I am not sure what you are looking for, but here are some resources that may help:
https://docs.mendix.com/howto/logic-business-rules/extending-your-application-with-custom-java/
https://docs.mendix.com/howto/logic-business-rules/java-api-tutorial/
https://docs.mendix.com/refguide/java-programming/
https://www.youtube.com/watch?v=mbC0zdk_h6o
Does this get you started?