Java Reference
In-Depth Information
9.1
Introduction
9.2
Superclasses and Subclasses
9.3
protected
Members
9.4
Relationship Between Superclasses and
Subclasses
9.4.1 Creating and Using a
CommissionEmployee
Class
9.4.2 Creating and Using a
BasePlusCommissionEmployee
Class
9.4.3 Creating a
CommissionEmployee
-
BasePlusCommissionEmployee
Inheritance Hierarchy
9.4.4
CommissionEmployee
-
BasePlusCommissionEmployee
Inheritance Hierarchy Using
protected
Instance Variables
9.4.5
CommissionEmployee
-
BasePlusCommissionEmployee
Inheritance Hierarchy Using
private
Instance Variables
9.5
Constructors in Subclasses
9.6
Class
Object
9.7
(Optional) GUI and Graphics
Case Study: Displaying Text and
Images Using
Label
s
9.8
Wrap-Up
Summary | Self-Review Exercises | Answers to Self-Review Exercises | Exercises
This chapter continues our discussion of object-oriented programming (OOP) by intro-
ducing
inheritance
, in which a new class is created by acquiring an existing class's mem-
bers and possibly embellishing them with new or modified capabilities. With inheritance,
you can save time during program development by basing new classes on existing proven
and debugged high-quality software. This also increases the likelihood that a system will
be implemented and maintained effectively.
When creating a class, rather than declaring completely new members, you can des-
ignate that the new class should
inherit
the members of an existing class. The existing class
is called the
superclass
, and the new class is the
subclass.
(The C++ programming language
refers to the superclass as the
base class
and the subclass as the
derived class
.) A subclass
can become a superclass for future subclasses.
A subclass can add its own fields and methods. Therefore, a subclass is
more specific
than its superclass and represents a more specialized group of objects. The subclass exhibits
the behaviors of its superclass and can modify those behaviors so that they operate appro-
priately for the subclass. This is why inheritance is sometimes referred to as
specialization
.
The
direct superclass
is the superclass from which the subclass explicitly inherits. An
indirect superclass
is any class above the direct superclass in the
class hierarchy
, which
defines the inheritance relationships among classes—as you'll see in Section 9.2, diagrams
help you understand these relationships. In Java, the class hierarchy begins with class
Object
(in package
java.lang
), which
every
class in Java directly or indirectly
extends
(or
“inherits from”). Section 9.6 lists the methods of class
Object
that are inherited by all
other Java classes. Java supports only
single inheritance
, in which each class is derived
from exactly
one
direct superclass. Unlike C++, Java does
not
support multiple inheritance
(which occurs when a class is derived from more than one direct superclass). Chapter 10,
Object-Oriented Programming: Polymorphism and Interfaces, explains how to use Java
interfaces
to realize many of the benefits of multiple inheritance while avoiding the associ-
ated problems.
