Polymorphism - Java Software Solutions: Foundations of Program Design - page 497

overridden, the Volunteer class would have been considered abstract and could
not have been instantiated.
Note that when a volunteer gets “paid” in the payday method of Staff , a
simple expression of thanks is printed. In all other situations, where the pay value
is greater than zero, the payment itself is printed.
The Employee class shown in Listing 10.5 represents an employee that gets paid
at a particular rate each pay period. The pay rate, as well as the employee's Social
Security number, is passed along with the other basic information to the Employee
constructor. The basic information is passed to the constructor of StaffMember
using the super reference.
The toString method of Employee is overridden to concatenate the additional
information that Employee manages to the information returned by the parent's
version of toString , which is called using the super reference. The pay method
of an Employee simply returns the pay rate for that employee.
The Executive class shown in Listing 10.6 represents an employee that may
earn a bonus in addition to his or her normal pay rate. The Executive class
is derived from Employee and therefore inherits from both StaffMember and
Employee . The constructor of Executive passes along its information to the
Employee constructor and sets the executive bonus to zero.
A bonus is awarded to an executive using the awardBonus method. This
method is called in the Staff constructor for the only executive that is part of the
staffList array. Note that the generic StaffMember reference must be cast into
an Executive reference to invoke the awardBonus method (which doesn't exist
for a StaffMember ).
The Executive class overrides the pay method so that it first determines
the payment as it would for any employee, then adds the bonus. The pay method
of the Employee class is invoked using super to obtain the normal payment
amount. This technique is better than using just the payRate variable, because
if we choose to change how Employee objects get paid, the change will auto-
matically be reflected in Executive . After the bonus is awarded, it is reset
to zero.
The Hourly class shown in Listing 10.7 represents an employee whose pay rate
is applied on an hourly basis. It keeps track of the number of hours worked in the
current pay period, which can be modified by calls to the addHours method. This
method is called from the payday method of Staff . The pay method of Hourly
determines the payment based on the number of hours worked and then resets
the hours to zero.