INHERITANCE AND POLYMORPHISM - Introduction to Java Programming - page 437

28 @Override
29

public String toString()

{

30 return "stack: " + list.toString();

31 }
32 }

An array list is created to store the elements in the stack (line 4). The isEmpty() method
(lines 6-8) returns list.isEmpty() . The getSize() method (lines 10-12) returns
list.size() . The peek() method (lines 14-16) retrieves the element at the top of the stack
without removing it. The end of the list is the top of the stack. The pop() method (lines
18-22) removes the top element from the stack and returns it. The push(Object element)
method (lines 24-26) adds the specified element to the stack. The toString() method (lines
28-31) defined in the Object class is overridden to display the contents of the stack by
invoking list.toString() . The toString() method implemented in ArrayList returns
a string representation of all the elements in an array list.

Design Guide
In Listing 11.10, MyStack contains ArrayList . The relationship between MyStack
and ArrayList is composition . While inheritance models an is-a relationship, compo-
sition models a has-a relationship. You could also implement MyStack as a subclass of
ArrayList (see Programming Exercise 11.4). Using composition is better, however,
because it enables you to define a completely new stack class without inheriting the
unnecessary and inappropriate methods from ArrayList .

composition
is-a
has-a

11.13 The protected Data and Methods
A protected member of a class can be accessed from a subclass.

Key
Point

So far you have used the private and public keywords to specify whether data fields and
methods can be accessed from outside of the class. Private members can be accessed only
from inside of the class, and public members can be accessed from any other classes.
Often it is desirable to allow subclasses to access data fields or methods defined in the
superclass, but not to allow nonsubclasses to access these data fields and methods. To accom-
plish this, you can use the protected keyword. This way you can access protected data
fields or methods in a superclass from its subclasses.
The modifiers private , protected , and public are known as visibility or accessibility
modifiers because they specify how classes and class members are accessed. The visibility of
these modifiers increases in this order:

why protected ?

Visibility increases

private, default (no modifier), protected, public

Table 11.2 summarizes the accessibility of the members in a class. Figure 11.5 illustrates how
a public, protected, default, and private datum or method in class C1 can be accessed from a
class C2 in the same package, from a subclass C3 in the same package, from a subclass C4 in
a different package, and from a class C5 in a different package.
Use the private modifier to hide the members of the class completely so that they cannot
be accessed directly from outside the class. Use no modifiers (the default) in order to allow
the members of the class to be accessed directly from any class within the same package but
not from other packages. Use the protected modifier to enable the members of the class to
be accessed by the subclasses in any package or classes in the same package. Use the public
modifier to enable the members of the class to be accessed by any class.
Your class can be used in two ways: (1) for creating instances of the class and (2) for defin-
ing subclasses by extending the class. Make the members private if they are not intended