newbie question about external classes

hi all, I'm new of GWT and I apologize for my newbie question but I
never found a response...
I built a simple GWT application that show some data, the data
actually is hardcoded on my class. Is it possibile, from my gwt
class(EntryPoint), to use external business classes(stored in jar) to
extract and elaborate my data before show it on web page? Compiling I
receive the error "did you forget to inherit a required module?" but I
can't inerit my classes because they are not gwt classes...
Summarizing: is it possible in a GWT project using non GWT classes?
how?

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