Java: is there more elegant way to extract a new list from an existing list using reduce?

Question

I'm trying to take a list of elements, make some manipulation on part of these elements and put the output of these manipulations in a new list. I want to do it with only one iteration on the list.

The only way I found to do it is:

List<Integer> newList = numList.stream().reduce(new ArrayList<Integer>(),
            (acc, value) -> {
                if (value % 2 == 0) {
                    acc.add(value * 10);
                }
                return acc;
            },
            (l1, l2) -> {
                l1.addAll(l2);
                return l1;
            }
        );

As you can see, it's very cumbersome.

I can of course use filter and then map, but in this case I iterate the list twice.

In other languages (Javascript for example) this kind of reduce operation is very straightforward, for example:

arr.reduce((acc, value) => { 
    if (value % 2 == 0) {
        acc.push(value * 10);
    }
    return acc;
}, new Array())

Amazing! I was thinking to myself whether Java has some nicer version for this reduce or the Java code I wrote is the shortest way to do such an operation.

That’s not only cumbersome, it’s wrong. Reduction must not modify the input objects. — Holger, Jun 10 '20 at 14:07

score 7 · Answer 1 · answered Jun 10 '20 at 08:53

7

I can of course use filter and they map, but in this case I iterate the list twice.

That's not true. The elements of the Stream are iterated once, no matter how many intermediate operations you have.

Besides, the correct way to perform mutable reduction is to use collect:

List<Integer> newList =
    numList.stream()
            .filter(v -> v % 2 == 0)
            .map(v -> v * 10)
            .collect(Collectors.toList());

answered Jun 10 '20 at 08:53

Eran

387,369
54
702
768

OP wants _using reduce_ – Hadi J Jun 10 '20 at 08:54
5

@HadiJ OP want a more elegant way. The more elegant (and correct) way is not to use reduce at all. – Eran Jun 10 '20 at 08:56
"The elements of the Stream are iterated once, no matter how many intermediate operations you have." - can you give me a link supporting that? – CrazySynthax Jun 10 '20 at 08:57
6

@CrazySynthax How about this : `The elements of a stream are only visited once during the life of a stream. Like an Iterator, a new stream must be generated to revisit the same elements of the source.` (source - https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html) – Eran Jun 10 '20 at 08:59

Andronicus · Answer 2 · 2020-06-10T09:07:21.307

2

What you're doing is the opposite of functional programming, because you're using functions only to get a side effect.

You can simply not use reduce to make it even more explicit:

List<Integer> newList = numList.stream()
    .filter(value -> value % 2 == 0)
    .map(value -> value * 10)
    .collect(Collectors.toList());

Edit: Streams iterate over the collection once by definition. According to the javadoc:

A stream should be operated on (invoking an intermediate or terminal stream operation) only once.

edited Jun 10 '20 at 09:07

answered Jun 10 '20 at 08:53

Andronicus

25,419
17
47
88

OP wants _using reduce_ – Hadi J Jun 10 '20 at 08:54
Sure! But this attitude has a penalty of iterating the list twice, and I want only one iteration. – CrazySynthax Jun 10 '20 at 08:56
3

@HadiJ OP didn't want to iterate over the collection twice, this way it doesn't – Andronicus Jun 10 '20 at 08:58
3

@CrazySynthax why do you think so? There is only a single iteration over the collection, that's the `Stream`'s definition – Andronicus Jun 10 '20 at 08:59

Java: is there more elegant way to extract a new list from an existing list using reduce?

2 Answers2