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Chapter 9 Object-Oriented Programming: Inheritance 361 20 21 // set Circle3’s x-y coordinates and radius to new values 22 circle.X = 2; 23 circle.Y = 2; 24 circle.Radius = 4.25; 25 26 // display Circle3’s string representation 27 output += “nn” + 28 “The new location and radius of circle are ” + 29 “n” + circle + “n”; 30 31 // display Circle3’s Diameter 32 output += “Diameter is ” + 33 String.Format( “{0:F}”, circle.Diameter() ) + “n”; 34 35 // display Circle3’s Circumference 36 output += “Circumference is ” + 37 String.Format( “{0:F}”, circle.Circumference() ) + “n”; 38 39 // display Circle3’s Area 40 output += “Area is ” + 41 String.Format( “{0:F}”, circle.Area() ); 42 43 MessageBox.Show( output, “Demonstrating Class Circle3″ ); 44 45 } // end method Main 46 47 } // end class CircleTest3 Fig. 9.11 Fig. 9.11Fig. 9.FiFi11g. 9.11g. 9.11CircleTest3demonstrates class Circle3functionality. (Part 2 of 2.) In the previous example, we declared the base-class instance variables as protected, so that a derived class could modify their values directly. The use of protectedvariables allows for a slight increase in performance, because we avoid incurring the overhead of a method call to a property s set or getaccessor. However, in most C# applications, in which user interaction comprises a large part of the execution time, the optimization offered through the use of protectedvariables is negligible. Using protected instance variables creates two major problems. First, the derived- class object does not have to use a property to set the value of the base-class s protected data. Therefore, a derived-class object can easily assign an illegal value to the protected
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360 Object-Oriented Programming: Inheritance Chapter 9 49 // calculate circumference 50 public double Circumference() 51 { 52 return Math.PI * Diameter(); 53 } 54 55 // calculate Circle area 56 public virtual double Area() 57 { 58 return Math.PI * Math.Pow( radius, 2 ); 59 } 60 61 // return string representation of Circle3 62 public override string ToString() 63 { 64 return “Center = [” + x + “, ” + y + “]” + 65 “; Radius = ” + radius; 66 } 67 68 } // end class Circle3 Fig. 9.10 Fig. 9.10Fig. 9.FiFi10g. 9.10g. 9.10Circle3class that inherits from class Point2. (Part 2 of 2.) Class CircleTest3(Fig. 9.11) performs identical tests on class Circle3as class CircleTest (Fig. 9.7) performed on class Circle (Fig. 9.6). Note that the outputs of the two programs are identical. We created class Circle without using inheritance and created class Circle3 using inheritance; however, both classes provide the same functionality. However, observe that the code listing for class Circle3, which is 68 lines, is considerably shorter than the code listing for class Circle, which is 98 lines, because class Circle3 absorbs part of its functionality from Point2, whereas class Circle does not. Also, there is now only one copy of the point functionality. 1 / Fig. 9.11: CircleTest3.cs 2 // Testing class Circle3. 3 4 using System; 5 using System.Windows.Forms; 6 7 // CircleTest3 class definition 8 class CircleTest3 9 { 10 // main entry point for application 11 static void Main( string[] args ) 12 { 13 // instantiate Circle3 14 Circle3 circle = new Circle3( 37, 43, 2.5 ); 15 16 // get Circle3’s initial x-y coordinates and radius 17 string output = “X coordinate is ” + circle.X + “n” + 18 “Y coordinate is ” + circle.Y + “nRadius is ” + 19 circle.Radius; Fig. 9.11 Fig. 9.11Fig. 9.FiFi11g. 9.11g. 9.11CircleTest3demonstrates class Circle3functionality. (Part 1 of 2.)
