Metadata

One question that is often raised by beginning modelers is why we don't use an inheritance relationship to relate the Course and Section classes, rather than using a simple association as we've chosen to do. On the surface, it does indeed seem tempting to want Section to be a derived class of Course, because all of the attributes listed for a Course—courseNo, courseName, and credits—also pertain to a Section; so, why wouldn't we want Section to inherit these, in the same way that Student and Professor inherit all of the attributes of Person? A simple example should quickly illustrate why inheritance isn't appropriate.

Let's say that, because Beginning Object Concepts is such a popular course, the university is offering three sections of the course for the Spring 2004 semester. So, we instantiate one Course object and three Section objects. If Section were a derived class of Course, then all four objects would carry courseNo, courseName, and credits attributes. Filling in the attribute values for these four objects, as shown in Table 10-3, we see that there is quite a bit of repetition in the attribute values across these four objects: we've repeated the same courseName, courseNumber, and creditValue attribute values four times! That's because the information contained within a Course object is common to, and hence describes, numerous Section objects.

Table 10-3: Duplication of Data Across Four Object Instances

Attribute Name	Attribute Values for the Course Object
courseName	Beginning Object Concepts
courseNumber	OBJECTS 101	?/td>	?/td>
creditValue	3	?/td>	?/td>
Attribute Name	Attr. Values for Section Object #1	Attr. Values for Section Object #2	Attr. Values for Section Object #3
courseName	Beginning Object Concepts	Beginning Object Concepts	Beginning Object Concepts
courseNumber	OBJECTS 101	OBJECTS 101	OBJECTS 101
creditValue	3	3	3
students Registered	(To be determined)	(To be determined)	(To be determined)
instructor	Reference to professor X	Reference to professor Y	Reference to professor Z
semester Offered	Spring 2004	Spring 2004	Spring 2004
dayOfWeek	Monday	Tuesday	Thursday
timeOfDay	7:00 p.m.	4:00 p.m.	6:00 p.m.
classroom	Hall A, Room 123	Hall B, Room 234	Hall A, Room 345

To reduce redundancy and to promote encapsulation, we should eliminate inheritance of these attributes, and instead create only one instance of a Course object for n instances of its related Section objects. We can then have each Section object maintain a handle on the common Course object so as to retrieve these shared values whenever necessary. This is precisely what we've modeled via the one-to-many offered as association.

Whenever an instance of some class A encapsulates information that describes numerous instances of some other class B (such as Course does for Section), we refer to the information contained by the A object (Course) as metadata relative to the B objects (Sections).