code: public static String sum(String... addends) {
  
          if (addends == null || addends.length == 0) {
              return null;
          }
  
          BigDecimal sum;
          try {
              sum = new BigDecimal(addends[0]);
              for (int i = 1; i < addends.length; i++) {
                  sum = sum.add(new BigDecimal(addends[i]));
              }
              return sum.toString();
          } catch (Exception e) {
              return null;
          }
      }
docs: Get the sum of all input numbers
  
  Any numeric string recognized by {@code BigDecimal} is supported.
  
  @param addends one or more valid number strings
  
  @return <code>addends[0] + addends[1] + ...</code>
  
  @see BigDecimal