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Chapter 9 Object-Oriented Programming: Inheritance 359 protected base-class data members. A derived class also can access protected methods in any of that derived class s base classes. 1 // Fig. 9.10: Circle3.cs 2 // Circle2 class that inherits from class Point2. 3 4 using System; 5 6 // Circle3 class definition inherits from Point2 7 public class Circle3 : Point2 8 { 9 private double radius; // Circle’s radius 10 11 // default constructor 12 public Circle3() 13 { 14 // implicit call to Point constructor occurs here 15 } 16 17 // constructor 18 public Circle3( 19 int xValue, int yValue, double radiusValue ) 20 { 21 // implicit call to Point constructor occurs here 22 x = xValue; 23 y = yValue; 24 Radius = radiusValue; 25 } 26 27 // property Radius 28 public double Radius 29 { 30 get 31 { 32 return radius; 33 } 34 35 set 36 { 37 if ( value >= 0 ) 38 radius = value; 39 } 40 41 } // end property Radius 42 43 // calculate Circle diameter 44 public double Diameter() 45 { 46 return radius * 2; 47 } 48 Fig. 9.10 Fig. 9.10Fig. 9.FiFi10g. 9.10g. 9.10Circle3class that inherits from class Point2. (Part 1 of 2.)
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358 Object-Oriented Programming: Inheritance Chapter 9 18 // constructor 19 public Point2( int xValue, int yValue ) 20 { 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 Point2 57 public override string ToString() 58 { 59 return “[” + x + “, ” + y + “]”; 60 } 61 62 } // end class Point2 Fig. 9.9 Fig. 9.9Fig. 9.FiFi9g. 9.9g. 9.9Point2class represents an x-y coordinate pair as protecteddata. (Part 2 of 2.) Class Circle3 (Fig. 9.10) modifies class Circle2 (Fig. 9.8) to inherit from class Point2 rather than inheriting from class Point. Because class Circle3 is a class derived from class Point2, class Circle3 can access class Point2 s protected member variables x and y directly, and the compiler does not generate errors when compiling Fig. 9.10. This shows the special privileges that a derived class is granted to access
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Chapter 9 Object-Oriented Programming: Inheritance 357 Fig. 9.8 Fig. 9.8Fig. 9.FiFi8g. 9.8g. 9.8Circle2class that inherits from class Point. (Part 3 of 3.) Lines 14 and 20 in the Circle2constructors (lines 12 24) invoke the default Point constructor implicitly to initialize the base-class portion (variables xand y, inherited from class Point) of a Circle2object to 0. However, because the parameterized constructor (lines 18 24) should set the x-y coordinate to a specific value, lines 21 22 attempt to assign argument values to xand ydirectly. Even though lines 21 22 attempt to set xand yvalues explicitly, line 20 first calls the Point default constructor to initialize these variables to their default values. The compiler generates syntax errors for lines 21 and 22 (and line 63, where Circle2 s method ToString attempts to use the values of x and y directly), because the derived class Circle2is not allowed to access the base class Point s privatemembers xand y. C# rigidly enforces restriction on accessing privatedata members, so that even a derived class (i.e., which is closely related to its base class) cannot access base-class privatedata. To enable class Circle2 to access Point member variables x and y directly, we can declare those variables as protected. As we discussed in Section 9.3, a base class s protected members can be accessed only in that base class or in any classes derived from that base class. Class Point2 (Fig. 9.9) modifies class Point (Fig. 9.4) to declare variables xand yas protected(line 10) instead of private. 1 // Fig. 9.9: Point2.cs 2 // Point2 class contains an x-y coordinate pair as protected data. 3 4 using System; 5 6 // Point2 class definition implicitly inherits from Object 7 public class Point2 8 { 9 // point coordinate 10 protected int x, y; 11 12 // default constructor 13 public Point2() 14 { 15 // implicit call to Object constructor occurs here 16 } 17 Fig. 9.9 Fig. 9.9Fig. 9.FiFi9g. 9.9g. 9.9Point2class represents an x-y coordinate pair as protecteddata. (Part 1 of 2.)
