code: public void freeze( String userId,
                          Map<String, String> userData,
                          DateTime timestamp ) {
          this.userId = userId;
          if (userData != null) {
              this.userData = Collections.unmodifiableMap(userData);
          }
          this.timestamp = timestamp;
      }
docs: Marks this change set as frozen (aka. closed). This means it should not accept any more changes.
  
  @param userId the username from the session which originated the changes; may not be null
  @param userData a Map which can contains arbitrary information; may be null
  @param timestamp a {@link DateTime} at which the changes set was created.