Android and Java

In Java, you write your Java source file, compile it into a Java byte code using the Java compiler, and then run this byte code on the Java VM. 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 the Dalvik compiler to Dalvik byte code. It is this Dalvik byte code that is then executed on the Dalvik VM. Figure 2-2 illustrates this comparison between standard Java (on the left)

in Android using Dalvik (on the right).

It might sound like you have to do a lot more work with Android when it comes to Java. However, all these compilation steps are automated by tools such as Eclipse or Ant, and you never notice the additional steps.

You may wonder, why not compile straight from Java into the Dalvik byte code? There are a couple of good reasons for the extra steps. 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 the appropriate libraries that are part of the SDK would need to be available. But it is likely that the open source community will come up with a solution to that in the future.

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

Java Standard Edition     Used for development on basic desktop-type applications Java Enterprise Edition (aka J2EE or JavaEE)     Used for development of enterprise applications Java Micro Edition (aka J2ME or JavaME)     Java for mobile applications

Android’s Java set of libraries is closest to Java Standard Edition. The major difference is that Java user interface libraries (AWT and Swing) have been taken out and replaced with Android-specific user interface libraries. Android also adds quite a few new fea- tures 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.