code: @Override
      public String[] getObjectHistory(String PID) {
          String[] dsIDs = ListDatastreamIDs(null);
          TreeSet<String> modDates = new TreeSet<String>();
          for (String element : dsIDs) {
              Date[] dsDates = getDatastreamVersions(element);
              for (Date element2 : dsDates) {
                  modDates.add(DateUtility.convertDateToString(element2));
              }
          }
          return modDates.toArray(EMPTY_STRING_ARRAY);
      }
docs: {@inheritDoc}