how can i run this code correctly?

hello, everybody:

    when  i run the code, i found that it dons't work,so ,i  hope somebody can help me!

    the Ball class is this :
    package {
    import flash.display.Sprite;
    [SWF(width = "550", height = "400")]
    public class Ball extends Sprite {
        private var radius:Number;
        private var color:uint;
        private var vx:Number;
        private var vy:Number;
        public function Ball(radius:Number=40, color:uint=0xff9900,
        vx:Number =0, vy:Number =0) {
            this.radius= radius;
            this.color = color;
            this.vx = vx;
            this.vy = vy;
            init();
        }
        private function init():void {
            graphics.beginFill(color);
            graphics.drawCircle(0,0,radius);
            graphics.endFill();
        }
    }
}

and the Chain class code is :

  
package {
    import flash.display.Sprite;
    import flash.events.Event;
    [SWF(width = "550", height = "400")]
   
    public class Chain extends Sprite {
        private var ball0:Ball;
        private var ball1:Ball;
        private var ball2:Ball;
        private var spring:Number = 0.1;
        private var friction:Number = 0.8;
        private var gravity:Number = 5;
        //private var vx:Number =0;
        //private var vy:Number = 0;
       
        public function Chain() {
            init();
        }
       
        public function init():void {
            ball0  = new Ball(20);
            addChild(ball0);
            ball1 = new Ball(20);
            addChild(ball1);
            ball2 = new Ball(20);
            addChild(ball2);
            addEventListener(Event.ENTER_FRAME, onEnterFrame);
        }
       
        private function onEnterFrame(event:Event):void {
            moveBall(ball0, mouseX, mouseY);
            moveBall(ball1, ball0.x, ball0.y);
            moveBall(ball2, ball1.x, ball1.y);
           
            graphics.clear();
            graphics.lineStyle(1);
            graphics.moveTo(mouseX, mouseY);
            graphics.lineTo(ball0.x, ball0.y);
            graphics.lineTo(ball1.x, ball1.y);
            graphics.lineTo(ball2.x, ball2.y);
           
        }
       
        private function moveBall(ball:Ball,
                                  targetX:Number,
                                  targetY:Number):void {
          
            ball.vx += (targetX - ball.x) * spring;
            ball.vy += (targetY - ball.y) * spring;
            ball.vy += gravity;
            ball.vx *= friction;
            ball.vy *= friction;
            ball.x += vx;
            ball.y += vy;
        }
    }
}

thanks every body's help!