这章的题目是目前为止所有章节中最简单的！
| Self-Review Exercises 9.1 Fill in the blanks in each of the following statements: a) Inheritance is a form of software reusability in which new classes acquire the members of existing classes and embellish those classes with new capabilities. b) A superclass’s public and protectd members can be accessed in the superclass declaration and in subclass declarations. c) In a(n) is-a (or inheritance) relationship, an object of a subclass can also be treated as an object of its superclass. d) In a(n) has-a (or composition) relationship, a class object has references to objects of other classes as members. e) In single inheritance, a class exists in a(n) hierachical relationship with its subclasses. f) A superclass’s public members are accessible anywhere that the program has a reference to an object of that superclass or to an object of one of its subclasses. g) When an object of a subclass is instantiated, a superclass constructor is called implicitly or explicitly. h) Subclass constructors can call superclass constructors via the keyword super. 9.2 State whether each of the following is true or false. If a statement is false, explain why. a) Superclass constructors are not inherited by subclasses. (T) b) A has-a relationship is implemented via inheritance. (F) c) A Car class has an is-a relationship with the SteeringWheel and Brakes classes. (F) d) When a subclass redefines a superclass method by using the same signature, the subclass is said to overload that superclass method. (F) Exercises 9.3 (Using Composition Rather Than Inheritance) Many programs written with inheritance could be written with composition instead, and vice versa. Rewrite class BasePlusCommissionEmployee(Fig. 9.11) of the CommissionEmployee–BasePlusCommissionEmployee hierarchy to use composition rather than inheritance. 9.4 (Software Reuse) Discuss the ways in which inheritance promotes software reuse, saves time during program development and helps prevent errors. 9.5 (Student Inheritance Hierarchy) Draw an inheritance hierarchy for students at a university similar to the hierarchy shown in Fig. 9.2. Use Student as the superclass of the hierarchy, then extend Student with classes UndergraduateStudent and GraduateStudent. Continue to extend the hierarchy as deep (i.e., as many levels) as possible. For example, Freshman, Sophomore, Junior and Senior might extend UndergraduateStudent, and DoctoralStudent and MastersStudent might be subclasses of GraduateStudent. After drawing the hierarchy, discuss the relationships that exist between the classes. [Note: You do not need to write any code for this exercise.] 9.6 (Shape Inheritance Hierarchy) The world of shapes is much richer than the shapes included in the inheritance hierarchy of Fig. 9.3. Write down all the shapes you can think of—both two-dimensional and three-dimensional—and form them into a more complete Shape hierarchy with as many levels as possible. Your hierarchy should have class Shape at the top. Classes TwoDimensionalShape and ThreeDimensionalShape should extend Shape. Add additional subclasses, such as Quadrilateral and Sphere, at their correct locations in the hierarchy as necessary. 9.7 (protected vs. private) Some programmers prefer not to use protected access, because they believe it breaks the encapsulation of the superclass. Discuss the relative merits of using protected access vs. using private access in superclasses. 9.8 (Quadrilateral Inheritance Hierarchy) Write an inheritance hierarchy for classes Quadrilateral, Trapezoid, Parallelogram, Rectangle and Square. Use Quadrilateral as the superclass of the hierarchy. Create and use a Point class to represent the points in each shape. Make the hierarchy as deep (i.e., as many levels) as possible. Specify the instance variables and methods for each class. The private instance variables of Quadrilateral should be the x-y coordinate pairs for the four endpoints of the Quadrilateral. Write a program that instantiates objects of your classes and outputs each object’s area (except Quadrilateral). 9.9 (What Does Each Code Snippet Do?) a) Assume that the following method call is located in an overridden earnings method in a subclass: super.earnings() b) Assume that the following line of code appears before a method declaration: @Override c) Assume that the following line of code appears as the first statement in a constructor’s body: super(firstArgument, secondArgument); 9.10 (Write a Line of Code) Write a line of code that performs each of the following tasks: a) Specify that class PieceWorker inherits from class Employee. b) Call superclass Employee’s toString method from subclass PieceWorker’s toString method. c) Call superclass Employee’s constructor from subclass PieceWorker’s constructor—assume that the superclass constructor receives three Strings representing the first name, last name and social security number. 9.11 (Using super in a Constructor’s Body) Explain why you would use super in the first statement of a subclass constructor’s body. 9.12 (Using super in an Instance Method’s Body) Explain why you would use super in the body of a subclass’s instance method. 9.13 (Calling get Methods in a Class’s Body) In Figs. 9.10–9.11 methods earnings and toString each call various get methods within the same class. Explain the benefits of calling these get methods within the classes. 9.14 (Employee Hierarchy) In this chapter, you studied an inheritance hierarchy in which class BasePlusCommissionEmployee inherited from class CommissionEmployee. However, not all types of employees are CommissionEmployees. In this exercise, you’ll create a more general Employee superclass that factors out the attributes and behaviors in class CommissionEmployee that are common to all Employees. The common attributes and behaviors for all Employees are firstName, lastName, socialSecurityNumber, getFirstName, getLastName, getSocialSecurityNumber and a portion of method toString. Create a new superclass Employee that contains these instance variables and methods and a constructor. Next, rewrite class CommissionEmployee from Section 9.4.5 as a subclass of Employee. Class CommissionEmployee should contain only the instance variables and methods that are not declared in superclass Employee. Class CommissionEmployee’s constructor should invoke class Employee’s constructor and CommissionEmployee’s toString method should invoke Employee’s toString method. Once you’ve completed these modifications, run the CommissionEmployeeTest and BasePlusCommissionEmployeeTest apps using these new classes to ensure that the apps still display the same results for a CommissionEmployee object and BasePlusCommissionEmployee object, respectively. 9.15 (Creating a New Subclass of Employee) Other types of Employees might include SalariedEmployees who get paid a fixed weekly salary, PieceWorkers who get paid by the number of pieces they produce or HourlyEmployees who get paid an hourly wage with time-and-a-half—1.5 times the hourly wage—for hours worked over 40 hours. Create class HourlyEmployee that inherits from class Employee (Exercise 9.14) and has instance variable hours (a double) that represents the hours worked, instance variable wage (a double) that represents the wages per hour, a constructor that takes as arguments a first name, a last name, a social security number, an hourly wage and the number of hours worked, set and get methods for manipulating the hours and wage, an earnings method to calculate an HourlyEmployee’s earnings based on the hours worked and a toString method that returns the HourlyEmployee’s String representation. Method setWage should ensure that wage is nonnegative, and setHours should ensure that the value of hours is between 0 and 168 (the total number of hours in a week). Use class HourlyEmployee in a test program that’s similar to the one in Fig. 9.5. |