Abstract Entities

[Christopher Gardner]

In the Clean Architecture,  UB talks about abstract entities that the database layer implements.  I found one abstract entity implementation in this overall case study:

http://cleancodejava.com/uncle-bob-payroll-case-study-full-implementation/

The author put a lot of effort in the whole case study.  The entity implementation looks very involved, but it looks like it works. 

Has anyone seen other approaches to implementing abstract entities?