code: public static String join(String[] input, char separator) {
          //simple case, no escaping
          StringBuilder stringBuilder = new StringBuilder();
          for (String s : input) {
              if (stringBuilder.length() > 0) {
                  stringBuilder.append(separator);
              }
              stringBuilder.append(s);
          }
          return stringBuilder.toString();
      }
docs: Join an array of strings with the given separator, without escaping
  
  @param input     input string
  @param separator separator
  
  @return joined string