code: private String getTimeZoneID(String tzID, String mzID) {
          String id = tzID;
          if (id == null) {
              assert (mzID != null);
              id = _tznames.getReferenceZoneID(mzID, getTargetRegion());
              if (id == null) {
                  throw new IllegalArgumentException("Invalid mzID: " + mzID);
              }
          }
          return id;
      }
docs: Private method returns a time zone ID. If tzID is not null, the value of tzID is returned.
  If tzID is null, then this method look up a time zone ID for the current region. This is a
  small helper method used by the parse implementation method
  
  @param tzID
  the time zone ID or null
  @param mzID
  the meta zone ID or null
  @return A time zone ID
  @throws IllegalArgumentException
  when both tzID and mzID are null