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Chapter 9 Object-Oriented Programming: Inheritance 351 16 // display point coordinates via X and Y properties 17 string output = “X coordinate is ” + point.X + 18 “n” + “Y coordinate is ” + point.Y; 19 20 point.X = 10; // set x-coordinate via X property 21 point.Y = 10; // set y-coordinate via Y property 22 23 // display new point value 24 output += “nnThe new location of point is ” + point; 25 26 MessageBox.Show( output, “Demonstrating Class Point” ); 27 28 } // end method Main 29 30 } // end class PointTest Fig. 9.5 Fig. 9.5Fig. 9.FiFi5g. 9.5g. 9.5PointTestclass demonstrates class Pointfunctionality. (Part 2 of 2.) We now discuss the second part of our introduction to inheritance by creating and testing (a completely new) class Circle (Fig. 9.6), which directly inherits from class System.Object and represents an x-y coordinate pair (representing the center of the circle) and a radius. Lines 9 10 declare the instance variables x, yand radius as private data. The public services of class Circle include two Circle constructors (lines 13 25), properties X, Yand Radius(lines 28 71), methods Diameter(lines 74 77), Circumference(lines 80 83), Area(lines 86 89) and ToString(lines 92 96). These properties and methods encapsulate all necessary features (i.e., the analytic geometry ) of a circle; in the next section, we show how this encapsulation enables us to reuse and extend this class. 1 // Fig. 9.6: Circle.cs 2 // Circle class contains x-y coordinate pair and radius. 3 4 using System; 5 6 // Circle class definition implicitly inherits from Object 7 public class Circle 8 { 9 private int x, y; // coordinates of Circle’s center 10 private double radius; // Circle’s radius 11 Fig. 9.6 Fig. 9.6Fig. 9.FiFi6g. 9.6g. 9.6Circleclass contains an x-y coordinate and a radius. (Part 1 of 3.)
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350 Object-Oriented Programming: Inheritance Chapter 9 class Object when invoked, method ToString of class Point returns a string containing an ordered pair of the values xand y (line 59), instead of returning a string containing the object s class and namespace. To override a base-class method definition, a derived class must specify that the derived-class method overrides the base-class method with keyword overridein the method header. Software Engineering Observation 9.4 The C# compiler sets the base class of a derived class to Object when the program does not specify a base class explicitly. In C#, a base-class method must be declared virtual if that method is to be overridden in a derived class. Method ToStringof class Objectis, in fact, declared virtual, which enables derived class Point to override this method. To view the method header for ToString, select Help > Index…, and enter Object.ToStringmethod (filtered by .Net Framework SDK) in the search text box. The page displayed contains a description of method ToString, which includes the following header: public virtual string ToString(); Keyword virtualallows programmers to specify those methods that a derived class can override a method that has not been declared virtual cannot be overridden. We use this later in this section to enable certain methods in our base classes to be overridden. Common Programming Error 9.1 A derived class attempting to override (using keyword override) a method that has not been declared virtual is a syntax error. Class PointTest(Fig. 9.5) tests class Point. Line 14 instantiates an object of class Pointand assigns 72as the x-coordinate value and 115as the y-coordinate value. Lines 17 18 use properties X and Y to retrieve these values, then append the values to string output. Lines 20 21 change the values of properties X and Y (implicitly invoking their set accessors), and line 24 calls Point s ToString method implicitly to obtain the Point s string representation. 1 // Fig. 9.5: PointTest.cs 2 // Testing class Point. 3 4 using System; 5 using System.Windows.Forms; 6 7 // PointTest class definition 8 class PointTest 9 { 10 // main entry point for application 11 static void Main( string[] args ) 12 { 13 // instantiate Point object 14 Point point = new Point( 72, 115 ); 15 Fig. 9.5 Fig. 9.5Fig. 9.FiFi5g. 9.5g. 9.5PointTestclass demonstrates class Pointfunctionality. (Part 1 of 2.)
