docs: public static int countOccurrencesOf(String str, String sub) {
          if (str == null || sub == null || str.length() == 0 || sub.length() == 0) {
              return 0;
          }
          int count = 0;
          int pos = 0;
          int idx;
          while ((idx = str.indexOf(sub, pos)) != -1) {
              ++count;
              pos = idx + sub.length();
          }
          return count;
      }