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G. Object-Oriented Programming: Inherita... > G.8. Demonstrating Polymorphic Behav...

G.8. Demonstrating Polymorphic Behavior

Section G.4 created a class hierarchy, in which class BasePlusCommissionEmployee inherited from CommissionEmployee. The examples in that section manipulated CommissionEmployee and BasePlusCommissionEmployee objects by using references to them to invoke their methods—we aimed superclass variables at superclass objects and subclass variables at subclass objects. These assignments are natural and straightforward—superclass variables are intended to refer to superclass objects, and subclass variables are intended to refer to subclass objects. However, as you’ll soon see, other assignments are possible.

In the next example, we aim a superclass reference at a subclass object. We then show how invoking a method on a subclass object via a superclass reference invokes the subclass functionality—the type of the referenced object, not the type of the variable, determines which method is called. This example demonstrates that an object of a subclass can be treated as an object of its superclass, enabling various interesting manipulations. A program can create an array of superclass variables that refer to objects of many subclass types. This is allowed because each subclass object is an object of its superclass. For instance, we can assign the reference of a BasePlusCommissionEmployee object to a superclass CommissionEmployee variable, because a BasePlusCommissionEmployee is a CommissionEmployee—we can treat a BasePlusCommissionEmployee as a CommissionEmployee.


  

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