code: public void setPosOnBand(PosOnBand value) {
          if (value == null) {
              getElement().removeClassName(STYLE_UI_BTN_RIGHT);
              getElement().removeClassName(STYLE_UI_BTN_LEFT);
          } else {
              switch (value) {
              case LEFT:
                  getElement().addClassName(STYLE_UI_BTN_LEFT);
                  break;
              case RIGHT:
                  getElement().addClassName(STYLE_UI_BTN_RIGHT);
                  break;
              }
          }
      }
docs: Works in case of this button placed on Header, Popup, ...