按两个字段分组然后汇总BigDecimal

我有一份税单:

TaxLine = title:"New York Tax", rate:0.20, price:20.00
TaxLine = title:"New York Tax", rate:0.20, price:20.00
TaxLine = title:"County Tax", rate:0.10, price:10.00

TaxLine类是

public class TaxLine {
    private BigDecimal price;
    private BigDecimal rate;
    private String title;
}

我想将它们结合起来的基础上独一无二的title和rate,然后加入price,预计:

 TaxLine = title:"New York Tax", rate:0.20, price:40.00
 TaxLine = title:"County Tax", rate:0.10, price:10.00

我怎样才能在Java 8中做到这一点？

在java 8中按多个字段名分组,不是对一个字段求​​和,它只能由两个字段组合.

主体与链接问题中的相同,您只需要一个不同的下游收集器来总结:

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());