protected String url(int zoomLevel,int xTile,int yTile){
  StringBuilder sb=new StringBuilder(_url);
  sb.append("/");
  sb.append(zoomLevel);
  sb.append("/");
  sb.append(xTile);
  sb.append("/");
  sb.append(yTile);
  sb.append(".png");
  return sb.toString();
}

protected String url ( int zoomLevel , int xTile , int yTile ) { StringBuilder sb = new StringBuilder ( _url ) ; sb . append ( "/" ) ; sb . append ( zoomLevel ) ; sb . append ( "/" ) ; sb . append ( xTile ) ; sb . append ( "/" ) ; sb . append ( yTile ) ; sb . append ( ".png" ) ; return sb . toString ( ) ; }
build a url request for a tile
