图像不在blackberry中的verticalfieldmanager中显示

Question

我将两个BitmapField和两个LabelField添加到垂直管理器中，但第一个BitmapField不可见。当我不添加任何两个字段时，第一个BitmapField变得可见。我错了什么代码如下

gridbackManager=new VerticalFieldManager(VerticalFieldManager.NO_HORIZONTAL_SCROLL|VerticalFieldManager.VERTICAL_SCROLL){
      protected void paintBackground(Graphics graphics)
      {  

                int c = graphics.getColor();
                //graphics.setColor(Color.WHITESMOKE);
                graphics.drawBitmap(0, 0, backbitmap.getWidth(), backbitmap.getHeight(),backbitmap,0,0);

                //graphics.setColor(c);

        super.paintBackground(graphics);
      }
      protected void sublayout( int maxWidth, int maxHeight )
      {
          int width = backbitmap.getWidth();
          int height = backbitmap.getHeight();
          //super.sublayout( width, height);

          Field f = getField(0);
          setPositionChild(f,10, 0);
          layoutChild(f,maxWidth, maxHeight);

          f = getField(1);
          setPositionChild(f,50, 0);
          layoutChild(f,maxWidth, maxHeight);

         f = getField(2);
          setPositionChild(f,50, 20);
          layoutChild(f,maxWidth, maxHeight);

          f = getField(3);
          setPositionChild(f,215, 10);
          layoutChild(f,maxWidth, maxHeight);

          setExtent( maxWidth, 61);
      }
    };
gridbackManager.add(bitmap);
gridbackManager.add(task);
gridbackManager.add(task2);
gridbackManager.add(bitmapdownload);