Lesson 24: Polymorphism

Introduction

When you are creating a class, if you anticipate that a certain member should be redefined in a derived class, you can indicate this in your code. On the other hand, while creating a class that is based on another, if you find out that you are customizing a member that already exists in the base class, you should indicate, in your code, that you are providing a new version. In both cases, the common member is said to be a virtual member of the class.

Practical Learning: Introcing Abstraction

Save the following picture somewhere on your computer and return to your programming environment:
Start Microsoft Visual Studio
On the main menu, click File -> New -> Project...
In the middle list, click ASP.NET Web Application (.NET Framework) and, in the Name text box, replace the name with Geometry12
Click OK
In the New ASP.NET Web Application dialog box, make sure Empty is hilighted and click OK
In the Solution Explorer, right-click Geometry12 -> Add -> Folder
Type Images and press Enter
Add the above picture to the images folder
In the Solution Explorer, right-click Geometry12, position the mouse on Add, position the mouse on Add ASP.NET Folder, and click App_Code
In the Solution Explorer, right-click App_Code -> Add -> Class...
Change the file name to Square
Click Add

Change the document as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Geometry12.App_Code
{
    public class Square
    {
        public double Side { get; set; }

        public Square(double side)
        {
            Side = side;
        }

        public double Perimeter
        {
            get
            {
                return Side * 4.00;
            }
        }
    }
}

Virtual Methods

A method is said to be virtual if it is anticipated to have different versions or implementations in classes that derive from it. To create a virtual method, in the base class, type the virtual keyword to the left of the return type of the method. If the method has an access level (private, public, or internal), the virtual keyword can appear before or after the access level.

Practical Learning: Creating Virtual Methods

In the Square class, create two virtual methods as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Geometry12.App_Code
{
    public class Square
    {
        public double Side { get; set; }

        public Square(double side)
        {
            Side = side;
        }

        public double Perimeter
        {
            get
            {
                return Side * 4.00;
            }
        }

        public virtual double CalculateInradius()
        {
            return Side / 2.00;
        }

        public virtual double CalculateCircumradius()
        {
            return Math.Sqrt(2.00) * Side / 2.00;
        }
    }
}

Virtual Properties

A property of a class is virtual if it may be implemented in a derived class. This means that the property should primarily be created in its original class.

To get a virtual property, when creating the class, add the virtual keyword before the return type of the property.

Practical Learning: Creating a Virtual Property

Create a virtual property in the Square class as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Geometry12.App_Code
{
    public class Square
    {
        public double Side { get; set; }

        public Square(double side)
        {
            Side = side;
        }

        public double Perimeter
        {
            get
            {
                return Side * 4.00;
            }
        }

        public virtual double Area
        {
            get
            {
                return Side * Side;
            }
        }
        
        public virtual double CalculateInradius()
        {
            return Side / 2.00;
        }

        public virtual double CalculateCircumradius()
        {
            return Math.Sqrt(2.00) * Side / 2.00;
        }
    }
}

In the Solution Explorer, right-click the name of the project -> Add -> New Item...
In the left list, expand Web, and click Razor
In the middle list, click Web Page (Razor)
Change the name to Index
Press Enter

Change the code as follows:

<!DOCTYPE html>
<html>
<head>
<style>
.container {
    margin: auto;
    width: 550px;
}

.col-empty {
    width: 40px
}

.col-format {
    width: 120px
}
</style>
<title>Geometry - Square</title>
</head>
<body>
<h2 style="text-align: center">Geometry - Square</h2>
<p style="text-align: center"><img src="~/Images/Square.png" alt="Geometry - Square" width="483" height="305" /></p>

@{ 
    double side = 0.00;
    Geometry12.App_Code.Square sqr = new Geometry12.App_Code.Square(0.00);

