ActionScript3.0 円を動かす

// Main.as
package{
  import flash.display.Sprite;
  import flash.events.MouseEvent;

[SWF(width="300", height="200", backgroundColor="0xFFFFFF")]

  public class Main extends Sprite
  {
    private const n:int = 50;
    private var clicked_flag:int = -1;
    private    var c:Array;
  
    public function Main()
    {
      c = new Array();
      for(var i:int = 0; i < n; i++)
      {
        c[i] = new Circle1();
        c[i].draw();

        addChild(c[i]);
      }

      stage.addEventListener(MouseEvent.MOUSE_MOVE, onMouseMove);
      stage.addEventListener(MouseEvent.MOUSE_DOWN, onMouseDown);
      stage.addEventListener(MouseEvent.MOUSE_UP, onMouseUp); 
    }

    private function onMouseMove(event:MouseEvent):void
    {
      if(clicked_flag != -1)
      {
        c[clicked_flag].erace();
        c[clicked_flag].setX(stage.mouseX);
        c[clicked_flag].setY(stage.mouseY);
        c[clicked_flag].draw();
      }
    }
  
    private function onMouseDown(event:MouseEvent):void
    {
      for(var i:int = 0; i < n; i++){
        if(c[i].hitTestPoint(stage.mouseX, stage.mouseY, true))
        {
          clicked_flag = i;
          addChildAt(c[i], n-1);
        }
      }
    }
  
    private function onMouseUp(event:MouseEvent):void
    {
      clicked_flag = -1;
    }
  }
}

// Circle1.as
package
{
  import flash.display.Shape;
  import flash.display.Graphics;
    
  public class Circle1 extends Shape
  {
    private const w:int = 300;  // 画面の幅
    private const h:int = 200;  // 画面の高さ
    private const r:int = 10;   // 円の半径
    private var px:int;
    private var py:int;
    private var g:Graphics;

    public function Circle1()
    {
      this.px = getRandom(r, this.w - this.r - 1);
      this.py = getRandom(r, this.h - this.r - 1);
      this.g = this.graphics;
    }

    public function setX(px:int):void { this.px = px; }
    public function setY(py:int):void { this.py = py; }
    public function getX():int { return this.px; }
    public function getY():int { return this.py; }

    public function draw():void
    {
      g.lineStyle(1, 0x000000, 1.0);   // 線の太さ、色、アルファ値
      g.beginFill(0xFFFFFF, 1.0);      // 面のスタイル設定
      g.drawCircle(this.getX(), this.getY() , this.r);
      g.endFill();
    }

    public function erace():void {
      g.clear();
    }

    private function getRandom(nMin:int, nMax:int):int {
      return Math.floor(Math.random() * (nMax-nMin + 1)) + nMin;
    }
  }
}