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Chapter 13 Building Information Systems 535

An object combines data and the specific processes that operate on those data.
Data encapsulated in an object can be accessed and modified only by the
operations, or methods, associated with that object. Instead of passing data to
procedures, programs send a message for an object to perform an operation that
is already embedded in it. The system is modeled as a collection of objects and
the relationships among them. Because processing logic resides within objects
rather than in separate software programs, objects must collaborate with each
other to make the system work.
Object-oriented modeling is based on the concepts of class and inheritance.
Objects belonging to a certain class, or general categories of similar objects,
have the features of that class. Classes of objects in turn can inherit all the
structure and behaviors of a more general class and then add variables and
behaviors unique to each object. New classes of objects are created by choosing
an existing class and specifying how the new class differs from the existing
class, instead of starting from scratch each time.
We can see how class and inheritance work in Figure 13.8, which illustrates
the relationships among classes concerning employees and how they are paid.
Employee is the common ancestor, or superclass, for the other three classes.
Salaried, Hourly, and Temporary are subclasses of Employee. The class name is
in the top compartment, the attributes for each class are in the middle portion
of each box, and the list of operations is in the bottom portion of each box.
The features that are shared by all employees (id, name, address, date hired,
position, and pay) are stored in the Employee superclass, whereas each subclass
stores features that are specific to that particular type of employee. Specific to
hourly employees, for example, are their hourly rates and overtime rates. A
solid line from the subclass to the superclass is a generalization path showing
that the subclasses Salaried, Hourly, and Temporary have common features
that can be generalized into the superclass Employee.
Object-oriented development is more iterative and incremental than
traditional structured development. During analysis, systems builders document

FIGURE 13.8 CLASS AND INHERITANCE

This figure illustrates how classes inherit the common features of their superclass.

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