Java Lambda Multiple operations on a single stream

Question

Scenario: I have a object with 2 functions -->

<Integer> getA(); <Integer> getB();

I have a list of objects, say List<MyObject> myObject.

Objective Iterate over the List and get separate sums of A and B's in the List of object.

My Solution

myObject.stream().map(a -> a.getA()).collect(Collectors.summingDouble());

myObject.stream().map(a -> a.getB()).collect(Collectors.summingDouble());

The Question: How can I do both of these at the same time? This way I will not have to iterate twice over the original list.

@Edit: I was doing this because some of the filters that I used were of O(n^3). Kind of really bad to do those twice.

Benchmark : It really matters if it is T or 2T when the program runs for half hour on an i5. This was on much lesser data and if I run on a cluster, my data would be larger too.

It does matter if you can do these in one line!.

Cheng Chen · Accepted Answer · 2016-08-15 06:07:59Z

3

You need to write another class to store the total values like this:

public class Total {
    private int totalA = 0;
    private int totalB = 0;

    public void addA(int a) {
        totalA += a;
    }

    public void addB(int b) {
        totalB += b;
    }

    public int getTotalA() {
        return totalA;
    }

    public int getTotalB() {
        return totalB;
    }
}

And then collect the values using

Total total = objects.stream()
        .map(o -> (A) o)
        .collect(Total::new,
                (t, a) -> {
                    t.addA(a.getA());
                    t.addB(a.getB());
                },
                (t1, t2) -> { });
//check total.getTotalA() and total.getTotalB()

You can also use AbstractMap.SimpleEntry<Integer, Integer> to replace Total to avoid writing a new class, but it's still kind of weird because A/B are not in a key-value relationship.

edited Aug 15, 2016 at 6:07

answered Aug 15, 2016 at 5:58

Cheng Chen

43.7k18 gold badges118 silver badges184 bronze badges

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

5 Comments

Pranay Over a year ago

So essentially you created another Collector? I could just extend my existing class MyObject to collect this by exposing addA/B() to it. Seems like a good way.

Cheng Chen Over a year ago

@user6184793 Of course you can use an existing class to replace my Total class.

ruakh Over a year ago

+1. Note that it's a good idea to benchmark afterward to see if this approach actually performs better than iterating once for A and once for B.

Pranay Over a year ago

@ruakh, This example is simple. Suppose I had a few filters in place. Something like .stream()..filter(myObj.x())....filter(myObj.y()).collect....). That would definitely mean doing things twice.

ruakh Over a year ago

@user6184793: And even so, it's worth benchmarking to see how much of a difference it really makes. You don't want to complicate your code for no benefit.

jcsahnwaldt Reinstate Monica · Accepted Answer · 2023-08-08 22:07:20Z

3

If you're running Java >= 12, this isn't hard at all to do in a way that's even compatible with parallel streams (unlike the other answers so far), using Collectors.teeing() and Collectors.summingInt():

int[] totals = myObject.stream()
   .collect(Collectors.teeing(
      Collectors.summingInt(MyObject::getA),
      Collectors.summingInt(MyObject::getB),
      (a, b) -> new int[] {a, b});

int sumA = totals[0];
int sumB = totals[1];

edited Aug 8, 2023 at 22:07

jcsahnwaldt Reinstate Monica

3,6564 gold badges39 silver badges35 bronze badges

answered Aug 8, 2023 at 21:27

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Comments

Joop Eggen · Accepted Answer · 2016-08-15 06:24:40Z

0

The most optimal probably still would be a for loop. Though the stream alternative has parallelism as option, and is likely to become as efficient soon. Combining two loops to one is not necessarily faster.

One improvement would be to use int instead of Integer.

List<String> list = Arrays.asList("tom","terry","john","kevin","steve");
int n = list.stream().collect(Collectors.summingInt(String::length));
int h = list.stream().collect(Collectors.summingInt(String::hashCode));

I favour this solution.

If one would make one loop, there are two alternatives:

putting both ints in their own class. You might abuse java.awt.Point class with int x and int y.
putting both ints in a long. When no overflow, then one can even sum in the loop on the long.

The latter:

List<String> list = Arrays.asList("tom","terry","john","kevin","steve");
long nh = list.stream()
    .collect(Collectors.summingLong((s) -> (s.hashCode() << 32) | s.length()));
    int n = (int) nh;
    int h = (int) (nh >> 32L);

answered Aug 15, 2016 at 6:24

Joop Eggen

110k8 gold badges89 silver badges142 bronze badges

1 Comment

Pranay Over a year ago

Combining two loops is not significantly faster, but if you are doing things like filter(), it will be definitely bad because of doing all the checks twice!

Collectives™ on Stack Overflow

Java Lambda Multiple operations on a single stream

3 Answers 3

5 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

5 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related