    if (IsPost)
    {
        side = Convert.ToDouble(Request["txtSide"]);
        sqr = new Geometry12.App_Code.Square(side);
    }
}
<div class="container">
    <form name="frmGeometry" method="post">
        <table>
            <tr>
                <td style="width: 80px">Side:</td>
                <td><input type="text" name="txtSide" style="width: 60px" value="@side" /></td>
                <td><input type="submit" name="btnCalculate" value="Calculate" /></td>
                <td>Perimter:</td>
                <td><input type="text" name="txtPerimter" value="@sqr.Perimeter" /></td>
            </tr>
            <tr>
                <td>&nbsp;</td>
                <td>&nbsp;</td>
                <td>&nbsp;</td>
                <td>Area:</td>
                <td><input type="text" name="txtArea" value="@sqr.Area" /></td>
            </tr>
            <tr>
                <td>&nbsp;</td>
                <td>&nbsp;</td>
                <td>&nbsp;</td>
                <td>Inscribed Radius:</td>
                <td><input type="text" name="txtInscribedRadius" value="@sqr.CalculateInradius()" /></td>
            </tr>
            <tr>
                <td>&nbsp;</td>
                <td>&nbsp;</td>
                <td>&nbsp;</td>
                <td>Circumscribed Radius:</td>
                <td><input type="text" name="txtCircumscribedRadius" value="@sqr.CalculateCircumradius()" /></td>
            </tr>
        </table>
    </form>
</div>
</body>
</html>

To execute the application to test the webpage, press Ctrl + F5:
In the Side text box, type a number such as 429.63:
Click the Calculate button:
Close the browser and return to your programming environment
Save the following picture somewhere on your computer and return to your programming environment:
On the main menu of Microsoft Visual Studio, click File -> New -> Project...
In the middle list, click ASP.NET Web Application (.NET Framework) and, in the Name text box, replace the name with Geometry13
Click OK
In the New ASP.NET Web Application dialog box, click Empty and click OK
In the Solution Explorer, right-click Geometry13, position the mouse on Add, position the mouse on Add ASP.NET Folder, and click App_Code
In the Solution Explorer, right-click Geometry13 -> Add -> Folder
Type Images and press Enter
Add the above picture to the images folder
In the Solution Explorer, right-click App_Code -> Add -> Class...
Change the name of the file to Trapezoid
Click Add

Change the class as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Geometry13.App_Code
{
    public class Trapezoid
    {
        public double ShortSide { get; set; }
        public double LongSide { get; set; }
        public double Edge { get; set; }

        public Trapezoid(double sSide, double lSide, double edge)
        {
            ShortSide = sSide;
            LongSide  = lSide;
            Edge      = edge;
        }

        public double Perimeter => LongSide + Edge + ShortSide + Edge;

        // http://mathworld.wolfram.com/IsoscelesTrapezoid.html
        public double Height
        {
            get
            {
                return Math.Sqrt((Edge * Edge) - (Math.Pow(LongSide - ShortSide, 2.00) / 4.00));
            }
        }

        virtual public double CalculateArea()
        {
            return (LongSide + ShortSide) * Height / 2.00;
        }
    }
}

In the Solution Explorer, right-click the name of the project -> Add -> New Item...
In the left list, expand Web, and click Razor
In the middle list, click Web Page (Razor)
Change the name to Index
Press Enter

Change the code as follows:

<!DOCTYPE html>
<html>
<head>
<title>Geometry - Trapezoid</title>
<style>
.container {
    margin: auto;
    width: 620px;
}
</style>
</head>
<body>
@{
    double shortSide = 0.00;
    double longSide  = 0.00;
    double border = 0.00;
    Geometry13.App_Code.Trapezoid trap = new Geometry13.App_Code.Trapezoid(sSide : 0.00, lSide : 0.00, edge : 0.00);

    if (IsPost)
    {
        shortSide = Convert.ToDouble(Request["txtShortSide"]);
        longSide  = Convert.ToDouble(Request["txtLongSide"]);
        border    = Convert.ToDouble(Request["txtEdge"]);

        trap = new Geometry13.App_Code.Trapezoid(sSide : shortSide, lSide : longSide, edge: 0.00);
    }
}

<div class="container">
    <h2 style="text-align: center">Geometry - Trapezoid</h2>

