a client and a service to communicate at the same level through interprocess communication. 

- The process involves breaking down objects into primitives that are Android understandable.

AIDL supports following data types:

- String

- List

- Map

- CharSequence

- All native Java data types like int,long, char and Boolean

- Notification messages to the user from an Android enabled device can be displayed using Intents.

- There are two types of Intents - Explicit Intent, Implicit Intent.

Implicit Intent:

- In case of Implicit Intent, an intent is just declared. 

- It is for the platform to find an activity that can respond to it.

- Since the target component is not declared, it is used for activating components of other applications.

Explicit Intent

- Explicit intent specifies the particular activity that should respond to the intent. 

- They are used for application internal messages.

The use of XML-based layouts provides a consistent and somewhat standard means of setting GUI definition format. In common practice, layout details are placed in XML files while other items are placed in source files.

In general, a Nine-patch image allows resizing that can be used as background or other image size requirements for the target device. The Nine-patch refers to the way you can resize the image: 4 corners that are unscaled, 4 edges that are scaled in 1 axis, and the middle one that can be scaled into both axes.