Inheritance - Java - page 415

Derived Classes

Suppose we are designing a record-keeping program that has records for salaried
employees and hourly employees. There is a natural hierarchy for grouping these
classes. These are all classes of people who share the property of being employees.
Employees who are paid an hourly wage are one subset of employees. Another sub-
set consists of salaried employees who are paid a fixed wage each month. Although the
program may not need any type corresponding to the set of all employees, thinking in
terms of the more general concept of employees can be useful. For example, all
employees have a name and a hire date (when they started working for the company),
and the methods for setting and changing names and hire dates will be the same for
salaried and hourly employees. The classes for hourly employees and salaried employ-
ees may be further subdivided as diagrammed in Display 7.1.
Within Java you can define a class called that includes all employees,

whether salaried or hourly, and then use this class to define classes for hourly employ-
ees and salaried employees. You can then, in turn, use classes like

Employee

to

HourlyEmployee

define classes like , and so forth.

Display 7.2 shows our definition for the class

PartTimeHourlyEmployee

is a
pretty ordinary class much like earlier classes we have seen. What will be interesting
about the class

. The class

Employee Employee

is how we use it to create a class for hourly employees and a
class for salaried employees. It is legal to create an object of the class

Employee

, but our

Employee

reason for defining the class is so that we can define derived classes for differ-

Employee

ent kinds of employees.

Display 7.1 A Class Hierarchy

Employee

HourlyEmployee SalariedEmployee

FullTimeHourlyEmployee PartTimeEmployee TechnicalStaff Executive

Engineer Technician ClericalStaff