code: public static boolean substringMatch(CharSequence str, int index, CharSequence substring) {
          for (int j = 0; j < substring.length(); j++) {
              int i = index + j;
              if (i >= str.length() || str.charAt(i) != substring.charAt(j)) {
                  return false;
              }
          }
          return true;
      }
docs: Test whether the given string matches the given substring
  at the given index.
  @param str the original string (or StringBuilder)
  @param index the index in the original string to start matching against
  @param substring the substring to match at the given index