Class getObjectIdClass(
  Class cls
)

Return the Class that implements the JDO Identity for the specified PersistenceCapable class. The application can use the returned Class to construct a JDO Identity instance for application identity PersistenceCapable classes. This JDO Identity instance can then be used to get an instance of the PersistenceCapable class for use in the application.

In order for the application to construct an instance of the ObjectId class it needs to know the class being used by the JDO implementation.