Java：将两个列表相乘，它们是 Stream<BigDecimal>

Question

I know my Question is a little for beginners, but in Internet I didn't find Answer, So It's good to have, I have a list which contain two value, first an Enum Type, second a big decimal Number. In my Enum Class, I have also two values related to each Enum, the first value is a string and the second is a BigDecimal.

Enum:

    private enum myEnumClass {
  BMW("Karl Rapp", new BigDecimal("1916")), BENZ("Carl Benz", new BigDecimal("1926")),
  private String founder;
  private BigDecimal yearOfFoundation;

List:

List<CarDetails> numberOfcars = new ArrayList<>();
numberOfcars.add(new CarDetails(BMW, new BigDecimal("3")));
CarDetails.add(new Receipt(BENZ, new BigDecimal("5")));

CarDetails Class:

    class CarDetails {
  myEnumClass carCompany;
  BigDecimal amountOfCar;

I want to multiply yearOfFundation to numberOfCar for each company, so like:

for BMW=1916*3;
for BENZ= 1926*5;
and then plus them: (1916*3)(1926*5);

I want to use Stream() I think it might be possible, but the problem is when I use stream I get this Datatyp: Stram, which not Allows me to use: BigDecimal.multiply(BigDecimal). any idea how can I write is correctly? Thanks a lot!

Answer 1

Using stream and reduce , You could do something like this

BigDecimal res = numberOfcars
    .stream()
    .reduce(
        BigDecimal.ZERO,
        (BigDecimal acc, CarDetails c) -> acc.add(
            c.getAmountOfCar().multiply(c.getCarCompany().getYearOfFoundation())
        ),
        BigDecimal::add
    );

Demo