code: public static String join(String joiner, String... parts) {
          StringBuilder builder = new StringBuilder();
          for (int i = 0; i < parts.length; i++) {
              builder.append(parts[i]);
              if (i < parts.length - 1) {
                  builder.append(joiner);
              }
          }
          return builder.toString();
      }
docs: Joins the strings in parts with joiner between each string
  @param joiner the string to insert between the strings in parts
  @param parts the parts to join