Drupal is a PHP web framework. The tools that is uses are those of the web. If you are trying to implement something with XML and Java you are not really using Drupal. There are skills that you have to learn in order to create web pages which are different from those on a phone.
Having said that. I believe that there are some Android front ends for Drupal. That may be a good starting point for you.
For certain applications, such as computations and interactive visualization of secure data, java is one of the only viable solutions. I use it to visualize secured datasets over public Internet connections via a Drupal Services based API. The java applet is passed the API end-point at launch, and communications with the applet take place RESTfully over https.
Any dataset visualized in most web technologies can have their data scrapped off the page; but not in a java applet. It is easy to imagine situations where this is useful: banking, market research, product designs, legal contracts and agreements, any secured data, really.
You can use headless Drupal as backend and like Jeremy French said, use Android as front end. Here is a Drupal 8 Android app https://github.com/onedrupal/One-Drupal-Android