getAbsoluteTop() / getOffsetHeight() always return 0

i create a custom panel class which draws threads of messages inside.
each message (several Labels) is wrapped in a FlowPanel. the flow
panels are added to a LayoutPanel which i intend to do some
animation. however, i failed to get neither the height nor the
position of the flow panels
through .getAbsoluteTop()/.getOffsetHeight(). they always return 0 so
i cant arrange the positions of the flow panels. may someone help to
tell me how the position can be caught with the following codes?

class MessageReply extends Composite {
private LayoutPanel lpanel = new LayoutPanel();

public MessageReply(){
initWidget(lReplyPanel);

FlowPanel fpanel = new FlowPanel();
fpanel.add(new Label("test message"));

lReplyPanel.add(fpanel);
int textHeight = fpanel.getOffsetHeight(); // i need this height
to put the flow panel to correct position but it always return 0
}
}

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