I took some methods (not all) from free open source desktop application under GPL. This application written in C++ language, I convert those methods from C++ into Java code and used them in my android application.
So, Should I open source my android application?
Agreed with what has been answered so far… you have to…
Now, if for some reason, you would prefer to not release the source code, include the copyright notice and be obligated under GPL, you would need to rewrite those functions from scratch as a “clean room implementation” (implement only from the specifications / description, without taking any “inspiration” from the original work). If you can do that and somehow prove in case there is a dispute that you did actually do it, then you are good to go.