same people will get irritated at you if your program “breaks” their phone by
• tying up the CPU so that calls can’t be received
• not working properly with the rest of the phone’s OS, such that your application doesn’t quietly fade to the background when a call comes in or needs to be placed
• crashing the phone’s operating system, such as by leaking memory like a sieve
Hence, developing programs for a phone is a different experience than developing desktop applications, Web sites, or back-end server processes. You wind up with different-looking tools, different-behaving frameworks, and “different than you’re used to” limitations on what you can do with your program.
What Android tries to do is meet you halfway:
• You get a commonly-used programming language (Java) with some commonly used libraries (e.g., some Apache Commons APIs) along with support for tools you may be used to (Eclipse).
• You get a fairly rigid and separate framework in which your programs need to run so they can be “good citizens” on the phone and not interfere with other programs or the operation of the phone itself.
As you may expect, much of this book deals with that framework and how you write programs that work within its confines and take advantage of its capabilities.
What Androids Are Made Of
When you write a desktop application, you are “master of your own domain.” You launch your main window and any child windows — like dialog boxes — that are needed. From your standpoint, you are your own world, leveraging features supported by the operating system, but largely ignorant of any other program that may be running on the computer at the same time. If you do interact with other programs, it is typically through an API, such as using JDBC (or frameworks atop it) to communicate with MySQL or another database.
Android has similar concepts, but packaged differently, and structured to make phones more crash- resistant.
Activities
The building block of the user interface is the activity. You can think of an activity as being the Android analogue for the window or dialog in a desktop application.
While it is possible for activities to not have a user interface, most likely your “headless” code will be packaged in the form of content providers or services, like the following described.
Content Providers
Content providers provide a level of abstraction for any data stored on the device that is accessible by multiple applications. The Android development model encourages you to make your own data available to other applications, as well as your own — building a content provider lets you do that, while maintaining complete control over how your data gets accessed.
Intents
Intents are system messages, running around the inside of the device, notifying applications of various events, from hardware state changes (e.g., an SD card was inserted), to incoming data (e.g., an SMS message arrived), to application events (e.g., your activity was launched from the device’s main menu). Not only can you respond to intents, but you can create your own, to launch other activities, or to let you know when specific situations arise (e.g., raise such-and-so intent when the user gets within 100 meters of this-and-such location).
Services
Activities, content providers, and intent receivers are all short-lived and can be shut down at any time. Services, on the other hand, are designed to keep running, if needed, independent of any activity. You might use a service for checking for updates to an RSS feed, or to play back music even if the controlling activity is no longer operating.
Stuff at Your Disposal
Android comes with a number of features to help you develop applications.
Storage
You can package data files with your application, for things that do not change, such as icons or help files. You also can carve out a small bit of space on the device itself, for databases or files containing user-entered or retrieved data needed by your application. If the user supplies bulk storage, like an SD card, you can read and write files on there as needed.
Network
Android devices will generally be Internet-ready, through one communications medium or another. You can take advantage of the Internet access at any level you wish, from raw Java sockets all the way up to a built-in WebKit-based Web browser widget you can embed in your application.
Multimedia
Android devices have the ability to play back and record audio and video. While the specifics may vary from device to device, you can query the device to learn its capabilities and then take advantage of the multimedia capabilities as you see fit, whether that is to play back music, take pictures with the camera, or use the microphone
