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Chapter 10 Object-Oriented Programming: Polymorphism 399 16 // constructor 17 public Cylinder2( int xValue, int yValue, double radiusValue, 18 double heightValue ) : base( xValue, yValue, radiusValue ) 19 { 20 Height = heightValue; 21 } 22 23 // property Height 24 public double Height 25 { 26 get 27 { 28 return height; 29 } 30 31 set 32 { 33 // ensure non-negative height value 34 if ( value >= 0 ) 35 height = value; 36 } 37 } 38 39 // calculate Cylinder2 area 40 public override double Area() 41 { 42 return 2 * base.Area() + base.Circumference() * Height; 43 } 44 45 // calculate Cylinder2 volume 46 public override double Volume() 47 { 48 return base.Area() * Height; 49 } 50 51 // return string representation of Circle2 object 52 public override string ToString() 53 { 54 return base.ToString() + “; Height = ” + Height; 55 } 56 57 // override property Name from class Circle2 58 public override string Name 59 { 60 get 61 { 62 return “Cylinder2″; 63 } 64 } 65 66 } // end class Cylinder2
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398 Object-Oriented Programming: Polymorphism Chapter 10 54 return Math.PI * Math.Pow( Radius, 2 ); 55 } 56 57 // return string representation of Circle2 object 58 public override string ToString() 59 { 60 return “Center = ” + base.ToString() + 61 “; Radius = ” + Radius; 62 } 63 64 // override property Name from class Point2 65 public override string Name 66 { 67 get 68 { 69 return “Circle2″; 70 } 71 } 72 73 } // end class Circle2 Fig. 10.6 Fig. 10.6Fig. 10FiFi.6g. 10.6g. 10.6Circle2class that inherits from class Point2. (Part 2 of 2.) Figure 10.7 defines class Cylinder2, which inherits from class Circle2. Class Cylinder2contains property Height(lines 24 37) for accessing the cylinder s height. Note that we do not declare property Heightas virtual, so classes derived from class Cylinder2 cannot override this property. A cylinder has different area and volume calculations from those of a circle, so this class overrides method Area(lines 40 43) to calculate the cylinder s surface area (i.e., 2pr2 + 2prh) and overrides method Volume (lines 46 49). Property Name(lines 58 64) overrides property Name of class Circle2. If this class did not override property Name, the class would inherit property Name of class Circle2, and this property would erroneously return Circle2. 1 // Fig. 10.7: Cylinder2.cs 2 // Cylinder2 inherits from class Circle2 and overrides key members. 3 using System; 4 5 // Cylinder2 inherits from class Circle2 6 public class Cylinder2 : Circle2 7 { 8 private double height; // Cylinder2 height 9 10 // default constructor 11 public Cylinder2() 12 { 13 // implicit call to Circle2 constructor occurs here 14 } 15 Fig. 10.7 Fig. 10.7Fig. 10FiFi.7g. 10.7g. 10.7Cylinder2class inherits from class Circle2. (Part 1 of 2.)
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Chapter 10 Object-Oriented Programming: Polymorphism 397 // Fig. 10.6: Circle2.cs // Circle2 inherits from class Point2 and overrides key members. using System; // Circle2 inherits from class Point2 public class Circle2 : Point2 { private double radius; // Circle2 radius // default constructor public Circle2() { // implicit call to Point2 constructor occurs here } // constructor public Circle2( int xValue, int yValue, double radiusValue ) : base( xValue, yValue ) { Radius = radiusValue; } // property Radius public double Radius { get { return radius; } set { // ensure non-negative radius value if ( value >= 0 ) radius = value; } } // calculate Circle2 diameter public double Diameter() { return Radius * 2; } // calculate Circle2 circumference public double Circumference() { return Math.PI * Diameter(); } // calculate Circle2 area public override double Area() { Fig. 10.6 Fig. 10.6Fig. 10FiFi.6g. 10.6g. 10.6Circle2class that inherits from class Point2. (Part 1 of 2.)
