Java Data Objects

Defining a Persistent Object Model

Figure 1-1 is a Unified Modeling Language (UML) diagram of the classes and interrelationships in the Media Mania object model. A Movie instance represents a particular movie. Each actor who has played a role in at least one movie is represented by an instance of Actor. The Role class represents the specific roles an actor has played in a movie and thus represents a relationship between Movie and Actor that includes an attribute (the name of the role). Each movie has one or more roles. An actor may have played a role in more than one movie or may have played multiple roles in a single movie.

Figure 1-1. UML diagram of the Media Mania object model

We will place these persistent classes and the application programs used to manage their instances in the Java com.mediamania.prototype package.

The Classes to Persist

We will make the Movie, Actor, and Role classes persistent, so their instances can be stored in a datastore. First we will examine the complete source code for each of these classes. An import statement is included for each class, so it is clear which package contains each class used in the example.

Example 1-1 provides the source code for the Movie class. JDO is defined in the javax.jdo package. Notice that the class does not require you to import any JDO-specific classes. Java references and collections defined in the java.util package are used ...