Unfortunately we need to deal with the object relational (O/R) impedance mismatch, and to do so you need to understand two things: the process of mapping objects to relational databases and how to implement those mappings.
Scott W. Amber, Mapping Objects to Relational Databases: O/R Mapping In Detail
This chapter covers
Most Java applications need to store data in persistent storage, and typically this storage is a relational database. There are many approaches to persisting the data, from executing low-level SQL statements directly to more sophisticated ...