List as an object field - how to handle it via Stream (Java 8)? - java-8

Question

There is a Waybill object that has a Set<Packing> field, the Packing object has a PRICE field.
I get a List<Waybill>.
Need to calculate the total cost of all Packing from the entire List<Waybill>.
How it competently to make through Stream?
Thank you.
class Waybill {
Set<Packing> setOfPacking;
}
class Packing {
int PRICE;
}
List<Waybill> allWaybills = ...

Answer 1

This worked for me:
double total = allWaybills.stream()
.flatMap(waybill -> waybill.setOfPacking.stream())
.mapToInt(packing -> packing.PRICE)
.sum();
I think it is easier to reason about because there aren't any multi-level stream operations.
I would be interested to see how to use flatMapToInt to replace both the flatMap and map operations with one operation without making it multi-level.
Here is a test program:
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);
}
}

Answer 2

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