interfaceでクラスの型指定

var a:hogeClass = new ClassA();
var b:hogeClass = new ClassB(); 

package 
{
	public interface hogeClass 
	{
		function commonFumc(val:String,num:Number):void
	}
	
}
package{
	public class  ClassA implements  hogeClass
	{
        }
}
package{
	public class  ClassB implements  hogeClass
	{
        }
}

BetweenAS3の勉強がてら書いたコード
まずinterface定義

package 
{
	import flash.display.MovieClip;
	public interface IBetween  
	{
		function startTween(target:MovieClip,param:Object=null):void
	}
	
}

んでIBetweenをimplementsしたclasssを2つほど

package 
{
	import flash.display.MovieClip;
	import org.libspark.betweenas3.BetweenAS3;
	import org.libspark.betweenas3.tweens.ITween;
	import org.libspark.betweenas3.easing.*;

	public class  Tween1 implements IBetween
	{
		public function Tween1() {
		}
		public function startTween(mc:MovieClip, param:Object = null):void {
			var nColor:int = 255;
			if (param) {
				nColor = param.nColor
			} 
			BetweenAS3.tween(mc, 
				{
					transform: {
						colorTransform: {
							redOffset: nColor
						}
					}
				}, null, 2.0, Cubic.easeOut
			).play();
		}
	}
	
}

package 
{
	import flash.display.MovieClip;
	import org.libspark.betweenas3.BetweenAS3;
	public class  Tween2 implements IBetween
	{
		private var _nx:Number=250;
		public function Tween2() {
		}
		private function setPos(mc:MovieClip,dx:Number, nx:Number):void{
			_nx = nx;
			mc.x = dx;
		}
		public function startTween(mc:MovieClip, param:Object = null):void {
				if (param) {
					setPos(mc, param.dx, param.nx )
				}
			   BetweenAS3.tween(mc, {x: _nx}).play();
		}
	}
	
}

として

			var id:int = Math.floor(Math.random() * 2);
			trace(id);
			var cTween:IBetween 
			switch(id) {
				case 0:
					cTween = new Tween1();
					cTween.startTween( _mc,{nColor:Math.ceil(Math.random() * 255)} );
				break;
				case 1:
					cTween = new Tween2();
					cTween.startTween( _mc,{dx:10, nx:220} );				
				break;
}

みたいな感じで呼んであげるとかいいかなあと。 この本参考にしたんだけどAmazonレビューでは誤字が多いといわれてますな。 考えかたとかは結構勉強になっていいんだけどねえ。