Put FieldManager at center of the screen in a BlackBerry app

I know about how to put field on the center of the screen with horizontally and vertically.

However I got success with the field, but I want to set the VerticalFieldManager or HorizontalFieldManage to the center of the screen.

My code is like below, but I am not able to set it at center of the screen.

    final Bitmap scale = new Bitmap(Display.getWidth(),Display.getHeight());
    _backgroundBitmap.scaleInto(scale, Bitmap.FILTER_LANCZOS);

    //==============================
    // this manager is used for the static background image
    mainManager = new VerticalFieldManager(Manager.USE_ALL_HEIGHT | Manager.USE_ALL_WIDTH | Manager.VERTICAL_SCROLL) {
        public void paint(Graphics graphics) {
            graphics.clear();
            graphics.drawBitmap(0, 0, deviceWidth, deviceHeight,scale, 0, 0);
            super.paint(graphics);
        }
    };


    // this manger is used for adding the componentes
    subManager = new VerticalFieldManager(Manager.VERTICAL_SCROLL | Manager.VERTICAL_SCROLLBAR) {

        /*protected void sublayout(int maxWidth, int maxHeight) {
            int displayWidth = deviceWidth;
            int displayHeight = deviceHeight;
            super.sublayout(displayWidth, displayHeight);
            setExtent(displayWidth, displayHeight);
        }*/
    };

    VerticalFieldManager dataManager = new VerticalFieldManager(){};

    dataManager.setMargin(20, 20, 20, 20);
    //CategoryListLayout 
    //===============================================================================
    //====================================
    HorizontalFieldManager categoryLayout = new HorizontalFieldManager(FIELD_VCENTER | USE_ALL_WIDTH | FIELD_HCENTER){
        protected void paint(Graphics graphics) {
            graphics.setColor(0xFFFFFFFF);
            super.paint(graphics);
        }
    };

    LabelField cateName = new LabelField("Category", FIELD_VCENTER){
        protected void layout(int width, int height) {
            super.layout(width, height);
            this.setExtent(150, this.getHeight());
        }
    };
    categoryLayout.setBorder(myBorder);
    //choice_country.setMinimalWidth(30);
    choice_country.setMargin(10, 10, 10, 10);
    categoryLayout.add(cateName);
    categoryLayout.add(new BitmapField(Bitmap.getBitmapResource("vertical_line.png"), FIELD_VCENTER));
    categoryLayout.add(choice_country);
    dataManager.add(categoryLayout);
    //===============================================================================


    //DistanceListLayout 
    //===============================================================================
    HorizontalFieldManager distanceLayout = new HorizontalFieldManager(FIELD_VCENTER |  USE_ALL_WIDTH | FIELD_HCENTER){
        protected void paint(Graphics graphics) {
            graphics.setColor(0xFFFFFFFF);
            super.paint(graphics);
        }
    };

    LabelField distName = new LabelField("Distance", FIELD_VCENTER){
        protected void layout(int width, int height) {
            super.layout(width, height);
            this.setExtent(150, this.getHeight());
        }
    };
    distanceLayout.setBorder(myBorder);
    //choice_distance.setMinimalWidth(300);
    choice_distance.setMargin(10, 10, 10, 10);
    distanceLayout.add(distName);
    distanceLayout.add(new BitmapField(Bitmap.getBitmapResource("vertical_line.png"), FIELD_VCENTER));
    distanceLayout.add(choice_distance);
    dataManager.add(distanceLayout);
    //===============================================================================

    listNames = new Vector();
    listImage = new Vector();
    //new GetList().execute(null);

    ButtonField b_search = new ButtonField("Search", ButtonField.CONSUME_CLICK | ButtonField.FIELD_HCENTER);


    b_search.setMargin(10,10,10,10);

    b_search.setChangeListener(new FieldChangeListener() {
        public void fieldChanged(Field field, int context) {
            if (choice_country.getSelectedIndex() != 0
                    || choice_distance.getSelectedIndex() != 0) {
                new SubmitSearch().execute(null);
            } else {
                Dialog.alert("Select atleast One of Two(Category/Distance).");
            }
        }
    });


    dataManager.add(b_search);
    removeAllScreen();
    subManager.add(dataManager);

    mainManager.add(subManager);
    add(mainManager);

What's wrong in my code?

Update

Here I am using mainManager to display the Background of the app. subManager is just for container. datamanager is to include mane HorizontalFieldManager.

Now I want is the datamanager is to be display vertically at center of the screen. It not depend on the HorizontalLayout I am going to add in it.

VerticalManager will ignore any vertical modificators like FIELD_VCENTER.

You have to implement your own Manager which will put submanager to center of the screen. Something like this:

protected void sublayout(int maxWidth, int maxHeight) {
        submanager.sublayout(displayWidth, displayHeight);
        //after sublayout submanager knows his real dimensions
        int submanagerWidth = submanager.getWidth();
        int submanagerHeight = submanager.getHeight();
        int x = (displayWidth - submanagerWidth) >> 1;
        int y = (displayHeight - submanagerHeight) >> 1;
        setPositionChild(submanager, x, y);
        setExtent(displayWidth, displayHeight);
}

In example I assumed that submanager is only one child. But is there are several it's clear how to modify example.

UPDATE

If there are several children (for example 2):

protected void sublayout(int maxWidth, int maxHeight) {
        int  height = 0;
        for (int i = 0; i < 2; i++) {
            submanager[i].sublayout(displayWidth, displayHeight);
            height += submanager[i].getHeight(); 
        }

        int y = (displayHeight - height) >> 1;
        if (y < 0)
            y = 0;
        for (int i = 0; i < 2; i++) {
            int submanagerWidth = submanager[i].getWidth();
            int submanagerHeight = submanager[i].getHeight();
            int x = (displayWidth - submanagerWidth) >> 1;
            setPositionChild(submanager[i], x, y);
            y += submanagerHeight;
        }
        setExtent(displayWidth, max(height, submanagerHeight));
}