How do you diff a directory for only files of a specific type?

Question

I have a question about the diff command if I want a recursive directory diff but only for a specific file type, how to do that?

I tried using the exclude option but can

Answer 1

The lack of a complementary --include makes it necessary to use such convoluted heuristic patterns as

*.[A-Zb-ik-uw-z]*

to find (mostly) java files!