code: public byte getKeyAsByte(int index) throws IOException {
          if (index >= structure.keySizes.size()) {
              throw new IOException("Index " + index + " is out of range.");
          }
          return key[structure.keyByteOffsets.get(index)];
      }
docs: Returns the key belonging to the given field as byte
  
  @param index
  the index of the requested field
  @return the requested value
  @throws IOException