Java 8 streams adding values from two or more lists

Question

I am trying to get into Java 8 and get my head around streams and lambdas to solve various problems and got stuck on this specific one which I normally use a forEach and store the values in a Map to solve.

How would you write the code to get the expected list using the new features in Java 8 ?

List<Integer> voterA = Arrays.asList(1,2,3,4,5);
List<Integer> voterB = Arrays.asList(1,2,3,4,5);
List<List<Integer>> votes = Arrays.asList(voterA, voterB);

// expected list = (2,4,6,8,10)
List<Integer> sumVotes = ...

How would you solve it with enhanced-for-loop(s)?

Pshemo
– Pshemo

2015-02-18 19:48:51 +00:00
Commented Feb 18, 2015 at 19:48 — Pshemo
– Pshemo, Commented Feb 18, 2015 at 19:48
Take a look to question

Denis Borovikov
– Denis Borovikov

2015-02-18 21:23:50 +00:00
Commented Feb 18, 2015 at 21:23 — Denis Borovikov
– Denis Borovikov, Commented Feb 18, 2015 at 21:23

Louis Wasserman · Accepted Answer · 2015-02-18 19:47:36Z

7

That one isn't really doable the way you're hoping. The closest you could get would probably be

IntStream.range(0, voterA.size())
    .mapToObj(i -> voterA.get(i) + voterB.get(i))
    .collect(toList());

...but there's no "zip" operation on streams, largely because two different streams can have backing spliterators that split at different points, so you can't line them up properly.

answered Feb 18, 2015 at 19:47

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

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

8 Comments

Jon Skeet Over a year ago

It's annoying that I can see exactly how I'd do this in C# in LINQ - in a pretty general way - but I'm still groping around when it comes to Java streams. Maybe when I get more practice...

Louis Wasserman Over a year ago

I mean...I look at the Spliterator interface, and I don't think there's any nice way to do zipping, just because trySplit allows the split to happen at an arbitrary point. You can do indexing efficiently with SUBSIZED spliterators, and fall back to a less-efficient sequential approach for other spliterators, but I don't think there's anything you can do with zipping.

Jon Skeet Over a year ago

Here's the C# version, complete with 3 voters with different lengths of lists: gist.github.com/jskeet/2c1db126e424cf58cb69

Louis Wasserman Over a year ago

I mean, that translates pretty directly to gist.github.com/lowasser/e1b57e1a9358cbf52cc6

Stuart Marks Over a year ago

@LouisWasserman Getting the spliterators to line up their splits, especially in parallel, is certainly a big issue. Another issue is that you really want the result of zipping two (or N) streams to be another stream. What would its element type be? A Pair or Tuple class? Or provide a BiFunction that combines elements pairwise? But we don't have N-ary functions. We're still wrestling with this, but it doesn't look like anything will emerge until we have value types (> Java 9).

|

user_3380739 · Accepted Answer · 2023-01-20 06:05:26Z

2

JDK doesn't provide the 'zip' API. But it can be done with third library abacus-common:

List<Integer> voterA = Arrays.asList(1, 2, 3, 4, 5);
List<Integer> voterB = Arrays.asList(1, 2, 3, 4, 5);

List<Integer> sumVotes = Stream.zip(voterA, voterB, (a, b) -> a + b).toList();

Disclosure： I'm the developer of abacus-common.

edited Jan 20, 2023 at 6:05

answered Dec 8, 2016 at 0:00

user_3380739

1,32416 silver badges15 bronze badges

Collectives™ on Stack Overflow

Java 8 streams adding values from two or more lists

2 Answers 2

8 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

8 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related