What's the significance of “as” keyword in this code

问题

package
{
    import flash.display.Sprite;
    import flash.media.Sound;
    import flash.media.SoundChannel;

    public class EmbeddedSoundExample extends Sprite
    {
        [Embed(source="smallSound.mp3")]
        public var soundClass:Class;

        public function EmbeddedSoundExample()
        {
          //WHAT DOES "as" keyword DO IN THE FOLLOWING LINE ??
          //*************************************************

            var smallSound:Sound = new soundClass() as Sound;

            //COULD BE WRITTEN AS : 
            //==>>>> var smallSound:Sound = new soundClass() ???
            // OR
            ////==>>>> var smallSound:Sound = new Sound() ???
         //******************************************************

            smallSound.play();
        }
    }
}

回答1:

This is a casting operator introduced in ActionScript 3. The difference with as (as opposed to Type(object) casting) is that if the cast fails, the result will be the default value for the type. More on this here

回答2:

as casting, use the data like the Type that your are using, no change the data. if you use Type(object) and you can't convert to this Type, return a default value( null,zero, or undefined)

来源：https://stackoverflow.com/questions/12390347/whats-the-significance-of-as-keyword-in-this-code