Group by two fields then summing BigDecimal

Question

I have a list of taxes:

TaxLine = title:\"New York Tax\", rate:0.20, price:20.00
TaxLine = title:\"New York Tax\", rate:0.20, price:20.00
TaxLine = title:\"Count         


        

Answer 1

The principle is the same as in the linked question, you just need a different downstream collector to sum:

List flattened = taxes.stream()
    .collect(Collectors.groupingBy(
        TaxLine::getTitle,
        Collectors.groupingBy(
            TaxLine::getRate,
            Collectors.reducing(
                BigDecimal.ZERO,
                TaxLine::getPrice,
                BigDecimal::add))))
    .entrySet()
    .stream()
    .flatMap(e1 -> e1.getValue()
         .entrySet()
         .stream()
         .map(e2 -> new TaxLine(e2.getValue(), e2.getKey(), e1.getKey())))
    .collect(Collectors.toList());