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1、AndroidAndroid, as a system, is a Java-based operating system that runs on the Linux 2.6 kernel. The system is very lightweight and full featured. Android applications are developed using Java and can be ported rather easily to the new platform. If you have not yet downloaded Java or are unsure abou
2、t which version you need, I detail the installation of the development environment in Chapter 2. Other features of Android include an accelerated 3-D graphics engine (based on hardware support), database support powered by SQLite, and an integrated web browser. If you are familiar with Java programm
3、ing or are an OOP developer of any sort, you are likely used to programmatic user interface (UI) developmentthat is, UI placement which is handled directly within the program code. Android, while recognizing and allowing for programmatic UI development, also supports the newer, XML-based UI layout.
4、XML UI layout is a fairly new concept to the average desktop developer. I will cover both the XML UI layout and the programmatic UI development in the supporting chapters of this book.One of the more exciting and compelling features of Android is that, because of its architecture, third-party applic
5、ationsincluding those that are “home grown”are executed with the same system priority as those that are bundled with the core system. This is a major departure from most systems, which give embedded system apps a greater execution priority than the thread priority available to apps created by third-
6、party developers. Also, each application is executed within its own thread using a very lightweight virtual machine.Aside from the very generous SDK and the well-formed libraries that are available to us to develop with, the most exciting feature for Android developers is that we now have access to
7、anything the operating system has access to. In other words, if you want to create an application that dials the phone, you have access to the phones dialer; if you want to create an application that utilizes the phones internal GPS (if equipped), you have access to it. The potential for developers
8、to create dynamic and intriguing applications is now wide open.On top of all the features that are available from the Android side of the equation, Google has thrown in some very tantalizing features of its own. Developers of Android applications will be able to tie their applications into existing
9、Google offerings such as Google Maps and the omnipresent Google Search. Suppose you want to write an application that pulls up a Google map of where an incoming call is emanating from, or you want to be able to store common search results with your contacts; the doors of possibility have been flung
10、wide open with Android.Chapter 2 begins your journey to Android development. You will learn the hows and whys of using specific development environments or integrated development environments (IDE), and you will download and install the Java IDE Eclipse.Application ComponentsA central feature of And
11、roid is that one application can make use of elements of other applications (provided those applications permit it). For example, if your application needs to display a scrolling list of images and another application has developed a suitable scroller and made it available to others, you can call up
12、on that scroller to do the work, rather than develop your own. Your application doesnt incorporate the code of the other application or link to it. Rather, it simply starts up that piece of the other application when the need arises.For this to work, the system must be able to start an application p
13、rocess when any part of it is needed, and instantiate the Java objects for that part. Therefore, unlike applications on most other systems, Android applications dont have a single entry point for everything in the application (no main() function, for example). Rather, they have essential components
14、that the system can instantiate and run as needed. There are four types of components:ActivitiesAn activity presents a visual user interface for one focused endeavor the user can undertake. For example, an activity might present a list of menu items users can choose from or it might display photogra
15、phs along with their captions. A text messaging application might have one activity that shows a list of contacts to send messages to, a second activity to write the message to the chosen contact, and other activities to review old messages or change settings. Though they work together to form a coh
16、esive user interface, each activity is independent of the others. Each one is implemented as a subclass of the Activity base class.An application might consist of just one activity or, like the text messaging application just mentioned, it may contain several. What the activities are, and how many there are depends, of course, on the application and its design. Typically, one of the activities is marked as the first one that should be presented to the user when t
