For my game I'd like to create another project that extends the functionality of libGDX and since it will be an independent project it could be easily reusable. How can I achieve this in Eclipse IDE? Do I have to use any building tools like Ant or Maven? I'd prefer not using them but I also don't want that the libgdx.jar file to get in the final jar twice. Do I have to create a full libGDX project (with gdx-setup-ui) for the extension project or is it enough to create a simpe Java Project and add the libGDX library?
I found the answer in Aurelien Ribbon's forum: http://www.aurelienribon.com/forum/viewtopic.php?f=7&t=2
This is easy to do:
- Create a project for your library (we'll refer to it as the "mylib" project),
- Add gdx.jar and tween-engine-api.jar to its build path (put these files in its "libs" folder, which you have to create),
- Add your source files to its "src" folder,
- Generate the 4 test projects with the ProjectSetup tool (tick the "Universal tween Engine" library),
- Add your "mylib" project to the build path of the main test project (check its checkbox to export it )
Now you should be able to work on your library in the mylib project, and to directly test it in the desktop/android/html projects.
 it is required for the android test project to work since the Android SDK rev17