code: public final int getSurrogateValue(char lead, char trail)
      {
          if (!UTF16.isLeadSurrogate(lead) || !UTF16.isTrailSurrogate(trail)) {
              throw new IllegalArgumentException(
                  "Argument characters do not form a supplementary character");
          }
          // get fold position for the next trail surrogate
          int offset = getSurrogateOffset(lead, trail);
  
          // get the real data from the folded lead/trail units
          if (offset > 0) {
              return m_data_[offset];
          }
  
          // return m_initialValue_ if there is an error
          return m_initialValue_;
      }
docs: Get the value associated with a pair of surrogates.
  @param lead a lead surrogate
  @param trail a trail surrogate