code: public static String join(Collection<String> strings, String separator) {
          StringBuilder sb = new StringBuilder();
          if (strings != null) {
              Iterator<String> i = strings.iterator();
              while (i.hasNext()) {
                  sb.append(i.next());
                  if (i.hasNext()) {
                      sb.append(separator);
                  }
              }
          }
          return sb.toString();
      }
docs: Joins the elements of the provided {@link Collection collection} into a
  single {@link String string}, using the provided {@code separator} and
  string elements.
  <p>
  For example: <br>
  <blockquote> {@code join(asList("foo", "bar"), "*")}<br>
  </blockquote> returns<br>
  <blockquote> {@code "foo*bar"} </blockquote>
  </p>
  
  @param strings {@link String Strings} to join together
  @param separator Separator {@link String string}
  @return String