Inheritance - Java - page 418

Display 7.3 contains the definition of a class for hourly employees. An hourly
employee is an employee, so we define the class to be a derived class of

HourlyEmployee

the class is a class defined by adding instance variables and

methods to an existing class. The existing class that the derived class is built upon is
called the

. A derived class

derived class

Employee

base class . In our example, is the base class and is

base class

Employee HourlyEmployee

the derived class. As you can see in Display 7.3, the way we indicate that

Hourly-

is a derived class of is by including the phrase Employee Employee extends Employee

on the first line of the class definition, like so:

extends public class HourlyEmployee extends Employee

A derived class is also called a subclass , in which case the base class is usually called

subclass and
superclass a superclass . However, we prefer to use the terms derived class and base class.

When you define a derived class, you give only the added instance variables and the
added methods. For example, the class

has all the instance variables

HourlyEmployee

and all the methods of the class , but you do not mention them in the defini- Employee

tion of . Every object of the class has instance vari-

HourlyEmployee HourlyEmployee

ables called and , but you do not specify the instance variable or name hireDate name

the instance variable in the definition of the class . The class

hireDate HourlyEmployee

the instance variables
and methods of the base class that it extends. For this reason, the topic of derived
classes is called

(or any other derived class) is said to inherit HourlyEmployee

.
Just as it inherits the instance variables of the class

inheritance

inheritance

, the class

Employee Hourly-

inherits all the methods from the class . So, the class Employee Employee Hourly-

inherits the methods , , , and ,

Employee getName getHireDate setName setHireDate

from the class .

For example, suppose you create a new object of the class

Employee

as follows:

HourlyEmployee

HourlyEmployee joe = new HourlyEmployee();

Then, the name of the object joe can be changed using the method setName , which
the class HourlyEmployee inherited from the class Employee . The inherited method
setName is used just like any other method; for example:

joe.setName("Josephine");

A small demonstration of this is given in Display 7.4.
Display 7.5 contains the definition of the class SalariedEmployee , which is also
derived from the class Employee . The class SalariedEmployee inherits all the instance
variables and methods of the class Employee . Inheritance allows you to reuse code,
such as the code in the class Employee , without needing to literally copy the code into
the definitions of the derived classes, such as HourlyEmployee and SalariedEmployee.