When you ask a button group for the currently selected radio button, it returns the selected radio button's model (rather than the selected radio button itself). Fortunately, the button group maintains the list of buttons and so you can iterate over this list looking for one with the same model.

    // This method returns the selected radio button in a button group

    public static JRadioButton getSelection(ButtonGroup group) {

        for (Enumeration e=group.getElements(); e.hasMoreElements(); ) {

            JRadioButton b = (JRadioButton)e.nextElement();

            if (b.getModel() == group.getSelection()) {

                return b;

            }

        }

        return null;

    }