Hi Joel,
There aren't any enum types in the Java client library for fields in a Report definition at the moment. I'll pass this feedback on to the team. You could also consider creating an new thread on the
library's issue tracker. Meanwhile, you can use
ReportDefinitionService.getReportFields() method to get the available report fields for a given report type. The
GetReportFields Java example shows how to use this service.
Regards,
Shwetha, AdWords API Team.