2.1. Entity Overview

The entity is not a new thing in data management. In fact, entities have been around longer than many programming languages and certainly longer than Java. They were first introduced by Peter Chen in his seminal paper on entity-relationship modeling.^[]He described entities as things that have attributes and relationships. The expectation was that the attributes and relationships would be persisted in a relational database.

^[] Peter P. Chen, "The entity-relationship model—toward a unified view of data," ACM Transactions on Database Systems 1, no. 1 (1976): 9–36.

Even now, the definition still holds true. An entity is essentially a noun, or a grouping of state associated together as a single unit. It may participate in relationships to any number of other entities in a number of standard ways. In the object-oriented paradigm, we would add behavior to it and call it an object. In JPA, any application-defined object can be an entity, so the important question might be this: What are the characteristics of an object that has been turned into an entity?