List as an object field - how to handle it via Stream (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 = ...

To clarify, by "Stream" are you referring to Java's Stream API - the Java equivalent of .NET's Linq, or do you mean an actual java.io.Stream object you must deserialize from? — Dai
– Dai, Commented Sep 13, 2017 at 23:52

Jerry Jeremiah · Accepted Answer · 2017-09-14 00:42:07Z

3

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);
    }
}

answered Sep 14, 2017 at 0:42

Jerry Jeremiah

9,6933 gold badges27 silver badges37 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Denis Over a year ago

Thank you! But tell me please, what is the difference? The better flatMap ()?

Jerry Jeremiah Over a year ago

@Denis I just think it is easier to understand because it isn't nested. Probably from the JVM's point of view it is exactly the same. Which one do you find easier to understand?

Denis Over a year ago

I understand both, and another) Interesting implementation is different, thank you for reacting so quickly!

Dai · Accepted Answer · 2017-09-13 23:57:37Z

0

import java.util.stream.*

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

answered Sep 13, 2017 at 23:57

Dai

158k31 gold badges314 silver badges436 bronze badges

1 Comment

charles-allen Over a year ago

I think you can optimise this with flatMapToInt() (should be able to remove one sum).

Collectives™ on Stack Overflow

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

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related