Is this a good implementation of Polymorphism?

Question

This is working, but is this a good example of polymorphism?

package mtm.test 
{
    import flash.display.Bitmap;
    import flash.display.BitmapData;
    import flash.display.MovieClip;

    public class TestDocument extends MovieClip
    {
        public function TestDocument() 
        {
            var image:Image = new Image();
            image.id = 0;
            image.bitmaps[BitmapFullSize] = new BitmapFullSize(new BitmapData(400, 400, false, 0x000000));
            image.bitmaps[BitmapMediumSize] = new BitmapMediumSize(new BitmapData(250, 250, false, 0x000000));
            image.bitmaps[BitmapThumbnail] = new BitmapThumbnail(new BitmapData(65, 65, false, 0x000000));

            var bmp:BitmapType = image.bitmaps[BitmapFullSize];
            if(bmp != null) addChild(bmp);
        }
    }
}

Here are the classes used above:

package mtm.test 
{
    import flash.display.Bitmap;
    public dynamic class Image extends Object
    {
        public var id:int;
        public var enabled:int;
        public var orderNum:int;
        public var filename:String;
        public var title:String;
        public var description:String;

        public var medExists:int;
        public var fullExists:int;
        public var thumbExists:int;

        public var bitmaps:Object = new Object();
    }
}

BitmapType Parent Class

package mtm.test 
{
    import flash.display.Bitmap;
    import flash.display.BitmapData;
    public class BitmapType extends Bitmap
    {
        public function BitmapType(bitmapData:BitmapData = null, pixelSnapping:String = 'auto', smoothing:Boolean = false)
        {
            super(bitmapData, pixelSnapping, smoothing);
        }
    }
}

Sub-Classes of BitmapType

package mtm.test 
{
    import flash.display.BitmapData;
    public class BitmapFullSize extends BitmapType 
    { 
        public function BitmapFullSize(bitmapData:BitmapData = null, pixelSnapping:String = 'auto', smoothing:Boolean = false)
        {
            super(bitmapData, pixelSnapping, smoothing);
        }
    }
}

package mtm.test 
{
    import flash.display.BitmapData;
    public class BitmapMediumSize extends BitmapType 
    { 
        public function BitmapMediumSize(bitmapData:BitmapData = null, pixelSnapping:String = 'auto', smoothing:Boolean = false)
        {
            super(bitmapData, pixelSnapping, smoothing);
        }
    }
}

package mtm.test 
{
    import flash.display.BitmapData;
    public class BitmapThumbnail extends BitmapType 
    {
        public function BitmapThumbnail(bitmapData:BitmapData = null, pixelSnapping:String = 'auto', smoothing:Boolean = false)
        {
            super(bitmapData, pixelSnapping, smoothing);
        }
    }
}

Answer 1

It's not really Polymorphism if the children are just calling their parent implementation. Polymorphism is if each subtype has its own implementation of some method. So if you had a Resize() method that employed different a different algorithm based on each type, then that would be polymorphism.

Your code for each of the three subtypes is identical.

---

Here is a classical example: Base class is called Vehicle
Subclasses are Car,Boat,Airplane.

Each of Car,Boat,Airplane would have a different Travel method.

Then you could say:

Vehicle myVehicle = new Boat();
myVehicle.Travel();

That's polymorphism.