code: private String getImageClass() {
  
          switch (m_state) {
              case on:
                  return CSS.triStateOn();
              case off:
                  return CSS.triStateOff();
              case middle:
              default:
                  return CSS.triStateMedium();
          }
  
      }
docs: Gets the image class to use for the check box.<p>
  
  @return the image class