How to convert java code into 'C' language?

Answer 1

Learn C so that you know the language. Once you learn the language, it'll be easy enough for you to translate the same concepts over to it.

Just note that you may have issues converting certain concepts from Java over to C, which may not natively have the same ability that Java has built in.

Answer 2

I asume that it is not so difficult to make C/C++ inserts in the Java app if performance is highly required in some cases of data processing e.t.c.
Also you could make 2 separate apps in Java and C/C++ and configure websocket connection between them for data processing/exchange.
Or implement some other kind of api.
May be it is possible to export or port your Java app to C/C++ language using some IDE, library or tool, but I think it is not the best way to do it (dependibg on your purpose).