    <form name="frmGeometry" method="post">
            <table>
                <tr>
                    <td style="width: 375px" rowspan="9">
                        <img src="~/images/trapezoid2.png" width="373" height="273" alt="Geometry - Trapezoid">
                    </td>
                    <td style="width: 150px">Short Side:</td>
                    <td><input type="text" name="txtShortSide" value="@shortSide" /></td>
                </tr>
                <tr>
                    <td>Long Side:</td>
                    <td><input type="text" name="txtLongSide" value="@longSide" /></td>
                </tr>
                <tr>
                    <td>Edge:</td>
                    <td><input type="text" name="txtEdge" value="@border" /></td>
                </tr>
                <tr>
                    <td>&nbsp;</td>
                    <td style="text-align: center"><input type="submit" name="btnSubmit" value="Calculate" /></td>
                </tr>
                <tr>
                    <td>Height:</td>
                    <td><input type="text" name="txtHeight" value="et.Height" /></td>
                </tr>
                <tr>
                    <td>Perimeter:</td>
                    <td><input type="text" name="txtPerimeter" value="et.Inradius" /></td>
                </tr>
                <tr>
                    <td>Area:</td>
                    <td><input type="text" name="txtArea" value="et.Area" /></td>
                </tr>
            </table>
    </form>
</div>
</body>
</html>

To execute the project, on the main menu, click Debug -> Start Without Debugging:
In the Short Side text box, enter a number such as 137.96
In the Long Side text box, enter a number such as 209.73
In the Edge text box, enter a number such as 87.59
Click the Calculate button:
Close the browser and return to your programming environment
On the main menu of Microsoft Visual Studio, click File -> New -> Project...
Make sure ASP.NET Web Application (.NET Framework) is selected in the middle list and, in the Name text box, replace the name with Geometry14
Click OK
In the New ASP.NET Web Application dialog box, make sure Empty is selected and press Enter
In the Solution Explorer, right-click the name of the project -> Add -> Add ASP.NET Folder -> App_Code
In the Solution Explorer, right-click the name of the project -> Add -> Folder
Type Images and press Enter
Add the tp.png picture from the previous lesson to the images folder of this project
In the Solution Explorer, right-click App_Code -> Add -> Class...
Change the name of the file to Trapezoid
Click Add
Fill the class as follows:
On the main menu, click Project -> Add Class...
Set the name of the class as Trapezoid and click Add

Fill the class as follows:

using static System.Math;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Geometry14.App_Code
{
    public class Trapezoid
    {
        public double ShortSide { get; set; }
        public double LongSide { get; set; }
        public double Edge { get; set; }

        public Trapezoid(double sSide, double lSide, double side)
        {
            ShortSide = sSide;
            LongSide = lSide;
            Edge = side;
        }

        public double Perimeter => LongSide + Edge + ShortSide + Edge;

        // http://mathworld.wolfram.com/IsoscelesTrapezoid.html
        public double Height
        {
            get
            {
                return Sqrt((Edge * Edge) - (Pow(LongSide - ShortSide, 2.00) / 4.00));
            }
        }

        virtual public double CalculateArea()
        {
            return (LongSide + ShortSide) * Height / 2.00;
        }
    }
}

In the Solution Explorer, right-click App_Code -> Add -> Class...
Set the name of the class as TrapezoidalPrism
Click Add

Change the class as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Geometry14.App_Code
{
    public class TrapezoidalPrism : Trapezoid
    {
        public double Length { get; set; }

        public TrapezoidalPrism(double sSide, double lSide, double side, double len)
            : base(sSide, lSide, side)
        {
            Length = len;
        }

        public double BaseArea => base.CalculateArea();

        public double Volume => BaseArea * Length;
    }
}

Overriding a Member in a Derived Class

As seen in previous sections, you can create a virtual method or property in a class and use it like any other method or property of its class. As mentioned in our introduction, the essence of creating a virtual method or property to provide a different implementation of that member in its own and in classes that derive from that class. Providing a different version of a member or property in a derived class is referred to as overriding the method or property.

To override a method, in a derived class, create a method with the same syntax (same return type, same name, and same parameter(s) if any) as the method in the parent class. This time, replace the virtual keyword with the override keyword. In the same way, to override a propertty, create one of the same return type and name in the derived class but use the override keyword instead of the the virtual keyword.