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396 Object-Oriented Programming: Polymorphism Chapter 10 32 set 33 { 34 x = value; // no validation needed 35 } 36 } 37 38 // property Y 39 public int Y 40 { 41 get 42 { 43 return y; 44 } 45 46 set 47 { 48 y = value; // no validation needed 49 } 50 } 51 52 // return string representation of Point2 object 53 public override string ToString() 54 { 55 return “[” + X + “, ” + Y + “]”; 56 } 57 58 // implement abstract property Name of class Shape 59 public override string Name 60 { 61 get 62 { 63 return “Point2″; 64 } 65 } 66 67 } // end class Point2 Fig. 10.5 Fig. 10.5Fig. 10FiFi.5g. 10.5g. 10.5Point2class inherits from abstractclass Shape. (Part 2 of 2.) Figure 10.6 defines class Circle2, which inherits from class Point2. Class Circle2contains property Radius(lines 24 37) for accessing the circle s radius. Note that we do not declare property Radius as virtual, so classes derived from this class cannot override this property. A circle has zero volume, so we do not override base-class method Volume. Rather, Circle2 inherits this method from class Point2, which inherited the method from Shape. However, a circle does have an area, so Circle2overrides Shapemethod Area(lines 52 55). Property Name(lines 65 71) of class Circle2 overrides property Name of class Point2. If this class did not override property Name, the class would inherit the Point2 version of property Name. In that case, Circle2 s Nameproperty would erroneously return Point2.
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Chapter 10 Object-Oriented Programming: Polymorphism 395 Class Shapedefines two concrete methods and one abstractproperty. All shapes have an area and a volume, so we include virtual methods Area(lines 8 11) and Volume (lines 14 17), which return the shape s area and volume, respectively. The volume of two- dimensional shapes is always zero, whereas three-dimensional shapes have a positive, nonzero volume. In class Shape, methods Area and Volume return zero, by default. Programmers can override these methods in derived classes when those classes should have different area calculations [e.g., classes Circle2 (Fig. 10.6) and Cylinder2 (Fig. 10.7)] and/or different volume calculations (e.g., Cylinder2). Read-only property Name(lines 20 23) is declared abstract, so derived classes must implement this property to become concrete classes. Note that abstractmethods and properties are implicitly virtual. Class Point2 (Fig. 10.5) inherits from abstract class Shape and overrides the abstract property Name, which makes Point2 a concrete class. A point s area and volume are zero, so class Point2 does not override base-class methods Area and Volume. Lines 59 65 implement property Name. If we did not provide this implementation, class Point2 would be an abstract class that would require keyword abstractin the first line of the class definition. 1 // Fig. 10.5: Point2.cs 2 // Point2 inherits from abstract class Shape and represents 3 // an x-y coordinate pair. 4 using System; 5 6 // Point2 inherits from abstract class Shape 7 public class Point2 : Shape 8 { 9 private int x, y; // Point2 coordinates 10 11 // default constructor 12 public Point2() 13 { 14 // implicit call to Object constructor occurs here 15 } 16 17 // constructor 18 public Point2( int xValue, int yValue ) 19 { 20 X = xValue; 21 Y = yValue; 22 } 23 24 // property X 25 public int X 26 { 27 get 28 { 29 return x; 30 } 31 Fig. 10.5 Fig. 10.5Fig. 10FiFi.5g. 10.5g. 10.5Point2class inherits from abstractclass Shape. (Part 1 of 2.)
