code: private Long getLastUpdateTime(String sessionId) {
          if (lastUpdateForSession != null && sessionId != null && lastUpdateForSession.containsKey(sessionId)) {
              return lastUpdateForSession.get(sessionId);
          } else {
              return -1L;
          }
      }
docs: Get last update time for given session ID, checking for null values
  @param sessionId session ID
  @return last update time for session if found, or {@code null}