code: public void setTrigger (AbstractButton trigger,
                              Icon collapsed, Icon uncollapsed)
      {
          _trigger = trigger;
          _trigger.setHorizontalAlignment(SwingConstants.LEFT);
          _trigger.setHorizontalTextPosition(SwingConstants.RIGHT);
          _downIcon = collapsed;
          _upIcon = uncollapsed;
          _trigger.addActionListener(this);
      }
docs: Set the trigger button.