code: public synchronized int getSignedValue(int from, int to) {
          try {
              // is synchronized so that values of bitSet and calculated can be
              // lazily
              // calculated and safely published (thread safe).
              SixBit.convertSixBitToBits(message, padBits, bitSet, calculated, from, to);
              return (int) SixBit.getSignedValue(from, to, bitSet);
          } catch (SixBitException e) {
              throw new AisParseException(e);
          }
      }
docs: Returns a signed integer value using the bits from character position
  start to position stop in the decoded message.
  
  @param from
  @param to
  @return