Practical Learning: Overriding a Method in a Derived Class

In the TrapezoidalPrism class, override the CalculateArea() method as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Geometry14.App_Code
{
    public class TrapezoidalPrism : Trapezoid
    {
        public double Length { get; set; }

        public TrapezoidalPrism(double sSide, double lSide, double side, double len)
            : base(sSide, lSide, side)
        {
            Length = len;
        }

        public double BaseArea => base.CalculateArea();

        override public double CalculateArea()
        {
            double shortArea = ShortSide * Length;
            double longArea = LongSide * Length;
            double sideArea = Edge * Length;
            return (BaseArea * 2.00) + shortArea + longArea + (sideArea * 2.00);
        }

        public double Volume => BaseArea * Length;
    }
}

In the Solution Explorer, right-click the name of the project -> Add -> New Item...
In the left list, expand Web, and click Razor
In the middle list, click Web Page (Razor)
Change the name to Index
Press Enter

Change the code as follows:

<!DOCTYPE html>
<html>
<head>
<style>
.container {
    margin: auto;
    width: 650px;
}
</style>
<title>Geometry - Trapezoidal Prism</title>
</head>
<body>
@{
    double shortSide = 0.00;
    double longSide = 0.00;
    double edge = 0.00;
    double length = 0.00;

    Geometry14.App_Code.TrapezoidalPrism tp = new Geometry14.App_Code.TrapezoidalPrism(shortSide, longSide, edge, length);

    if (IsPost)
    {
        shortSide = Convert.ToDouble(Request["txtShortSide"]);
        longSide = Convert.ToDouble(Request["txtLongSide"]);
        edge = Convert.ToDouble(Request["txtEdge"]);
        length = Convert.ToDouble(Request["txtLength"]);

        tp = new Geometry14.App_Code.TrapezoidalPrism(shortSide, longSide, edge, length);
    }
}

<div class="container">
    <h2 style="text-align: center">Geometry - Trapezoid</h2>

    <form name="frmGeometry" method="post">
        <table>
            <tr>
                <td>
                    <img src="~/images/tp.png" width="373" height="273" alt="Geometry - Trapezoid"></td>
                <td>
                    <p>Base</p>
                    <table>
                        <tr>
                            <td style="width: 20px">&nbsp;</td>
                            <td style="width: 120px">Short Side:</td>
                            <td><input type="text" name="txtShortSide" value="@shortSide" /></td>
                        </tr>
                        <tr>
                            <td>&nbsp;</td>
                            <td>Long Side:</td>
                            <td><input type="text" name="txtLongSide" value="@longSide" /></td>
                        </tr>
                        <tr>
                            <td>&nbsp;</td>
                            <td>Edge:</td>
                            <td><input type="text" name="txtEdge" value="@edge" /></td>
                        </tr>
                    </table>
                    <p>Prism</p>
                    <table>
                        <tr>
                            <td style="width: 20px">&nbsp;</td>
                            <td style="width: 120px">Length:</td>
                            <td><input type="text" name="txtLength" value="@length" /></td>
                        </tr>
                    </table>

                    <p style="text-align: center"><input type="submit" name="btnSubmit" value="Calculate" /></p>

                    <p>Base</p>

                    <table>
                        <tr>
                            <td style="width: 20px">&nbsp;</td>
                            <td style="width: 120px">Height:</td>
                            <td><input type="text" name="txtHeight" value="@tp.Height" /></td>
                        </tr>
                        <tr>
                            <td>&nbsp;</td>
                            <td>Perimeter:</td>
                            <td><input type="text" name="txtPerimeter" value="@tp.Perimeter" /></td>
                        </tr>
                        <tr>
                            <td>&nbsp;</td>
                            <td>Base Area:</td>
                            <td><input type="text" name="txtBaseArea" value="@tp.BaseArea" /></td>
                        </tr>
                    </table>

                    <p>Prism</p>

                    <table>
                        <tr>
                            <td style="width: 20px">&nbsp;</td>
                            <td style="width: 120px">Total Area:</td>
                            <td><input type="text" name="txt>TotalArea" value="@tp.CalculateArea()" /></td>
                        </tr>
                        <tr>
                            <td>&nbsp;</td>
                            <td>Volume:</td>
                            <td><input type="text" name="txtVolume" value="@tp.Volume" /></td>
                        </tr>
                    </table>
                </td>
            </tr>
        </table>
    </form>
</div>
</body>
</html>

To execute the project, on the main menu, click Debug -> Start Without Debugging:
In the Short Side text box, enter a number such as 229.47
In the Long Side text box, enter a number such as 167.66
In the Edge text box, enter a number such as 98.59
In the Length text box, enter a number such as 214.85
Click the Calculate button:
Close the browser and return to your programming environment
Close your programming environment


Previous	Copyright © 2002-2019, FunctionX	Next