Ketai
Although it helps if you understand how an application interacts with the underlying mobile platform, a lot of time and effort can be saved by using libraries. The Ketai library simplifies many tasks related to the creation of UI features, access to sensor values and wireless communications. It also has good documentation and comes with many worked examples.
The task in this section is to familiarise yourself with the Ketai library by modifying the game application from the last section to use Ketai instead of basic Processing commands. The first link in the resources box will take you to the Ketai documentation where you will be able to find the relevant explanations and examples. The second link leads to an online book on rapid application development for Android. As well as a basic introduction to Android, Processing and sensors, it also has many examples which make use of Ketai. Some of the more advanced topics such as networking, database interaction and 3D graphics are not relevant at this stage but may be of use in other situations.
Ketai.org