Android Programming Basic Concepts

1. An activity is an instance of ACTIVITY, it is responsible for managing user interaction with a screen of information.
2. A layout defines a set of user interface objects and their position on the screen, which is written in XML.
3. Widgets are the building blocks you use to compose a user interface. Every widgets is an instance of the View class or one of its subclasses. Every widgets has a corresponding XML element. Each element has a set of XML attributes. Each attribute is an instruction about how the widget should be configured.
4. FrameLayout, TableLayout, RelativeLayout and LinearLayout are subclasses of ViewGroup, which is a subclass of View.
5. A resource is a piece of your application that is not code, such as image files, audio files and XML files. Android generates a resource ID for the entire layout and for each string, but it does not generate IDs for the individual widgets, since not every widget needs a resource ID.
6. The manifest is an XML file containing metadata that describes your application to the Android OS.
The file is always named AndroidManifest.xml, and it lives in the root directory of your project. Every activity in an application must be declared in the manifest so that the OS can access it.
7.  An intent is an object that a component can use to communicate with the OS. Activities, services, broadcast receivers and content providers are all components. When you create an Intent with a context and a class object, you are creating an explicit intent. Explicit intents are used to start activities within your application. public Intent(Context packageContext, Class<?> cls). When an activity in your application wants to start an activity in another application, you create an implicit intent.