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Chapter 9 Object-Oriented Programming: Inheritance 349 21 // implicit call to Object constructor occurs here 22 X = xValue; 23 Y = yValue; 24 } 25 26 // property X 27 public int X 28 { 29 get 30 { 31 return x; 32 } 33 34 set 35 { 36 x = value; // no need for validation 37 } 38 39 } // end property X 40 41 // property Y 42 public int Y 43 { 44 get 45 { 46 return y; 47 } 48 49 set 50 { 51 y = value; // no need for validation 52 } 53 54 } // end property Y 55 56 // return string representation of Point 57 public override string ToString() 58 { 59 return “[” + x + “, ” + y + “]”; 60 } 61 62 } // end class Point Fig. 9.4 Fig. 9.4Fig. 9.FiFi4g. 9.4g. 9.4Pointclass represents an x-y coordinate pair. (Part 2 of 2.) Note that method ToString (lines 57 60) contains the keyword override in its declaration. Every class in C# (such as class Point) inherits either directly or indirectly from class System.Object, which is the root of the class hierarchy. As we mentioned previously, this means that every class inherits the eight methods defined by class Object. One of these methods is ToString, which returns a stringcontaining the object s type preceded by its namespace this method obtains an object s string representation and sometimes is called implicitly by the program (such as when an object is concatenated to a string). Method ToString of class Point overrides the original ToString from
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348 Object-Oriented Programming: Inheritance Chapter 9 which directly inherits from class Point(i.e., class Circle2 is a Pointbut also contains a radius) and attempts to use the Pointprivatemembers this results in compilation errors, because the derived class does not have access to the base-class s private data. We then show that if Point s data is declared as protected, a Circle3 class that inherits from class Point can access that data. Both the inherited and non-inherited Circleclasses contain identical functionality, but we show how the inherited Circle3 class is easier to create and manage. After discussing the merits of using protecteddata, we set the Point data back to private (to enforce good software engineering), then show how a separate Circle4 class (which also inherits from class Point) can use Pointmethods to manipulate Point s private data. Let us first examine the Point (Fig. 9.4) class definition. The public services of class Pointinclude two Pointconstructors (lines 13 24), properties Xand Y(lines 27 54) and method ToString (lines 57 60). The instance variables x and y of Point are specified as private(line 10), so objects of other classes cannot access xand ydirectly. Technically, even if Point s variables x and y were made public, Point can never maintain an inconsistent state, because the x-y coordinate plane is infinite in both directions, so xand ycan hold any intvalue. In general, however, declaring data as private, while providing non-private properties to manipulate and perform validation checking on that data, enforces good software engineering. We mentioned in Section 9.2 that constructors are not inherited. Therefore, Class Point does not inherit class Object s constructor. However, class Point s constructors (lines 13 24) call class Object s constructor implicitly. In fact, the first task of any derived-class constructor is to call its direct base class s constructor, either implicitly or explicitly. (The syntax for calling a base-class constructor is discussed later in this section.) If the code does not include an explicit call to the base-class constructor, an implicit call is made to the base class s default (no-argument) constructor. The comments in lines 15 and 21 indicate where the implicit calls to the base-class Object s default constructor occur. 1 // Fig. 9.4: Point.cs 2 // Point class represents an x-y coordinate pair. 3 4 using System; 5 6 // Point class definition implicitly inherits from Object 7 public class Point 8 { 9 // point coordinates 10 private int x, y; 11 12 // default (no-argument) constructor 13 public Point() 14 { 15 // implicit call to Object constructor occurs here 16 } 17 18 // constructor 19 public Point( int xValue, int yValue ) 20 { Fig. 9.4 Fig. 9.4Fig. 9.FiFi4g. 9.4g. 9.4Pointclass represents an x-y coordinate pair. (Part 1 of 2.)
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Chapter 9 Object-Oriented Programming: Inheritance 347 TwoDimensionalShape Circle Square Shape ThreeDimensionalShape Triangle Sphere Cube Cylinder Fig. 9.3 Fig. 9.Fig..Fi 93g. 9.39.3Portion of a Shapeclass hierarchy. Fig. 9.3 protectedand internalMembers Chapter 8 discussed public and private member access modifiers. A base class s publicmembers are accessible anywhere that the program has a reference to an object of that base class or one of its derived classes. A base class s privatemembers are accessible only within the body of that base class. In this section, we introduce two additional member access modifiers, protected and internal. Using protected access offers an intermediate level of protection between public and private access. A base class s protected members can be accessed only in that base class or in any classes derived from that base class. Another intermediate level of access is known as internal access. A base class s internalmembers can be accessed only by objects declared in the same assembly. Note that an internal member is accessible in any part of the assembly in which that internalmember is declared. Derived-class methods normally can refer to public, protectedand internal members of the base class simply by using the member names. When a derived-class method overrides a base-class member, the base-class member can be accessed from the derived class by preceding the base-class member name with keyword base, followed by the dot operator (.). We discuss keyword basein Section 9.4. 9.4 Relationship between Base Classes and Derived Classes In this section, we use a point-circle hierarchy1 to discuss the relationship between a base class and a derived class. We divide our discussion of the point-circle relationship into several parts. First, we create class Point, which directly inherits from class System.Object and contains as private data an x-y coordinate pair. Then, we create class Circle, which also directly inherits from class System.Objectand contains as privatedata an x-y coordinate pair (representing the location of the center of the circle) and a radius. We do not use inheritance to create class Circle; rather, we construct the class by writing every line of code the class requires. Next, we create a separate Circle2class, 1. The point-circle relationship may seem unnatural when we discuss it in the context of a circle is a point. This example teaches what is sometimes called structural inheritance; the example focuses on the mechanics of inheritance and how a base class and a derived class relate to one another. In Chapter 10, we present more natural inheritance examples.
