code: @Override
      protected String escapeCharacter(char ch, char previousChar) {
          Character key = Character.valueOf(ch);
          String replacement = replacements.get(key);
          if (replacement == null) {
              replacement = StreamingHTMLEncoderHelper.convertToReference(ch);
              replacements.put(key, replacement != null ? replacement : NULL_MARKER);
          }
          return replacement != NULL_MARKER ? replacement : null;
      }
docs: /*
  (non-Javadoc)
  @see
  AbstractCharReplacementEncoder
  #escapeCharacter(char, char)