Linux is a portable platform relatively easy to compile on various hardware architectures. What Linux brings to Android is a level of hardware abstractions. By basing Android on Linux, we don’t have to worry too much about underlying hardware features. Most of low level parts of Linux have been written in fairly portable C code, which allows for third-parties to port Android to variety of devices.

Linux is a highly secure system, having been tried and tested through some very harsh environments over the decades. Android heavily relies on Linux for security. All Android applications run as separate linux processes with permissions set by the Linux system. As such, Android passes a lot of security concerns to the underlying Linux system.

Linux comes with a lot of very useful features. Android leverages many of them, such as support for memory management, power management, and networking.

Figure 2.2. Java versus Dalvik

In Java, you write your Java source file, you compile it into a Java byte code using Java compiler and you run this byte code on the Java Virtual Machine. In Android things are different. You still write the Java source file and you still compile it to Java byte code using the same Java compiler. But at that point, you recompile it once again using Dalvik compiler to Dalvik byte code. It is Dalvik byte code that is then executed on Dalvik virtual machine.

You may wonder why not compile straight from Java into the Dalvik byte code. There are couple of good reasons for that. Back in 2005 when work on Dalvik started, the Java language was going through frequent changes. But the Java byte code was more or less set in stone. So, the Android team chose to base Dalvik on Java byte code instead of Java source code.

A side effect of this is that in theory you could write Android applications in any other language that compiles down to Java byte code. For example, you could use Python or Ruby. I say in theory because in practice you’d still need appropriate libraries that are part of the SDK to be available. But it is likely that the open source community will come up with solution to that in the future.

Another thing to keep in mind is that Android Java is a non-standard collection of Java classes. Java typically ships in:

Android’s Java set of libraries is closest to Java Standard Edition. The major difference is that Java libraries for user interface (AWT and Swing) have been taken out and replaced with Android-specific user interface libraries. Android also adds quite a few new features to standard Java while supporting most of Java’s standard features. So, you have most of your favorite Java libraries at your disposal, plus many new ones.

An application is a single APK (application package) file. An APK file roughly has three main components. It consists of all your Dalvik executable code (Java code compiled automatically to Dalvik executable) plus all your resources (everything that is not code, such as images, layouts, etc.). Additionally, it may contain some native code as well.

Android applications must be signed before they can be installed on a device. For development purposes, we’ll be signing applications with a debug key - a key that you already have on your development platform. However, when you want to distribute your application commercially, you’ll want to sign it with your own key. The details how to do that are at Signing Your Application page. We’ll also discuss it in Going Live chapter.

One place Android is quite different than other platforms is the distribution of the apps. On most other platforms, such as iPhone, a single vendor has the monopoly over the distribution of applications. On Android, there can be many different Android stores or markets. Each one can have its own set of policies with respect to what is allowed, how the revenue is split, and so on. As such, Android is much more of a free market space with vendors competing for business.

In practice, the biggest market currently is Android Market run by Google. It is unclear if Google’s motivation is to just seed the market space while other stores develop, or they will strive to make it a profitable venture for them.

Applications can also be distributed via the web. When you download an APK file from a website by using the Browser, the application represented by the APK file automatically gets installed on your phone.