Tip of the Week: Modeling with Interfaces

Eitan Suez — Wed, 06 Sep 2006 22:44:00 -0500

We are taught that interface-based modeling is preferred over models that do not employ interfaces. I concur. In this short tip, I'd like to demonstrate how to do interface-based modeling in JMatter.

As an illustration, let's take the original Sympster application and extend it. Sympster is a conference manager demo application. Up until now, one could use this application to define a symposium, speakers, their talks, and to schedule sessions. A session is defined as a talk given at a specific date and time, and with a specific duration, at a specific location.

I'd like to broaden the definition of a session to include events other than speakers' presentations. What about BOF's (Birds of a Feature session) for example? Or maybe a panel discussion including a number of participants. At NFJS symposia, we actually hold informal BOFs with 2-3 participants where we discuss a particular topic.

Accompanying this blog entry are the code changes committed to Sympster in subversion revision #84, which essentially implements this enhancement. Let's walk through the code.

The first thing I do is define the interface Event:

  public interface Event extends ComplexEObject
  {
  }

It's not much of an interface but it turns out that ComplexEObject already includes the contract that I will need: the requirement that every event have a title:

  public Title title();

I still want to define this interface as it serves as a mechanism for qualifying event types.

Next, let's go ahead and define a new object to model a BOF:

  public class BOF
        extends AbstractComplexEObject implements Event
  {
     private final StringEO title = new StringEO();

     private final RelationalList participants = new RelationalList(Speaker.class);
     public static Class participantsType = Speaker.class;

     public static String[] fieldOrder = {"title", "participants"};

     public BOF() { }

     public StringEO getTitle() { return title; }
     public RelationalList getParticipants() { return participants; }

     public Title title()
     {
        return title.title().append(" with", participants);
     }
  }

Finally, let's retrofit our existing class Talk:

  eitan@ubuntu:~/build-area/jmatter-ba/demo-apps/Sympster/src/com/u2d/sympster$ svn diff -r83:84 Talk.java
  Index: Talk.java
  ===================================================================
  --- Talk.java   (revision 83)
  +++ Talk.java   (revision 84)
  @@ -7,7 +7,7 @@
   import com.u2d.reflection.FieldAt;
   import java.awt.Color;

  -public class Talk extends AbstractComplexEObject
  +public class Talk extends AbstractComplexEObject implements Event
   {
      private final StringEO title = new StringEO();
      private final TextEO talkAbstract = new TextEO();

As you can see from the above diff, all I had to do was specify that Talk implement Event.

Finally, remaining is broadening Session's definition. Rather than define an association to a Talk, we'll revise that to be be an association to an Event. I won't show you all the edits. Here's the gist of the change:

  -   public Talk getTalk() { return talk; }
  -   public void setTalk(Talk talk)
  +   public Event getEvent() { return event; }
  +   public void setEvent(Event event)

And that's it. Now let's take a look at how these changes manifest themselves in our application:

Polymorphic Queries: Listing Events

Discussion:

JMatter models the polymorphic association using Hibernate's implicit polymorphism mechanism, otherwise known as any associations.
Polymorphic Instantiation: Creating a new Event

Discussion:

JMatter will prompt the user whether they wish to create a talk or a BOF.
The calendar:

Discussion:

We see two events on this calendar. The first is a talk on JMatter, and the second is a BOF on Programming. :-)

JMatter Blog: Tip of the Week: Modeling with Interfaces

Tip of the Week: Modeling with Interfaces