code: public static String join(final String[] strings, final String separator) throws IllegalArgumentException {
          if (strings == null) {
              throw new IllegalArgumentException("Input array 'strings' can't be null");
          }
          if (separator == null) {
              throw new IllegalArgumentException("separator can't be null");
          }
          StringJoiner joiner = new StringJoiner(separator);
          for (String el : strings) {
              joiner.add(el);
          }
          return joiner.toString();
      }
docs: Join concatenates all the elements of the strings array into a single String. The separator string is placed between elements in the resulting string.
  
  @param strings   The input array to concatenate
  @param separator The separator to use
  @return Concatenated String