List作为一个对象字段-如何通过Stream (Java 8)处理它？

Question

有一个Waybill对象，它有一个Set字段，Packing对象有一个Set字段。我得到了一个List。需要从整个计算出所有包装的总成本。如何胜任通过流？谢谢。

class Waybill {
    Set<Packing> setOfPacking;
}

class Packing {
    int PRICE;
}

List<Waybill> allWaybills = ...

Answer 1

import java.util.stream.*

List<Waybill> allWaybills = ...
int totalCost = allWaybills
    .stream()
    .mapToInt(w -> w.setOfPacking
        .stream()
        .mapToInt(p -> p.PRICE)
        .sum()
    )
    .sum();

Answer 2

这对我起了作用：

double total = allWaybills.stream()
    .flatMap(waybill -> waybill.setOfPacking.stream())
    .mapToInt(packing -> packing.PRICE)
    .sum();

我认为这更容易推理，因为没有任何多层次的流操作。

我想了解如何使用flatMapToInt将flatMap和map操作替换为一个操作，而不使其成为多级操作。

下面是一个测试程序：

import java.util.Set;
import java.util.List;
import java.util.HashSet;
import java.util.ArrayList;
import java.util.stream.Collectors;

public class HelloWorld
{
    public static class Packing
    {
        public int PRICE = 0;
    }

    public static class Waybill
    {
        public Set<Packing> setOfPacking = new HashSet<Packing>();
    }

    public static void main(String []args){
        List<Waybill> allWaybills = new ArrayList<Waybill>();

        Waybill w1 = new Waybill();
        Packing p1 = new Packing(); p1.PRICE = 1; w1.setOfPacking.add(p1);
        Packing p2 = new Packing(); p2.PRICE = 2; w1.setOfPacking.add(p2);
        allWaybills.add(w1);

        Waybill w2 = new Waybill();
        Packing p3 = new Packing(); p3.PRICE = 3; w2.setOfPacking.add(p3);
        Packing p4 = new Packing(); p4.PRICE = 4; w2.setOfPacking.add(p4);
        allWaybills.add(w2);

        double total = allWaybills.stream()
            .flatMap(waybill -> waybill.setOfPacking.stream())
            .mapToInt(packing -> packing.PRICE)
            .sum();

        System.out.println("total = "+total);
    }
}