Multiple instances of VisAD class

I am trying to use VisAD to display some data sets.  I have the
display working, but i run into problems when I want to load a
new data set.  Here are the specifics:

I have an application that displays the data, and allows for
some manipulation of the data (which is working).  I would like
to allow the user to load a new data set without restarting the
application.   To do this, I am trying to create a new instance
of the VisAD class in question.

EG:

visad_class_name = new visad_class();
[some code ...]

// Time to create a new instance with the new data:
visad_class_name = null;  // first, i erase the old instance
(since i have static variables)
visad_class_name = new visad_class();

It compiles alright, but i get the runtime exception:
visad.TypeException: ScalarType: name already used

I have seen this exception before when i've tried to create more
than one class with the same scalar-maps (as when multiple
classes are mapping data on latitude and longitude axis).  I
don't quite understand when/why this exception arises.  As a
result, I'm not sure how to solve the problem.

Any information or help would be appreciated.

Thank you for your help and patience,

megan