The version we used during most of the book was 'L', but it will all work with 'M' which was released at around the same time as the book. We concentrate on the core concepts, so the truth is the contents should still be valid in 2-3 years.
The future of Java on Android is an interesting question. There's two things:
- Java as a language
- Java as a platform
Both are likely to continue for a very long time. But Java as a platform (i.e. other languages compiling to Java bytecodes, and then being converted to Dalvik executables) will--we think--be an increasingly interesting area in the next 5 years. Regardless of the language you use (and with Android you can use almost any) a thorough understanding of the core concepts is vitally important for creating great apps.
Thanks for the question.
Dawn and David
Head First Android Development