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394 Object-Oriented Programming: Polymorphism Chapter 10 interface appropriate for all device drivers. Then, through inheritance from that abstract base class, derived classes are formed that all operate similarly. The capabilities (i.e., the public services) offered by the device drivers are provided as abstract methods in the abstract base class. The implementations of these abstract methods are provided in the derived classes that correspond to the specific types of device drivers. It is common in object-oriented programming to define an iterator class that can traverse all the objects in a container (such as an array). For example, a program can print a list of objects in a linked list by creating an iterator object, then using the iterator to obtain the next element of the list each time the iterator is called. Iterators often are used in polymorphic programming to traverse an array or a linked list of objects from various levels of a hierarchy. The references in such a list are all base-class references. (See Chapter 23, Data Structures, to learn more about linked lists.) A list of objects of base class TwoDimensionalShape could contain objects from classes Square, Circle, Triangle and so on. Using polymorphism to send a Draw message to each object in the list would draw each object correctly on the screen. 10.6 Case Study: Inheriting Interface and Implementation Our next example (Fig. 10.4 Fig. 10.8) reexamines the Point, Circle, Cylinderhierarchy that we explored in Chapter 9. In this example, the hierarchy begins with abstract base class Shape (Fig. 10.4). This hierarchy mechanically demonstrates the power of polymorphism. In the exercises, we explore a more substantial shape hierarchy. 1 // Fig. 10.4: Shape.cs 2 // Demonstrate a shape hierarchy using an abstract base class. 3 using System; 4 5 public abstract class Shape 6 { 7 // return Shape’s area 8 public virtual double Area() 9 { 10 return 0; 11 } 12 13 // return Shape’s volume 14 public virtual double Volume() 15 { 16 return 0; 17 } 18 19 // return Shape’s name 20 public abstract string Name 21 { 22 get; 23 } 24 } Fig. 10.4 Fig. 10.4Fig. 10FiFi.4g. 10.4g. 10.4Abstract Shapebase class.
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Chapter 10 Object-Oriented Programming: Polymorphism 393 syntax error. Every concrete derived class must override all base-class abstract methods and properties (using keyword override) and provide concrete implementations of those methods or properties. Any class with an abstract method in it must be declared abstract. The difference between an abstract method and a virtual method is that a virtual method has an implementation and provides the derived class with the option of overriding the method; by contrast, an abstract method does not provide an implementation and forces the derived class to override the method (for that derived class to be concrete). Common Programming Error 10.5 Defining an abstract method in a class that has not been declared as abstract results is a syntax error. Common Programming Error 10.6 Attempting to instantiate an object of an abstract class results in a compilation error. Common Programming Error 10.7 Failure to override an abstract method in a derived class is a syntax error, unless the derived class also is an abstract class. Software Engineering Observation 10.7 An abstract class can have instance data and non-abstract methods (including constructors), which are subject to the normal rules of inheritance by derived classes. Although we cannot instantiate objects of abstract base classes, we can use abstract base classes to declare references; these references can refer to instances of any concrete classes derived from the abstract class. Programs can use such references to manipulate instances of the derived classes polymorphically. Let us consider another application of polymorphism. A screen manager needs to display a variety of objects, including new types of objects that the programmer will add to the system after writing the screen manager. The system might need to display various shapes, such as Circle, Triangle or Rectangle, which are derived from abstract class Shape. The screen manager uses base-class references of type Shape to manage the objects that are displayed. To draw any object (regardless of the level at which that object s class appears in the inheritance hierarchy), the screen manager uses a base-class reference to the object to invoke the object s Draw method. Method Draw is an abstract method in base class Shape; therefore, each derived class must implement method Draw. Each Shape object in the inheritance hierarchy knows how to draw itself. The screen manager does not have to worry about the type of each object or whether the screen manager has ever encountered objects of that type. Polymorphism is particularly effective for implementing layered software systems. In operating systems, for example, each type of physical device could operate quite differently from the others. Even so, commands to read or write data from and to devices may have a certain uniformity. The write message sent to a device-driver object needs to be interpreted specifically in the context of that device driver and how that device driver manipulates devices of a specific type. However, the write call itself really is no different from the write to any other device in the system place some number of bytes from memory onto that device. An object-oriented operating system might use an abstract base class to provide an