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356 Object-Oriented Programming: Inheritance Chapter 9 17 // constructor 18 public Circle2( int xValue, int yValue, double radiusValue ) 19 { 20 // implicit call to Point constructor occurs here 21 x = xValue; 22 y = yValue; 23 Radius = radiusValue; 24 } 25 26 // property Radius 27 public double Radius 28 { 29 get 30 { 31 return radius; 32 } 33 34 set 35 { 36 if ( value >= 0 ) 37 radius = value; 38 } 39 40 } // end property Radius 41 42 // calculate Circle diameter 43 public double Diameter() 44 { 45 return radius * 2; 46 } 47 48 // calculate Circle circumference 49 public double Circumference() 50 { 51 return Math.PI * Diameter(); 52 } 53 54 // calculate Circle area 55 public virtual double area() 56 { 57 return Math.PI * Math.Pow( radius, 2 ); 58 } 59 60 // return string representation Circle 61 public override string ToString() 62 { 63 return “Center = [” + x + “, ” + y + “]” + 64 “; Radius = ” + radius; 65 } 66 67 } // end class Circle2 Fig. 9.8 Fig. 9.8Fig. 9.FiFi8g. 9.8g. 9.8Circle2class that inherits from class Point. (Part 2 of 3.)
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Chapter 9 Object-Oriented Programming: Inheritance 355 45 46 } // end class CircleTest Fig. 9.7 Fig. 9.7Fig. 9.FiFi7g. 9.7g. 9.7CircleTestdemonstrates class Circlefunctionality. (Part 2 of 2.) In the next examples we answer that question, we use a more elegant class construction approach emphasizing the benefits of inheritance. Now, we create and test a class Circle2 (Fig. 9.8) that inherits variables x and y and properties X and Y from class Point (Fig. 9.4). This class Circle2 is a Point (because inheritance absorbs the capabilities of class Point), but also contains radius (line 9). The colon (:) symbol in the class declaration (line 7) indicates inheritance. As a derived class, Circle2 inherits all the members of class Point, except for the constructors. Thus, the public services to Circle2 include the two Circle2constructors (lines 12 24); the public methods inherited from class Point; property Radius(lines 27 40); and the Circle2methods Diameter, Circumference, Areaand ToString(lines 43 65). We declare method Area as virtual, so that derived classes (such as class Cylinder, as we will see in Section 9.5) can override this method to provide a more appropriate implementation. 1 // Fig. 9.8: Circle2.cs 2 // Circle2 class that inherits from class Point. 3 4 using System; 5 6 // Circle2 class definition inherits from Point 7 class Circle2 : Point 8 { 9 private double radius; // Circle2’s radius 10 11 // default constructor 12 public Circle2() 13 { 14 // implicit call to Point constructor occurs here 15 } 16 Fig. 9.8 Fig. 9.8Fig. 9.FiFi8g. 9.8g. 9.8Circle2class that inherits from class Point. (Part 1 of 3.)
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354 Object-Oriented Programming: Inheritance Chapter 9 It appears that we literally copied code from class Point, pasted this code in the code from class Circle, then modified class Circle to include a radius. This copy-andpaste approach is often error-prone and time-consuming. Worse yet, it can result in many physical copies of the code existing throughout a system, creating a code-maintenance nightmare. Is there a way to absorb the attributes and behaviors of one class in a way that makes them part of other classes without duplicating code? 1 // Fig. 9.7: CircleTest.cs 2 // Testing class Circle. 3 4 using System; 5 using System.Windows.Forms; 6 7 // CircleTest class definition 8 class CircleTest 9 { 10 // main entry point for application. 11 static void Main( string[] args ) 12 { 13 // instantiate Circle 14 Circle circle = new Circle( 37, 43, 2.5 ); 15 16 // get Circle’s initial x-y coordinates and radius 17 string output = “X coordinate is ” + circle.X + 18 “nY coordinate is ” + circle.Y + “nRadius is ” + 19 circle.Radius; 20 21 // set Circle’s x-y coordinates and radius to new values 22 circle.X = 2; 23 circle.Y = 2; 24 circle.Radius = 4.25; 25 26 // display Circle’s string representation 27 output += “nnThe new location and radius of ” + 28 “circle are n” + circle + “n”; 29 30 // display Circle’s diameter 31 output += “Diameter is ” + 32 String.Format( “{0:F}”, circle.Diameter() ) + “n”; 33 34 // display Circle’s circumference 35 output += “Circumference is ” + 36 String.Format( “{0:F}”, circle.Circumference() ) + “n”; 37 38 // display Circle’s area 39 output += “Area is ” + 40 String.Format( “{0:F}”, circle.Area() ); 41 42 MessageBox.Show( output, “Demonstrating Class Circle” ); 43 44 } // end method Main Fig. 9.7 Fig. 9.7Fig. 9.FiFi7g. 9.7g. 9.7CircleTestdemonstrates class Circlefunctionality. (Part 1 of 2.)
