Extending Rapidminer: Where can I find information on class hierarchy, methods and so on?

In the tutorial for creating a new extension there isn't a lot of information about which classes we need to import or which methods or variables are in each class. For example in this page, the information about which classes to import is present only in the "The complete operator class now looks like this." tab. However in the following steps that information is missing with no indication of where it is available. 

I found this page with some information, but it seems to be regarding rapidminer 5.2.

Maybe this is obvious to someone else, but I can't seem to figure it out.

Thanks in advance,
Ana