Unless someone has written a library for this you will probably have to read in the obj geometry and create boundaries based on what is in the file.
If for example the obj has only triangles then you would write a triangle boundary function and input each triangle from the obj file. You'd probably want to store this in an ArrayList and when you want to test if something is touching the obj geometry you'd test against all the elements in the ArrayList.
This is not a fast implementation, it is just a very general one. It would be a good idea to surround you obj geometry with a primitive (cube, sphere, etc.) and do boundary testing with that. If it comes up true, then test against the obj geometry.
If you have a lot of geometry in the obj file then maybe separate it into different files and contain each one inside a primitive boundary so that it runs at a reasonable speed.
