Here is the how-to and reference guide on using Java actions with Mendix:
https://docs.mendix.com/howto/logic-business-rules/extending-your-application-with-custom-java
https://docs.mendix.com/refguide/java-programming
The examples should help you, and feel free to ask specific questions in a new thread if you get stuck!
Hi,
Adding one more documention which explains in detail about parameters used in java action
https://docs.mendix.com/refguide/java-actions
https://www.youtube.com/watch?v=PZ718BAUPmU