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346 Object-Oriented Programming: Inheritance Chapter 9 in which they share the eight methods defined by class Object. We discuss some of these methods inherited from Objectthroughout the text. Another inheritance hierarchy is the Shapehierarchy of Fig. 9.3. To specify that class TwoDimensionalShape is derived from (or inherits from) class Shape, class TwoDimensionalShapecould be defined in C# as follows: class TwoDimensionalShape : Shape In Chapter 8, we briefly discussed has-a relationships, in which classes have as members references to objects of other classes. Such relationships create classes by composition of existing classes. For example, given the classes Employee, BirthDateand TelephoneNumber, it is improper to say that an Employee is a BirthDate or that an Employee is a TelephoneNumber. However, it is appropriate to say that an Employeehas a BirthDateand that an Employeehas a TelephoneNumber. With inheritance, privatemembers of a base class are not accessible directly from that class s derived classes, but these privatebase-class members are still inherited. All other base-class members retain their original member access when they become members of the derived class (e.g., publicmembers of the base class become public members of the derived class, and, as we will soon see, protected members of the base class become protected members of the derived class). Through these inherited base-class members, the derived class can manipulate privatemembers of the base class (if these inherited members provide such functionality in the base class). It is possible to treat base-class objects and derived-class objects similarly; their commonalities are expressed in the member variables, properties and methods of the base class. Objects of all classes derived from a common base class can be treated as objects of that base class. In Chapter 10, Object-Oriented Programming: Polymorphism we consider many examples that take advantage of this relationship. Software Engineering Observation 9.3 Constructors never are inherited they are specific to the class in which they are defined. Employee Student Faculty Staff Administrator Teacher Alumnus CommunityMember Fig. 9.2 Fig. 9.Fig..Fi 92g. 9.29.2Inheritance hierarchy for university CommunityMembers. Fig.
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Chapter 9 Object-Oriented Programming: Inheritance 345 Base class Derived classes Student GraduateStudent UndergraduateStudent Shape Circle Triangle Rectangle Loan CarLoan HomeImprovementLoan MortgageLoan Employee FacultyMember StaffMember Account CheckingAccount SavingsAccount Fig. 9.1 Fig. 9.Fig..Fi 91g. 9.19.1Inheritance examples. Fig. Every derived-class object is an object of its base class, and one base class can have many derived classes; therefore, the set of objects represented by a base class typically is larger than the set of objects represented by any of its derived classes. For example, the base class Vehiclerepresents all vehicles, including cars, trucks, boats, bicycles and so on. By contrast, derived-class Carrepresents only a small subset of all Vehicles. Inheritance relationships form tree-like hierarchical structures. A class exists in a hierarchical relationship with its derived classes. Although classes can exist independently, once they are employed in inheritance arrangements, they become affiliated with other classes. A class becomes either a base class, supplying data and behaviors to other classes, or a derived class, inheriting its data and behaviors from other classes. Let us develop a simple inheritance hierarchy. A university community has thousands of members. These members consist of employees, students and alumni. Employees are either faculty members or staff members. Faculty members are either administrators (such as deans and department chairpersons) or teachers. This organizational structure yields the inheritance hierarchy, depicted in Fig. 9.2. Note that the inheritance hierarchy could contain many other classes. For example, students can be graduate or undergraduate students. Undergraduate students can be freshmen, sophomores, juniors and seniors. Each arrow in the hierarchy represents an is-a relationship. For example, as we follow the arrows in this class hierarchy, we can state, an Employee is a CommunityMember and a Teacher is a Faculty member. CommunityMember is the direct base class of Employee, Student and Alumnus. In addition, CommunityMember is an indirect base class of all the other classes in the hierarchy diagram. Starting from the bottom of the diagram, the reader can follow the arrows and apply the is-a relationship to the topmost base class. For example, an Administrator is a Facultymember, is an Employee and is a CommunityMember. In C#, an Administrator also is an Object, because all classes in C# have Objectas either a direct or indirect base class. Thus, all classes in C# are connected via a hierarchical relationship
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344 Object-Oriented Programming: Inheritance Chapter 9 grammers focus on the commonalities among objects in the system, rather than on the special cases. This process is called abstraction. We distinguish between the is-a relationship and the has-a relationship. Is-a represents inheritance. In an is-a relationship, an object of a derived class also can be treated as an object of its base class. For example, a car is a vehicle. By contrast, has-a stands for composition (composition is discussed in Chapter . In a has-a relationship, a class object contains one or more object references as members. For example, a car has a steering wheel. Derived-class methods might require access to their base-class instance variables, properties and methods. A derived class can access the non-private members of its base class. Base-class members that should not be accessible to properties or methods of a class derived from that base class via inheritance are declared private in the base class. A derived class can effect state changes in private base-class members, but only through