...
The Entity Manager is a required guideline, (see G125 of the guidelines).
Implementing an Entity
As stated above, the implementation of an entity consists of two Java classes: the entity implementation and the interface. The interface contains the public methods that may be invoked by any external class, even Java classes from another plugin. The interface must extend the Identifiable interface, which holds getters and setters for the WmId. The WmId is the unique identifier for the entity inside the Entity Manager’s registry.
...