code: public synchronized Boolean isChecked(V object) {
          if(object == null)
              return null;
  
          if(indexOf(object) == -1)
              return null;
  
          return checkedIndex == indexOf(object);
      }
docs: This method will see if an object is the currently selected item in this RadioCheckBoxList
  @param object Object to test if it's the selected one
  @return {@code true} if the supplied object is what's currently selected in the list box,
  {@code false} otherwise. Returns null if the supplied object is not an item in the list box.