Hi again.
It looks to me as though you could create a B4A library from the Jbox2d Java library.
Look at the Jbox2d reference:
jbox2d-library 2.1.2.2 API.
For each Jbox2d class that you want to use within B4A you'd need to create a wrapper class in the library.
So click through the various classes under the top left
All Classes frame and look at the ones you'd want to use.
Now look at the methods and properties of the classes you want to use.
Each class you want to use will require that you create a wrapper class to enable you to use it within B4A.
And in that wrapper class you need to create methods and properties that mirror the methods and properties of the original class.
If you understand the basics of how Jbox2d works then you could start off with a library that provides minimal functionality and then update as and when you need to use more classes.
That's not something that i understand though.
Martin.