Object-Oriented Programming: Inheritance - Java: How to Program Early Objects - page 364

Not every class relationship is an inheritance relationship. In Chapter 8, we discussed
the has-a relationship, in which classes have members that are references to objects of other
classes. Such relationships create classes by composition of existing classes. For example,
given the classes Employee , BirthDate and TelephoneNumber , it's improper to say that an
Employee is a BirthDate or that an Employee is a TelephoneNumber . However, an
Employee has a BirthDate , and an Employee has a TelephoneNumber .
It's possible to treat superclass objects and subclass objects similarly—their common-
alities are expressed in the superclass's members. Objects of all classes that extend a
common superclass can be treated as objects of that superclass—such objects have an is-a
relationship with the superclass. Later in this chapter and in Chapter 10, we consider
many examples that take advantage of the is-a relationship.
A subclass can customize methods that it inherits from its superclass. To do this, the
subclass overrides ( redefines ) the superclass method with an appropriate implementation,
as we'll see in the chapter's code examples.
9.3 protected Members
Chapter 8 discussed access modifiers public and private . A class's public members are
accessible wherever the program has a reference to an object of that class or one of its sub-
classes . A class's private members are accessible only within the class itself. In this section,
we introduce the access modifier protected . Using protected access offers an intermedi-
ate level of access between public and private . A superclass's protected members can be
accessed by members of that superclass, by members of its subclasses and by members of
other classes in the same package — protected members also have package access .
All public and protected superclass members retain their original access modifier
when they become members of the subclass— public members of the superclass become
public members of the subclass, and protected members of the superclass become pro-
tected members of the subclass. A superclass's private members are not accessible outside
the class itself. Rather, they're hidden from its subclasses and can be accessed only through
the public or protected methods inherited from the superclass.
Subclass methods can refer to public and protected members inherited from the
superclass simply by using the member names. When a subclass method overrides an inher-
ited superclass method, the superclass version of the method can be accessed from the sub-
class by preceding the superclass method name with keyword super and a dot ( . )
separator. We discuss accessing overridden members of the superclass in Section 9.4.
Software Engineering Observation 9.1
Methods of a subclass cannot directly access private members of their superclass. A
subclass can change the state of private superclass instance variables only through non-
private methods provided in the superclass and inherited by the subclass.
Software Engineering Observation 9.2
Declaring private instance variables helps you test, debug and correctly modify systems.
If a subclass could access its superclass's private instance variables, classes that inherit
from that subclass could access the instance variables as well. This would propagate access
to what should be private instance variables, and the benefits of information hiding
would be lost.