Defining Classes I - Absolute Java - page 209

EXAMPLE: Yet Another Date Class
Display 4.9 contains another, much improved, definition of a class for a date. Note
that all instance variables are private and that two methods are private. We made the
methods dateOK and monthString private because they are just helping methods
used in the definitions of other methods. A user of the class DateFifthTry would
not (in fact, cannot) use either of the methods dateOK or monthString . This is
all hidden information that need not concern a programmer using the class. The
method monthString was public in previous versions of our date classes because we
had not yet discussed the private modifier. It is now marked private because it is
just a helping method.
Note that the class DateFifthTry uses the method dateOK to make sure that
any changes to instance variables make sense. Because the methods of the class
DateFifthTry use the method dateOK to check for impossible dates, you cannot
use any methods, such as readInput or setDate , to set the instance variables so that
they represent an impossible date such as January 63, 2005. If you try to do so, your
program would end with an error message. (To make our definition of the method
dateOK simple, we did not check for certain impossible dates, such as February 31,
but it would be easy to exclude these dates as well.)
The methods dateOK and equals each return a value of type boolean . That
means they return a value that is either true or false and so can be used as
the Boolean expression in an if-else statement, while statement, or other loop
statement. This is illustrated by the following, which is taken from the definition of
the method setDate in Display 4.9 :
if (dateOK(month, day, year))
{
this .month = monthString(month);
this .day = day;
this .year = year;
}
else
{
System.out.println("Fatal Error");
System.exit(0);
}
Note that, although all the instance variables are private, a programmer using the
class can still change or access the value of an instance variable using the methods that
start with set or get . This is discussed more fully in the next subsection, “Accessor
and Mutator Methods.”
(continued)