Java How to Program (early objects), 9/e

10.3 Demonstrating Polymorphic Behavior

Section 9.4 created a class hierarchy, in which class BasePlusCommissionEmployee inherited from CommissionEmployee. The examples in that section manipulated CommissionEmployee and BasePlusCommission-Employee 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—super-class 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 ...