Inheritance - Absolute Java - page 432

Display 7.3 contains the definition of a class for hourly employees. An hourly
employee is an employee, so we define the class HourlyEmployee to be a derived
class of the class Employee . A derived 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 base class . In our example, Employee is the base class and
HourlyEmployee is the derived class. As you can see in Display 7.3, the way we
indicate that HourlyEmployee is a derived class of Employee is by including the phrase
extends Employee on the first line of the class definition, like so:

derived class

base class

extends public class HourlyEmployee extends Employee

A derived class is also called a subclass , in which case the base class is usually called 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 HourlyEmployee has all the instance variables
and all the methods of the class Employee , but you do not mention them in the
definition of HourlyEmployee . Every object of the class HourlyEmployee has instance
variables called name and hireDate , but you do not specify the instance variable name or
the instance variable hireDate in the definition of the class HourlyEmployee . The class
HourlyEmployee (or any other derived class) is said to inherit the instance variables and
methods of the base class that it extends. For this reason, the topic of derived classes is
called inheritance .
Just as it inherits the instance variables of the class Employee , the class
HourlyEmployee inherits all the methods from the class Employee . So, the class
HourlyEmployee inherits the methods getName ,

subclass and
superclass

inheritance

getHireDate , setName , and

setHireDate , from the class Employee .
For example, suppose you create a new object of the class HourlyEmployee as
follows:

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.

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Inheritance
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