Subclasses - Multimedia Introduction to Programming Using Java - page 141

Java syntax: Subclass definition
public class subclass-name extends superclass-name {
declarations of methods and fields

}
Purpose : To define a new file drawer, named subclass-name , and
describe the contents of its manila folders (instances of the class). They
have the methods and fields that are declared in superclass superclass-
name as well as the methods and fields being declared in the subclass.

Actually, the subclass is not new to you. It was discussed in some detail in
Sec.1.4, and you have already learned that the subclass is a mechanism for cus-
tomizing a class to fit new needs. An instance of the subclass has all the compo-
nents (instance variables and instance methods) that the superclass does, but it
can define new ones. In Sec. 1.4, we also showed how to draw an instance of a
subclass (see Fig. 1.9). In this chapter, we review the concepts discussed in
Sec.1.4 and go into more detail on some of the issues concerning subclasses.

4.1 The subclass definition

Figure 4.1 contains a definition of a class Executive . Because of the extends
clause

Get class Em-
ployee and its
subclasses
from a footnote
on lesson page
4-1 of the CD.

extends Employee

in the first line, class Executive is called a subclass of class Employee and class
Employee is called a superclass of Executive . The presence of this clause
means that every instance component of class Employee is also an instance com-
ponent of class Executive . We say that Executive inherits the instance vari-
ables and methods of Employee . But subclass Executive can have additional
instance components.
Figure 4.1 contains uses of keyword super in the constructor and functions
toString and getCompensation . Keyword super is explained in subsections
4.1.1 and 4.1.3.
Figure 4.2 shows how we draw an instance of subclass Executive . The
folder has two partitions. At the top is the partition for components that are inher-
ited from class Employee ; the name Employee is in a box in the upper righthand
corner of the partition. At the bottom is the partition for components that are
defined in the subclass, Executive ; the name of the subclass is in a box in the
upper righthand corner of the partition.
We draw every instance of a subclass in this fashion. Having a standard way
to draw such manila folders makes it easier to communicate.
Subclass Executive contains a private field, bonus . Since only Execu-
tives , and not other Employees , get bonuses, this field is placed in the subclass.
Superclass Employee contains only fields that represent properties that all
employees have.

Activities 4-1.2
and 4-1.3 dis-
cuss subclasses
with a slight
variation of
Employee and
its subclasses.