code: @Override
      public int compareTo(CurrencyUnit currency) {
          Objects.requireNonNull(currency);
          return getCurrencyCode().compareTo(currency.getCurrencyCode());
      }
docs: Compares two instances, based on {@link #getCurrencyCode()}.
  
  @param currency The instance to be compared with.
  @see java.lang.Comparable#compareTo(java.lang.Object)