code: private JsonParserException createHelpfulException(char first, char[] expected, int failurePosition)
        throws JsonParserException {
      // Build the first part of the token
      StringBuilder errorToken = new StringBuilder(first
          + (expected == null ? "" : new String(expected, 0, failurePosition)));
  
      // Consume the whole pseudo-token to make a better error message
      while (isAsciiLetter(peekChar()) && errorToken.length() < 15)
        errorToken.append((char)advanceChar());
  
      return createParseException(null, "Unexpected token '" + errorToken + "'"
          + (expected == null ? "" : ". Did you mean '" + first + new String(expected) + "'?"), true);
    }
docs: Throws a helpful exception based on the current alphanumeric token.