Wrap-Up

Answers to Self Review Exercises

10.1	a) switch. b) abstract. c) concrete. d) Polymorphism. e) abstract. f) downcasting.
10.2	a) True. b) False. An abstract class can include methods with implementations and abstract methods. c) False. Trying to invoke a subclass-only method with a superclass variable is dangerous. d) False. Only a concrete subclass must implement the method. e) True.

Exercises

10.3	How does polymorphism enable you to program "in the general" rather than "in the specific"? Discuss the key advantages of programming "in the general."
10.4	A subclass can inherit "interface" or "implementation" from a superclass. How do inheritance hierarchies designed for inheriting interface differ from those designed for inheriting implementation?
10.5	What are abstract methods? Describe the circumstances in which an abstract method would be appropriate.
10.6	How does polymorphism promote extensibility?
10.7	Discuss four ways in which you can assign superclass and subclass references to variables of superclass and subclass types.
10.8	Compare and contrast abstract classes and interfaces. Why would you use an abstract class? Why would you use an interface?
10.9	(Payroll System Modification) Modify the payroll system of Fig. 10.4Fig. 10.9 to include private instance variable birthDate in class Employee. Use class Date of Fig. 8.7 to represent an employee's birthday. Add get methods to class Date and replace method toDateString with method toString. Assume that payroll is processed once per month. Create an array of Employee variables to store references to the various employee objects. In a loop, calculate the payroll for each Employee (polymorphically), and add a $100.00 bonus to the person's payroll amount if the current month is the month in which the Employee's birthday occurs.
10.10	(Shape Hierarchy) Implement the Shape hierarchy shown in Fig. 9.3. Each TwoDimensionalShape should contain method getArea to calculate the area of the two-dimensional shape. Each ThreeDimensionalShape should have methods getArea and getVolume to calculate the surface area and volume, respectively, of the three-dimensional shape. Create a program that uses an array of Shape references to objects of each concrete class in the hierarchy. The program should print a text description of the object to which each array element refers. Also, in the loop that processes all the shapes in the array, determine whether each shape is a TwoDimensionalShape or a ThreeDimensionalShape. If a shape is a TwoDimensionalShape, display its area. If a shape is a ThreeDimensionalShape, display its area and volume.
10.11	(Payroll System Modification) Modify the payroll system of Fig. 10.4Fig. 10.9 to include an additional Employee subclass PieceWorker that represents an employee whose pay is based on the number of pieces of merchandise produced. Class PieceWorker should contain private instance variables wage (to store the employee's wage per piece) and pieces (to store the number of pieces produced). Provide a concrete implementation of method earnings in class PieceWorker that calculates the employee's earnings by multiplying the number of pieces produced by the wage per piece. Create an array of Employee variables to store references to objects of each concrete class in the new Employee hierarchy. For each Employee, display its string representation and earnings.
10.12	(Accounts Payable System Modification) In this exercise, we modify the accounts payable application of Fig. 10.11Fig. 10.15 to include the complete functionality of the payroll application of Fig. 10.4Fig. 10.9. The application should still process two Invoice objects, but now should process one object of each of the four Employee subclasses. If the object currently being processed is a BasePlusCommissionEmployee, the application should increase the BasePlusCommissionEmployee's base salary by 10%. Finally, the application should output the payment amount for each object. Complete the following steps to create the new application: Modify classes HourlyEmployee (Fig. 10.6) and CommissionEmployee (Fig. 10.7) to place them in the Payable hierarchy as subclasses of the version of Employee (Fig. 10.13) that implements Payable. [Hint: Change the name of method earnings to getPaymentAmount in each subclass so that the class satisfies its inherited contract with interface Payable.] Modify class BasePlusCommissionEmployee (Fig. 10.8) such that it extends the version of class CommissionEmployee created in Part a. Modify PayableInterfaceTest (Fig. 10.15) to polymorphically process two Invoices, one SalariedEmployee, one HourlyEmployee, one CommissionEmployee and one BasePlusCommissionEmployee. First output a string representation of each Payable object. Next, if an object is a BasePlusCommissionEmployee, increase its base salary by 10%. Finally, output the payment amount for each Payable object.