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392 Object-Oriented Programming: Polymorphism Chapter 10 Software Engineering Observation 10.5 Polymorphism promotes extensibility. Software used to invoke polymorphic behavior is written to be independent of the types of the objects to which messages (i.e., method calls) are sent. Thus, programmers can include into a system additional types of objects that respond to existing messages and can do this without modifying the base system. 10.5 Abstract Classes and Methods When we think of a class as a type, we assume that programs will create objects of that type. However, there are cases in which it is useful to define classes for which the programmer never intends to instantiate any objects. Such classes are called abstract classes. Because such classes normally are used as base classes in inheritance hierarchies, we refer to such classes as abstract base classes. These classes cannot be used to instantiate objects, since abstract classes are incomplete. Derived classes must define the missing pieces. Abstract classes normally contain one or more abstract methods or abstract properties, which are methods and properties that do not provide implementations. Derived classes must override inherited abstract methods and properties to enable objects of those derived classes to be instantiated. We discuss abstract classes extensively in Section 10.6 and Section 10.8. The purpose of an abstract class is to provide an appropriate base class from which other classes may inherit. Classes from which objects can be instantiated are called concrete classes. Such classes provide implementations of every method and property they define. We could have an abstract base class TwoDimensionalObject and derive such concrete classes as Square, Circle and Triangle. We could also have an abstract base class ThreeDimensionalObject and derive such concrete classes as Cube, Sphere and Cylinder. Abstract base classes are too generic to define real objects; we need to be more specific before we can think of instantiating objects. For example, if someone tells you to draw the shape, what shape would you draw? Concrete classes provide the specifics that make it reasonable to instantiate objects. A class is made abstract by declaring it with keyword abstract. An inheritance hierarchy does not need to contain any abstract classes, but, as we will see, many good object- oriented systems have class hierarchies headed by abstract base classes. In some cases, abstract classes constitute the top few levels of the hierarchy. A good example of this is the shape hierarchy in Fig. 9.3. The hierarchy begins with abstract base-class Shape. On the next level of the hierarchy, we have two more abstract base classes, namely TwoDimensionalShape and ThreeDimensionalShape. The next level of the hierarchy would define concrete classes for two-dimensional shapes, such as Circle and Square, and for three-dimensional shapes, such as Sphere and Cube. Software Engineering Observation 10.6 An abstract class defines a common set of public methods and properties for the various members of a class hierarchy. An abstract class typically contains one or more abstract methods and properties that derived classes will override. All classes in the hierarchy can use this common set of public methods and properties. Abstract classes must specify signatures for their abstract methods and properties. C# provides keyword abstract to declare a method or property as abstract. Methods and properties that are abstract do not provide implementations attempting to do so is a
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Chapter 10 Object-Oriented Programming: Polymorphism 391 Testing and Debugging Tip 10.1 An interesting consequence of using polymorphism is that programs take on a simplified appearance. They contain less branching logic and more simple, sequential code. This simplification facilitates testing, debugging and program maintenance. 10.4 Polymorphism Examples In this section, we discuss several examples of polymorphism. If class Rectangle is derived from class Quadrilateral, then a Rectangle object is a more specific version of a Quadrilateralobject. Any operation (such as calculating the perimeter or the area) that can be performed on an object of class Quadrilateral also can be performed on an object of class Rectangle. Such operations also can be performed on other kinds of Quadrilaterals, such as Squares, Parallelograms and Trapezoids. When a program invokes a derived-class method through a base-class (i.e., Quadrilateral) reference, C# polymorphically chooses the correct overriding method in the derived class from which the object was instantiated. We investigate this behavior in later examples. Suppose that we design a video game that manipulates objects of many different types, including objects of classes Martian, Venutian, Plutonian, SpaceShip and LaserBeam. Also, imagine that each of these classes inherits from the common base class called SpaceObject, which contains method DrawYourself. Each derived class implements this method. A screen-manager program would maintain a container (such as a SpaceObject array) of references to objects of the various classes. To refresh the screen, the screen manager would periodically send each object the same message namely, DrawYourself. However, each