Check out the about avast, from the tray icon, it gives acknowledgements of some elements used.
I don't see JAVA in that list, but I don't believe that lists the language the program is actually written in.
Given that JAVA is a massive target for malware I would doubt that it is the case. Plus one of the avast team has responded (lukor) in the link given by Eddy "mosty C/C++". Mind you that is a very old topic.
Many people don't have JAVA installed as it is much depreciated and because it is targeted by malware.