code: public static String multiply(String f1, String f2) {
          BigDecimal factor1;
          BigDecimal factor2;
  
          try {
              factor1 = new BigDecimal(f1);
              factor2 = new BigDecimal(f2);
          } catch (Exception ex) {
              return null;
          }
  
          return factor1.multiply(factor2).toString();
      }
docs: Multiply two numbers together
  
  Any numeric string recognized by {@code BigDecimal} is supported.
  
  @param f1 A valid number string
  @param f2 A valid number string
  
  @return <code>f1*f2</code>
  
  @see BigDecimal