code: @SuppressWarnings("unchecked")
      public static <T> String join(String separator, T... items) {
          StringBuilder result = new StringBuilder();
          for (int i = 0; i < items.length; ++i) {
              T item = items[i];
              if (item instanceof Number) {
                  result.append(Op.str((Number) item));
              } else {
                  result.append(item.toString());
              }
              if (i + 1 < items.length) {
                  result.append(separator);
              }
          }
          return result.toString();
      }
docs: Joins a variadic number of elements by the given separator into a single
  string. The elements are represented as strings utilizing the Op.str()
  method on each element
  
  @tparam T determines the type of elements in the array
  @param separator is the string to add between the elements
  @param items are the elements to join
  @return a single string joining the variadic number of elements by the
  given separator