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Chapter 9 Object-Oriented Programming: Inheritance 353 65 set 66 { 67 if ( value >= 0 ) // validation needed 68 radius = value; 69 } 70 71 } // end property Radius 72 73 // calculate Circle diameter 74 public double Diameter() 75 { 76 return radius * 2; 77 } 78 79 // calculate Circle circumference 80 public double Circumference() 81 { 82 return Math.PI * Diameter(); 83 } 84 85 // calculate Circle area 86 public double Area() 87 { 88 return Math.PI * Math.Pow( radius, 2 ); 89 } 90 91 // return string representation of Circle 92 public override string ToString() 93 { 94 return “Center = [” + x + “, ” + y + “]” + 95 “; Radius = ” + radius; 96 } 97 98 } // end class Circle Fig. 9.6 Fig. 9.6Fig. 9.FiFi6g. 9.6g. 9.6Circleclass contains an x-y coordinate and a radius. (Part 3 of 3.) Class CircleTest (Fig. 9.7) tests class Circle. Line 14 instantiates an object of class Circle, assigning 37as the x-coordinate value, 43as the y-coordinate value and 2.5 as the radius value. Lines 17 19 use properties X, Yand Radiusto retrieve these values, then concatenate the values to stringoutput. Lines 22 24 use Circle s X, Yand Radius properties to change the x-y coordinates and the radius, respectively. Property Radius ensures that member variable radius cannot be assigned a negative value. Line 28 calls Circle s ToStringmethod implicitly to obtain the Circle s string representation, and lines 32 40 call Circle s Diameter, Circumferenceand Areamethods. After writing all the code for class Circle (Fig. 9.6), we note that a major portion of the code in this class is similar, if not identical, to much of the code in class Point. For example, the declaration in Circleof privatevariables xand yand properties Xand Y are identical to those of class Point. In addition, the class Circleconstructors and method ToString are almost identical to those of class Point, except that they also supply radiusinformation. The only other additions to class Circleare privatemember variable radius, property Radiusand methods Diameter, Circumferenceand Area.
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352 Object-Oriented Programming: Inheritance Chapter 9 12 // default constructor 13 public Circle() 14 { 15 // implicit call to Object constructor occurs here 16 } 17 18 // constructor 19 public Circle( int xValue, int yValue, double radiusValue ) 20 { 21 // implicit call to Object constructor occurs here 22 x = xValue; 23 y = yValue; 24 Radius = radiusValue; 25 } 26 27 // property X 28 public int X 29 { 30 get 31 { 32 return x; 33 } 34 35 set 36 { 37 x = value; // no need for validation 38 } 39 40 } // end property X 41 42 // property Y 43 public int Y 44 { 45 get 46 { 47 return y; 48 } 49 50 set 51 { 52 y = value; // no need for validation 53 } 54 55 } // end property Y 56 57 // property Radius 58 public double Radius 59 { 60 get 61 { 62 return radius; 63 } 64 Fig. 9.6 FiFiFiFig.g.g.g. 9.9.9.9.6666Circleclass contains an x-y coordinate and a radius. (Part 2 of 3.)
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