On the plus side, Android-style smartphones are sexy. Offering Internet services over mobile devices dates back to the mid-1990s and the Handheld Device Markup Language (HDML). However, only in recent years have phones capable of Internet access taken off. Now, thanks to trends like text messaging and products like Apple's iPhone, phones that can serve as Internet-access devices are rapidly gaining popularity. So, working on Android applications gives you experience with an interesting technology (Android) in a fast-moving market segment (Internet-enabled phones), which is always a good thing.
The problem comes when you actually have to program the darn things. Anyone with experience in programming for PDAs or phones has felt the pain of phones simply being small in all sorts of dimensions, such as the following:
Screens are small (you won't get comments like, 'Is that a 24-inch LCD in your pocket, or...?').
Keyboards, if they exist, are small.
Pointing devices, if they exist, are annoying (as anyone who has lost a stylus will tell you) or inexact (large fingers and 'multitouch' LCDs are not a good mix).
CPU speed and memory are limited compared with what are available on desktops and servers.
You can have any programming language and development framework you want, as long as it was what the device manufacturer chose and burned into the phone's silicon.
People with mobile phones tend to get very irritated when those phones don't work, which is why the 'Can you hear me now?' ad campaign from Verizon Wireless has been popular for the past few years. Similarly, those same people will get angry with you if your program 'breaks' their phone:
By tying up the CPU so that calls can't be received
By not quietly fading into the background when a call comes in or needs to be placed, because it does not work properly with the rest of the phone's operating system
By 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. The tools look different, the frameworks behave differently, and there are more limitations on what you can do with your programs.
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), with support for tools you may be used to using (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 might expect, much of this book deals with that framework and how to 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 Java Database Connectivity (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. Here are the main components used in an Android application:
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 box 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.
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 is accessed.
Services: Activities and content providers are 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.
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).
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 bit of space on the device itself, for databases or files containing user-entered or retrieved data needed by your application. And, if the user supplies bulk storage, like an SD card, you can read and write files 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 for audio note-taking.
Global positioning system (GPS): Android devices will frequently have access to location providers, such as a GPS, which can tell your applications where the device is on the face of the Earth. In turn, you can display maps or otherwise take advantage of the location data, such as tracking a device's movements if the device has been stolen.
Phone services: Of course, since Android devices are typically phones, your software can initiate calls, send and receive Short Message Service (SMS) messages, and everything else you expect from a modern bit of telephony technology.