DATABASE MANAGEMENT APPROACHES - Concepts of Database Management - page 305

a superclass of the first class. The relationship between a superclass and a subclass is called a generalization ,
which is shown in Figure 9-26. This class diagram represents the relationship between the class of students
and the subclass of students who live in dorms.

Student

Super Superclass

#StudentNum : char(9)
#LastName : char(15)

#FirstName : char(15)
#Status : char(1)

+AddStudent( )
+DeleteStudent( )
+ChangeStudent( )

305

Generalization

Constraint

StudentDorm
#DormNum : char(2)
+AddDorm( )
+RemoveDorm( )

Subclass

(Only students living in dorms
have StudentDorm record)

FIGURE 9-26 Class diagram with a generalization and a constraint

Rules for OODBMSs
Just as rules specify desired characteristics for DDBMSs and OLAP, OODBMSs also have a set of rules. These
rules serve as a benchmark against which you can measure object-oriented systems. The rules are as follows:
1. Complex objects. An OODBMS must support the creation of complex objects from simple objects
such as integers and characters.
2. Object identity. An OODBMS must provide a way to identify objects; that is, the OODBMS must
provide a way to distinguish between one object and another.
3. Encapsulation. An OODBMS must encapsulate data and associated methods together in the
database.
4. Information hiding. An OODBMS must hide from the users of the database the details concern-
ing the way data is stored and the actual implementation of the methods.
5. Types or classes. You are already familiar with the idea of a class. Types are very similar to classes
and correspond to abstract types in programming languages. The differences between the two are
subtle and will not be explored here. It is important to know, however, that an OODBMS sup-
ports either abstract types or classes (it doesn't matter which).
6. Inheritance. An OODBMS must support inheritance.
7. Late binding. In this case, binding refers to the association of operations to actual program code.
With late binding, this association does not happen until runtime, that is, until some user actu-
ally invokes the operation. Late binding lets you use the same name for different operations, which
is called polymorphism in object-oriented systems. For example, an operation to display an
object on the screen requires different program code when the object is a picture than when it is
text. With late binding, you can use the same name for both operations. At the time a user invokes
this “display” operation, the system determines the object being displayed and then binds the
operation to the appropriate program code.
8. Computational completeness. You can use functions in the language of the OODBMS to per-
form various computations.
9. Extensibility. Any DBMS, object-oriented or not, comes with a set of predefined data types, such
as numeric and character. An OODBMS should be extensible , meaning that it is possible to define
new data types. Furthermore, the OODBMS should make no distinction between the data types
provided by the system and the new data types.