A Closer Look at Methods and Classes - Java: A Beginner's Guide - page 248

Let's look closely at this example. Inside FailSoftArray are defined three private mem-
bers. The first is a , which stores a reference to the array that will actually hold information.
The second is errval , which is the value that will be returned when a call to get( ) fails.
The third is the private method indexOK( ) , which determines whether an index is within
bounds. Thus, these three members can be used only by other members of the FailSoftAr-
ray class. Specifically, a and errval can be used only by other methods in the class, and
indexOK( ) can be called only by other members of FailSoftArray . The rest of the class
members are public and can be called by any other code in a program that uses FailSoftAr-
ray .
When a FailSoftArray object is constructed, you must specify the size of the array and
the value that you want to return if a call to get( ) fails. The error value must be a value that
would otherwise not be stored in the array. Once constructed, the actual array referred to
by a and the error value stored in errval cannot be accessed by users of the FailSoftArray
object. Thus, they are not open to misuse. For example, the user cannot try to index a dir-
ectly, possibly exceeding its bounds. Access is available only through the get( ) and put( )
methods.
The indexOK( ) method is private mostly for the sake of illustration. It would be harm-
less to make it public because it does not modify the object. However, since it is used in-
ternally by the FailSoftArray class, it can be private .
Notice that the length instance variable is public . This is in keeping with the way Java
implements arrays. To obtain the length of a FailSoftArray , simply use its length member.
To use a FailSoftArray array, call put( ) to store a value at the specified index. Call get(
) to retrieve a value from a specified index. If the index is out-of-bounds, put( ) returns
false and get( ) returns errval .
For the sake of convenience, the majority of the examples in this topic will continue to
use default access for most members. Remember, however, that in the real world, restrict-
ing access to members—especially instance variables—is an important part of successful
object-oriented programming. As you will see in Chapter 7 , access control is even more
vital when inheritance is involved.
Try This 6-1 Improving the Queue Class
You can use the private modifier to make a rather important improvement to the Queue
class developed in Chapter 5 , Try This 5-2 . In that version, all members of the Queue class