code: private static BigDecimal doSubtract(final BigDecimal v1, final BigDecimal v2) {
          BigDecimal diff = v1;
          if (v1 != null && v2 != null) {
              diff = v1.subtract(v2);
          } else if (v2 != null) {
              diff = v2.negate();
          }
          return diff;
      }
docs: Subtract 2 BigDecimal safely (i.e. handles nulls) v1 - v2