DrawOval layering

Hi,
I'm trying to build a simple application using smartgwt that does the following:
- draws circles using DrawOval at Random positions
- By clicking a button I should bring to front the selected circle
If the followings are circles: a,b,c,d where d is the circle on top. If I'm selecting b circle and press the button, circle b should move on top of c and under the d circle.
How can I achive this?
thank you