Java™ How To Program (Early Objects), Tenth Edition

10.9.1 Developing a Payable Hierarchy

To build an application that can determine payments for employees and invoices alike, we first create interface Payable, which contains method getPaymentAmount that returns a double amount that must be paid for an object of any class that implements the interface. Method getPaymentAmount is a general-purpose version of method earnings of the Employee hierarchy—method earnings calculates a payment amount specifically for an Employee, while getPaymentAmount can be applied to a broad range of possibly unrelated objects. After declaring interface Payable, we introduce class Invoice, which implements interface Payable. We then modify class Employee such that it also implements interface Payable. Finally, we update ...