Trying to understand this code

1

I’m working on creating a liquid Flash layout and I found one tutorial that seems pretty helpful at http://www.adobe.com/devnet/flash/articles/liquid_gui_04.html

But can someone help me understand part of this code? My question about the code is down below the code

inside BodyUI Class


    ....
    public static var _instance:BodyUI;
    public var bg:Sprite;
    private var _parent:Sprite;
    public function BodyUI (p:Sprite)
    {
      _parent = p;
      bg = new BodyBG ();
      addChild (bg);
    }
    public static function getInstance (p:Sprite):BodyUI
    {
      if (_instance == null)
        _instance = new BodyUI (p);
      return _instance;
    }....

then inside LiquidGUI_v3 Class

public function LiquidGUI_v3 ()
    {
      // Tell the player not to scale assets
      stage.scaleMode = StageScaleMode.NO_SCALE;
      // Tell the player to put coords 0,0 to the top left corner
      stage.align = StageAlign.TOP_LEFT;
      // Listen for resizing events
      stage.addEventListener(Event.RESIZE, onResize);
      // Create and add the Header UI
      _header = HeaderUI.getInstance (this);
      addChild (_header);
      // Create and add the Body UI
      _body = BodyUI.getInstance (this);
      addChild (_body);
      _body.y = _header.height;
      // Create and add the Footer UI
      _footer = FooterUI.getInstance (this);
      addChild (_footer);
      // Size everything after creation to ensure the app is drawn
      // properly the first time it is seen prior to any user-
      // initiated resizing
      onResize (null);
    }

my question is: why is there a static variable called “_instance” in the BodyUI class? I see that in the LiquidGUI class the _body is actually retrieved by using the _instance variable in BodyUI…but why not just say

_body = new BodyUI()

instead of saying

_body = BodyUI.getInstance()?

I just don’t understand how it makes sense to have a static variable here…it seems to be short-circuiting the OOP principles, not extending them.

Thanks