Polymorphism - Java Software Solutions: Foundations of Program Design - page 490

the child class overrode the definition it inherited). The following invocation calls
the move method, but the particular version of the method it calls is determined
at run time:

creature.move();

When this line is executed, if creature currently refers to an Animal object, the
move method of the Animal class is invoked. Likewise, if creature currently refers
to a Mammal object, the Mammal version of move is invoked. Likewise if it currently
refers to a Horse object.
Of course, since Animal and Mammal represent general concepts, they may be
defined as abstract classes. This situation does not eliminate the ability to have
polymorphic references. Suppose the move method in the Mammal class is abstract
and is given unique definitions in the Horse , Dog , and Whale classes (all derived
from Mammal ). A Mammal reference variable can be used to refer to any objects cre-
ated from any of the Horse , Dog , and Whale classes, and can be used to execute
the move method on any of them.
Let's look at another situation. Consider the class hierarchy shown in Figure 10.1.
The classes in it represent various types of employees that might be employed at
a particular company. Let's explore an example that uses this hierarchy to pay a
set of employees of various types.
The Firm class shown in Listing 10.1 contains a main driver that creates a
Staff of employees and invokes the payday method to pay them all. The program
output includes information about each employee and how much each is paid (if
anything).
The Staff class shown in Listing 10.2 maintains an array of objects that repre-
sent individual employees of various kinds. Note that the array is declared to hold
StaffMember references, but it is actually filled with objects created from several
other classes, such as Executive and Employee . These classes are all descendants
of the StaffMember class, so the assignments are valid. The staffList array is
filled with polymorphic references.
The payday method of the Staff class scans through the list of employees,
printing their information and invoking their pay methods to determine how
much each employee should be paid. The invocation of the pay method is poly-
morphic, because each class has its own version of the pay method.
The StaffMember class shown in Listing 10.3 is abstract. It does not represent
a particular type of employee and is not intended to be instantiated. Rather,
it serves as the ancestor of all employee classes and contains information that
applies to all employees. Each employee has a name, address, and phone number,
so variables to store these values are declared in the StaffMember class and are
inherited by all descendants.

VideoNote

Exploring the Firm
program.