How to find a mid point of an android textview

I have a sub-class of TextView and want to draw a horizontal line by making its height to be 1px. It is ok but i also want to make it center vertically by setting the top margin as half of the height. However it turns out to be failed. Any solution to it? thank you

        LayoutParams lp = new LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, 1);
        int h = getHeight();
        lp.setMargins(0, h/2, 0, 0);