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304 PART THREE CONVENTIONAL METHODS FOR SOFTWARE ENGINEERING

FIGURE 12.3 Naming Ties one data object to another,
Tabular attributes in this case, owner
representation
of data objects Descriptive Referential
Identifier attributes attributes

Make Model ID# Body type Color Owner
Lexus LS400 AB123. . . Sedan White RSP
Instance Chevy Corvette X456. . . Sports Red CCD
BMW 750iL XZ765. . . Coupe White LJL
Ford Taurus Q12A45. . . Sedan Blue BLF

Attributes. Attributes define the properties of a data object and take on one of three
different characteristics. They can be used to (1) name an instance of the data object,
(2) describe the instance, or (3) make reference to another instance in another table.
Attributes name a data In addition, one or more of the attributes must be defined as an identifier—that is, the
object, describe its identifier attribute becomes a "key" when we want to find an instance of the data
characteristics, and in object. In some cases, values for the identifier(s) are unique, although this is not a
some cases, make
reference to another requirement. Referring to the data object car, a reasonable identifier might be the ID
object. number.
The set of attributes that is appropriate for a given data object is determined through
an understanding of the problem context. The attributes for car might serve well for
an application that would be used by a Department of Motor Vehicles, but these attri-
butes would be useless for an automobile company that needs manufacturing con-
trol software. In the latter case, the attributes for car might also include ID number,
body type and color, but many additional attributes (e.g., interior code, drive train
type, trim package designator, transmission type) would have to be added to make
car a meaningful object in the manufacturing control context.

Relationships. Data objects are connected to one another in different ways. Con-
sider two data objects, book and bookstore. These objects can be represented using
Relationships indicate the simple notation illustrated in Figure 12.4a. A connection is established between
the manner in which book and bookstore because the two objects are related. But what are the rela-
data objects are tionships? To determine the answer, we must understand the role of books and book-
“connected” to one
another. stores within the context of the software to be built. We can define a set of
object/relationship pairs that define the relevant relationships. For example,
• A bookstore orders books.
• A bookstore displays books.
• A bookstore stocks books.
• A bookstore sells books.
• A bookstore returns books.