object responds in a unique way. For example, a Martian object would draw itself in red with the appropriate number of antennae. A SpaceShip object would draw itself as a bright, silver flying saucer. A LaserBeam object would draw itself as a bright red beam across the screen. Thus the same message sent to a variety of objects would have many forms of results hence the term polymorphism. A polymorphic screen manager facilitates adding new classes to a system with minimal modifications to the system s code. Suppose we want to add class Mercurians to our video game. To do so, we must build a class Mercurian that inherits from SpaceObject, but provides its own definition of method DrawYourself. Then, when objects of class Mercurian appear in the container, the programmer does not need to modify the code for the screen manager. The screen manager invokes method Draw- Yourself on every object in the container, regardless of the object s type, so the new Mercurian objects simply plug right in. Thus, without modifying the system (other than to build and include the classes themselves), programmers can use polymorphism to include additional types of classes that were not envisioned when the system was created. With polymorphism, one method can cause different actions to occur, depending on the type of the object on which the method is invoked. This gives the programmer tremendous expressive capability. In the next several sections, we provide examples that demonstrate polymorphism. Software Engineering Observation 10.4 With polymorphism, the programmer can deal in generalities and let the execution-time environment concern itself with the specifics. The programmer can command a wide variety of objects to behave in manners appropriate to those objects, even if the programmer does not know the objects types.
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Chapter 10 Object-Oriented Programming: Polymorphism 391 Testing and Debugging Tip 10.1 An interesting consequence of using polymorphism is that programs take on a simplified appearance. They contain less branching logic and more simple, sequential code. This simplification facilitates testing, debugging and program maintenance. 10.4 Polymorphism Examples In this section, we discuss several examples of polymorphism. If class Rectangle is derived from class Quadrilateral, then a Rectangle object is a more specific version of a Quadrilateralobject. Any operation (such as calculating the perimeter or the area) that can be performed on an object of class Quadrilateral also can be performed on an object of class Rectangle. Such operations also can be performed on other kinds of Quadrilaterals, such as Squares, Parallelograms and Trapezoids. When a program invokes a derived-class method through a base-class (i.e., Quadrilateral) reference, C# polymorphically chooses the correct overriding method in the derived class from which the object was instantiated. We investigate this behavior in later examples. Suppose that we design a video game that manipulates objects of many different types, including objects of classes Martian, Venutian, Plutonian, SpaceShip and LaserBeam. Also, imagine that each of these classes inherits from the common base class called SpaceObject, which contains method DrawYourself. Each derived class implements this method. A screen-manager program would maintain a container (such as a SpaceObject array) of references to objects of the various classes. To refresh the screen, the screen manager would periodically send each object the same message namely, DrawYourself. However, each object responds in a unique way. For example, a Martian object would draw itself in red with the appropriate number of antennae. A SpaceShip object would draw itself as a bright, silver flying saucer. A LaserBeam object would draw itself as a bright red beam across the screen. Thus the same message sent to a variety of objects would have many forms of results hence the term polymorphism. A polymorphic screen manager facilitates adding new classes to a system with minimal modifications to the system s code. Suppose we want to add class Mercurians to our video game. To do so, we must build a class Mercurian that inherits from SpaceObject, but provides its own definition of method DrawYourself. Then, when objects of class Mercurian appear in the container, the programmer does not need to modify the code for the screen manager. The screen manager invokes method Draw- Yourself on every object in the container, regardless of the object s type, so the new Mercurian objects simply plug right in. Thus, without modifying the system (other than to build and include the classes themselves), programmers can use polymorphism to include additional types of classes that were not envisioned when the system was created. With polymorphism, one method can cause different actions to occur, depending on the type of the object on which the method is invoked. This gives the programmer tremendous expressive capability. In the next several sections, we provide examples that demonstrate polymorphism. Software Engineering Observation 10.4 With polymorphism, the programmer can deal in generalities and let the execution-time environment concern itself with the specifics. The programmer can command a wide variety of objects to behave in manners appropriate to those objects, even if the programmer does not know the objects types.
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