Another primary concept of object-oriented programming is Polymorphism. It allows you to invoke derived class methods through a base class reference during run-time. This is handy when you need to assign a group of objects to an array and then invoke each of their methods. They won't necessarily have to be the same object type. However, if they're related by inheritance, you can add them to the array as the inherited type. Then if they all share the same method name, that method of each object can be invoked. This lesson will show you how to accomplish this.

using System;

public class DrawingObject
{
    public virtual void Draw()
    {
        Console.WriteLine("I'm just a generic drawing object.");
    }
}

The code shows the DrawingObject class. This will be the base class for other objects to inherit from. It has a single method named Draw(). The Draw() method has a virtual modifier. The virtual modifier indicates to derived classes that they can override this method. The Draw() method of the DrawingObject class performs a single action of printing the statement, "I'm just a generic drawing object.", to the console.

using System;

public class Line : DrawingObject
{
    public override void Draw()
    {
        Console.WriteLine("I'm a Line.");
    }
}

public class Circle : DrawingObject
{
    public override void Draw()
    {
        Console.WriteLine("I'm a Circle.");
    }
}

public class Square : DrawingObject
{
    public override void Draw()
    {
        Console.WriteLine("I'm a Square.");
    }
}

The code shows three classes. These classes inherit the DrawingObject class. Each class has a Draw() method and each Draw() method has an override modifier. The override modifier allows a method to override the virtual method of its base class at run-time. The override will happen only if the class is referenced through a base class reference. Overriding methods must have the same signature, name and parameters, as the virtual base class method it is overriding.

using System;

public class DrawDemo
{
    public static int Main( )
    {
        DrawingObject[] dObj = new DrawingObject[4];

        dObj[0] = new Line();
        dObj[1] = new Circle();
        dObj[2] = new Square();
        dObj[3] = new DrawingObject();

        foreach (DrawingObject drawObj in dObj)
        {
            drawObj.Draw();
        }

        return 0;
    }
}

The Code shows a program that uses the classes defined in Listing 9-1 and Listing 9-2. This program implements polymorphism. In the Main() method of the DrawDemo class, there is an array being created. The type of object in this array is the DrawingObject class. The array is named dObj and is being initialized to hold four objects of type DrawingObject.

Next the dObj array is initialized. Because of their inheritance relationship with the DrawingObject class, the Line, Circle, and Square classes can be assigned to the dObj array. Without this capability, you would have to create an array for each type. Inheritance allows derived objects to act like their base class, which saves work.

After the array is initialized, there is a foreach loop that looks at each element of the array. Within the foreach loop the Draw() method is invoked on each element of the dObj array. Because of polymorphism, the run-time type of each object is invoked. The type of the reference object from the dObj array is a DrawingObject. However, that doesn't matter because the derived classes override the virtual Draw() method of the DrawingObject class. This makes the overriden Draw() methods of the derived classes execute when the Draw() method is called using the DrawingObject base class reference from the dObj array. Here's what the output looks like:

We can easily create Facebook-Like textbox for leaving comments in an easy and cool way using jQuery, I am leaving an example below.

You can see an implementation working here

HTML

   <textarea class="comment_empty">Write a comment</textarea><br />
   <input type="submit" id="submit" value="Submit" style="display: none" />

jQuery

$(document).ready(function(){
    var submit = $("#submit");

    $("textarea").blur(function() {
        if ($(this).val() == "") {
            $(this).val("Write a comment")
                   .removeClass("comment_filled")
                   .addClass("comment_empty");
            submit.hide();
        }
    }).focus(function() {
        if ($(this).val() == "Write a comment") {
            $(this).val("")
                   .removeClass("comment_empty")
                   .addClass("comment_filled");
            submit.show();
        }
    });
});

CSS

.comment_empty {
   color: gray;
   height: 30px;
}

.comment_filled {
   color: black;
   height: 100px;
}

One of the most powerful JavaScript keywords is this. Unfortunately it is hard to use if you don't exactly know how it works.

Below I explain how to use it in event handling.

Owner

The question that we'll discuss for the remainder of the page is: What does this refer to in the function doSomething()?

 
function doSomething() {
   this.style.color = '#cc0000';
}

In JavaScript this always refers to the "owner" of the function we are executing, or rather, to the object that a function is a method of. When we define our faithful function doSomething() in a page, its owner is the page, or rather, the window object (or global object) of JavaScript. An onclick property, though, is owned by the HTML element it belongs to.

This "ownership" is the result of JavaScript's object oriented approach.

 
------------ window --------------------------------------
|                                          / \           |
|                                           |            |
|                                          this          |
|   ----------------                        |            |
|   | HTML element | <-- this         -----------------  |
|   ----------------      |           | doSomething() |  |
|               |         |           -----------------  |
|          --------------------                          |
|          | onclick property |                          |
|          --------------------                          |
|                                                        |
----------------------------------------------------------

If we execute doSomething() without any more preparation the this keyword refers to the window and the function tries to change the style.color of the window. Since the window doesn't have a style object the function fails miserably and produces JavaScript errors.

I hope you find them useful

using System.Globalization;

// gets a CurrentYear Julian Date (ddd) Returns DateTime
// ddd = day number
// 001 = January 1st Current Year
// 365 = December 25 Current Year
public DateTime parseJulianDate(String txt) {
    var year = DateTime.Now.Year;
    var days = int.Parse(txt);

    var time = new DateTime(year, 1, 1).AddDays(days - 1);
    
    return time.ToString("MM/dd/yyyy");
}

// gets a Current Year Julian Date (wwd) Returns DateTime
// ww = Week of the year
// d = Day of the week
// 011 = January First Current Year
public DateTime parseJulianWeeksDays(String txt) {
    var yearw = DateTime.Now.Year;
    var daysw = int.Parse(txt.Substring(2, 1));
    var weeksw = int.Parse(txt.Substring(0, 2));

    var cal = CultureInfo.InvariantCulture.Calendar;

    var timew = new DateTime(yearw, 1, 1);
    timew = cal.AddWeeks(timew, weeksw - 1);
    timew = cal.AddDays(timew, daysw - 1);
    
    return timew.ToString("MM/dd/yyyy");
}