non-private methods and properties provided in the base class and inherited into the derived class. Software Engineering Observation 9.1 Properties and methods of a derived class cannot directly access private members of their base class. Software Engineering Observation 9.2 Hiding private members helps programmers test, debug and correctly modify systems. If a derived class could access its base class s private members, classes that inherit from that derived class could access that data as well. This would propagate access to what should be private data, and the benefits of information hiding would be lost. One problem with inheritance is that a derived class can inherit properties and methods it does not need or should not have. It is the class designer s responsibility to ensure that the capabilities provided by a class are appropriate for future derived classes. Even when a base-class property or method is appropriate for a derived class, that derived class often requires the property or method to perform its task in a manner specific to the derived class. In such cases, the base-class property or method can be overridden (redefined) in the derived class with an appropriate implementation. New classes can inherit from abundant class libraries. Organizations develop their own class libraries and can take advantage of other libraries available worldwide. Someday, the vast majority of new software likely will be constructed from standardized reusable components, as most hardware is constructed today. This will facilitate the development of more powerful and abundant software. 9.2 Base Classes and Derived Classes Often, an object of one class is an object of another class, as well. For example, a rectangle is a quadrilateral (as are squares, parallelograms and trapezoids). Thus, class Rectangle can be said to inherit from class Quadrilateral. In this context, class Quadrilateral is a base class, and class Rectangle is a derived class. A rectangle is a specific type of quadrilateral, but it is incorrect to claim that a quadrilateral is a rectangle the quadrilateral could be a parallelogram or some other type of Quadrilateral. Figure 9.1 lists several simple examples of base classes and derived classes.
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Chapter 9 Object-Oriented Programming: Inheritance 343 Outline 9.1 Introduction 9.2 Base Classes and Derived Classes 9.3 protected Members 9.4 Relationship between Base Classes and Derived Classes 9.5 Case Study: Three-Level Inheritance Hierarchy 9.6 Constructors and Destructors in Derived Classes 9.7 Software Engineering with Inheritance Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises 9.1 Introduction In this chapter, we begin our discussion of object-oriented programming (OOP) by introducing one of its main features inheritance. Inheritance is a form of software reusability in which classes are created by absorbing an existing class s data and behaviors and embellishing them with new capabilities. Software reusability saves time during program development. It also encourages the reuse of proven and debugged high-quality software, which increases the likelihood that a system will be implemented effectively. When creating a class, instead of writing completely new instance variables and methods, the programmer can designate that the new class should inherit the class variables, properties and methods of another class. The previously defined class is called the base class, and the new class is referred to as the derived class. (Other programming languages, such as Java, refer to the base class as the superclass, and the derived class as the subclass.) Once created, each derived class can become the base class for future derived classes. A derived class, to which unique class variables, properties and methods normally are added, is often larger than its base class. Therefore, a derived class is more specific than its base class and represents a more specialized group of objects. Typically, the derived class contains the behaviors of its base class and additional behaviors. The direct base class is the base class from which the derived class explicitly inherits. An indirect base class is inherited from two or more levels up the class hierarchy. In the case of single inheritance, a class is derived from one base class. C#, unlike C++, does not support multiple inheritance (which occurs when a class is derived from more than one direct base class). (We explain in Chapter 10 how C# can use interfaces to realize many of the benefits of multiple inheritance while avoiding the associated problems.) Every object of a derived class is also an object of that derived class s base class. However, base-class objects are not objects of their derived classes. For example, all cars are vehicles, but not all vehicles are cars. As we continue our study of object-oriented programming in Chapters 9 and 10, we take advantage of this relationship to perform some interesting manipulations. Experience in building software systems indicates that significant amounts of code deal with closely related special cases. When programmers are preoccupied with special cases, the details can obscure the big picture. With object-oriented programming, pro
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9 Object-Oriented Programming: Inheritance Objectives To understand inheritance and software reusability. To understand the concepts of base classes and derived classes. To understand member access modifier protected and internal. To be able to use the base reference to access base- class members To understand the use of constructors and finalizers in base classes and derived classes. To present a case study that demonstrates the mechanics of inheritance. Say not you know another entirely, till you have divided an inheritance with him. Johann Kasper Lavater This method is to define as the number of a class the class of all classes similar to the given class. Bertrand Russell Good as it is to inherit a library, it is better to collect one. Augustine Birrell
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