Right now you cannot export microlfows logic directly to java code, Mendix will hide the default functionalities like for activities which are used in Microflows. You need to achieve this by using SDK, something like that.
Refer Mendix SDK : https://docs.mendix.com/apidocs-mxsdk/mxsdk/
Do you mean you need to call a Mendix Microflow from Java? If you do, I wrote a short article on how to do this that may help you.