Android Beginners Guide


Android Operation System:

Android is an operating system based on Linux with a Java programming interface. It provides tools, e.g. a compiler, debugger and a device emulator as well as its own Java Virtual machine (Dalvik Virtual Machine - DVM). Android is created by the Open Handset Alliance which is lead by Google.
Android uses a special virtual machine, e.g. the Dalvik Virtual Machine. Dalvik uses special bytecode. Therefore you cannot run standard Java bytecode on Android. Android provides a tool "dx" which allows to convert Java Class files into "dex" (Dalvik Executable) files. Android applications are packed into an .apk (Android Package) file by the program "aapt" (Android Asset Packaging Tool) To simplify development Google provides the Android Development Tools (ADT) for Eclipse. The ADT performs automatically the conversion from class to dex files and creates the apk during deployment.
Android supports 2-D and 3-D graphics using the OpenGL libraries and supports data storage in a SQLite database.
Every Android applications runs in its own process and under its own userid which is generated automatically by the Android system during deployment. Therefore the application is isolated from other running applications and a misbehaving application cannot easily harm other Android applications
Important Android components

An Android application consists out of the following parts:
Activity - Represents the presentation layer of an Android application, e.g. a screen which the user sees. An Android application can have several activities and it can be switched between them during runtime of the application.
Views - The User interface of an Activity is build with widgets classes which inherent from "android.view.View". The layout of the views is managed by "android.view.ViewGroups"
Services - perform background tasks without providing an UI. They can notify the user via the notification framework in Android.
Content Provider - provides data to applications, via a content provider your application can share data with other applications. Android contains a SQLite DB which can serve as data provider
Intents:-are asynchronous messages which allow the application to request functionality from other services or activities. An application can call directly a service or activity (explicit intent) or asked the Android system for registered services and applications for intent (implicit intents). For example the application could ask via intent for a contact application. Application registers them self to an intent via an Intent Filter. Intents are a powerful concept as they allow creating loosely coupled applications.
Broadcast Receiver - receives system messages and implicit intents, can be used to react to changed conditions in the system. An application can register as a broadcast receiver for certain events and can be started if such an event occurs
Security and permissions
Android defines certain permissions for certain tasks. For example if the application want to access the Internet it must define in its configuration file that it would like to use the related permission. During the installation of an Android application the user get a screen in which he needs to confirm the required permissions of the application.
An Android application is described the file "AndroidManifest.xml". This file must declare all activities, services, broadcast receivers and content provider of the application. It must also contain the required permissions for the application. For example if the application requires network access it must be specified here. "AndroidManifest.xml" can be thought as the deployment descriptor for an Android application.

Features and Run Time Environment of Android:
Android includes a set of core libraries that provides most of the functionality available in the core libraries of the Java programming language.
Every Android application runs in its own process, with its own instance of the Dalvik virtual machine. Dalvik has been written so that a device can run multiple VMs efficiently. The Dalvik VM executes files in the Dalvik Executable (.dex) format which is optimized for minimal memory footprint. The VM is register-based, and runs classes compiled by a Java language compiler that have been transformed into the .dex format by the included "dx" tool.
The Dalvik VM relies on the Linux kernel for underlying functionality such as threading and low-level memory management. 

XML (Extensible Markup Language)
XML files are text files, which can be managed by any text editor.
XML is very simple, because it has less than 10 syntax rules.
XML is extensible, because it only specifies the structural rules of tags. No specification on tags them self
XML provides a basic syntax that can be used to share information between different kinds of computers, different applications, and different organizations. XML data is stored in plain text format. This software- and hardware-independent way of storing data allows different incompatible systems to share data without needing to pass them through many layers of conversion. This also makes it easier to expand or upgrade to new operating systems, new applications, or new browsers, without losing any data
With XML, your data can be available to all kinds of "reading machines" (Handheld computers, voice machines, news feeds, etc), and make it more available for blind people, or people with other disabilities.
It supports Unicode, allowing almost any information in any written human language to be communicated.
It can represent common computer science data structures: records, lists and trees

XML on Android
The Android platform is an open source mobile development platform. It gives you access to all aspects of the mobile device that it runs on, from low level graphics, to hardware like the camera on a phone. With so many things possible using Android, you might wonder why you need to bother with XML. It is not that working with XML is so interesting; it is working with the things that it enables. XML is commonly used as a data format on the Internet. If you want to access data from the Internet, chances are that the data will be in the form of XML. If you want to send data to a Web service, you might also need to send XML. In short, if your Android application will leverage the Internet, then you will probably need to work with XML. Luckily, you have a lot of options available for working with XML on Android. 

Android 3.0  Android 2.3   Android 2.2   Android 2.0    Android 1.6



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