Convert Iterable to Stream using Java 8 JDK

Question

I have an interface which returns java.lang.Iterable<T>.

I would like to manipulate that result using the Java 8 Stream API.

However Iterable can't "stream".

Any idea how to use the Iterable as a Stream without converting it to List?

If you can iterate, why not simly use a loop to check its condition or value or what so ever? — Afzaal Ahmad Zeeshan
– Afzaal Ahmad Zeeshan, Commented May 29, 2014 at 11:18
As I said, I need to do some maniuplations on that list (filters, mapping). I would like to use the new Java 8 JDK API - > Stream. but Iterable isnot "SteamAble" — rayman
– rayman, Commented May 29, 2014 at 11:19
@Guillaume: Yes, but Stream.of(iterable) produces Stream<Iterable<Object>>. — Matthias Braun
– Matthias Braun, Commented Oct 4, 2018 at 16:16

M. Justin · Accepted Answer · 2024-01-18 05:36:00Z

724

+300

Iterable has a spliterator() method, which you can pass to StreamSupport.stream to create a stream:

StreamSupport.stream(iterable.spliterator(), false)
             .filter(...)
             .moreStreamOps(...);

This is a much better answer than using spliteratorUnknownSize directly, as it is both easier and gets a better result. In the worst case, it's the same code (the default implementation uses spliteratorUnknownSize), but in the more common case, where your Iterable is already a collection, you'll get a better spliterator, and therefore better stream performance (maybe even good parallelism). It's also less code.

As you can see, getting a stream from an Iterable (see also this question) is not very painful.

edited Jan 18, 2024 at 5:36

M. Justin

23.1k12 gold badges133 silver badges167 bronze badges

answered May 29, 2014 at 15:15

Brian Goetz

96.3k26 gold badges164 silver badges172 bronze badges

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

7 Comments

robinst Over a year ago

A static method on Stream would have been nice, e.g. Stream.ofIterable(iterable).

Brian Goetz Over a year ago

@robinst This wasn't an omission; it was a deliberate (and highly debated) choice. The challenge is that were this to exist, it would just be too easy to reach for it without understanding the tradeoffs that come with it. Because Iterable is so abstract, such a method would result in the worst-performing stream possible (no parallel support, no size information or characteristics (used to optimize execution choices)). Forcing more thought results in better APIs across the entire ecosystem. This was a tradeoff of "what's best for XYZ code" vs "what's best for all Java code."

qualidafial Over a year ago

Based on your explanation, I'm curious why we got Collection.stream() but not Iterable.stream(). It seems like the reasoning to omit Iterable.stream() (or Stream.ofIterable()) applies equally to Collection.stream().

Brian Goetz Over a year ago

@qualidafial See stackoverflow.com/questions/23114015/….

user_3380739 Over a year ago

@BrianGoetz It looks most of people are not in the level to understand you said above or don't care about. they only want to write the simple code by calling simple API. On the other hand, those things(parallel...) maybe not important to most of daily iterable operations.

|

numéro6 · Accepted Answer · 2017-01-16 09:11:18Z

99

If you can use Guava library, since version 21, you can use

Streams.stream(iterable)

answered Jan 16, 2017 at 9:11

numéro6

4,3101 gold badge21 silver badges18 bronze badges

2 Comments

Jacob van Lingen Over a year ago

Or, if you are stuck at an older version, use Lists.newArrayList(Iterable).

Vadzim Over a year ago

Current Guava implementation is not worse than accepted answer: github.com/google/guava/blob/master/guava/src/com/google/common/…

Victor Stafusa · Accepted Answer · 2015-02-08 13:53:59Z

25

You can easily create a Stream out of an Iterable or Iterator:

public static <T> Stream<T> stream(Iterable<T> iterable) {
    return StreamSupport.stream(
        Spliterators.spliteratorUnknownSize(
            iterable.iterator(),
            Spliterator.ORDERED
        ),
        false
    );
}

edited Feb 8, 2015 at 13:53

Victor Stafusa

14.7k13 gold badges63 silver badges77 bronze badges

answered May 29, 2014 at 11:26

nosid

50.3k13 gold badges117 silver badges146 bronze badges

3 Comments

gexicide Over a year ago

You have to write this function once and then just call it. Why does a call to stream(...) clutter your code?

rayman Over a year ago

Wanted to do it Inline short and elegant.. you right I can write this function once.. but I am into dropping code (and not adding code). anyway this answer is right because thats the way to convert this.

aepurniet Over a year ago

static import said function. short and elegant. (though not necessarily transparent)

Saikat · Accepted Answer · 2020-07-11 19:39:27Z

11

I would like to suggest using JOOL library, it hides spliterator magic behind the Seq.seq(iterable) call and also provides a whole bunch of additional useful functionality.

edited Jul 11, 2020 at 19:39

Saikat

17.5k22 gold badges125 silver badges146 bronze badges

answered Nov 5, 2015 at 16:18

Shaggie

16.2k1 gold badge17 silver badges8 bronze badges

1 Comment

Pawel Zieminski Over a year ago

Wow there is a whole JOO* family. I was only familiar with JOOQ. Thanks!

Alexander Oh · Accepted Answer · 2017-11-03 12:09:53Z

9

So as another answer mentioned Guava has support for this by using:

Streams.stream(iterable);

I want to highlight that the implementation does something slightly different than other answers suggested. If the Iterable is of type Collection they cast it.

public static <T> Stream<T> stream(Iterable<T> iterable) {
  return (iterable instanceof Collection)
    ? ((Collection<T>) iterable).stream()
    : StreamSupport.stream(iterable.spliterator(), false);
}

public static <T> Stream<T> stream(Iterator<T> iterator) {
  return StreamSupport.stream(
    Spliterators.spliteratorUnknownSize(iterator, 0),
    false
  );
}

edited Nov 3, 2017 at 12:09

answered Nov 3, 2017 at 12:02

Alexander Oh

26k17 gold badges81 silver badges84 bronze badges

Comments

g-t · Accepted Answer · 2016-04-21 10:31:49Z

6

I've created this class:

public class Streams {
    /**
     * Converts Iterable to stream
     */
    public static <T> Stream<T>  streamOf(final Iterable<T> iterable) {
        return toStream(iterable, false);
    }

    /**
     * Converts Iterable to parallel stream
     */
    public static <T> Stream<T> parallelStreamOf(final Iterable<T> iterable) {
        return toStream(iterable, true);
    }

    private static <T> Stream<T> toStream(final Iterable<T> iterable, final boolean isParallel) {
        return StreamSupport.stream(iterable.spliterator(), isParallel);
    }
}

I think it's perfectly readable because you don't have to think about spliterators and booleans (isParallel).

answered Apr 21, 2016 at 10:31

g-t

1,53312 silver badges17 bronze badges

Comments

OldCurmudgeon · Accepted Answer · 2017-08-09 10:27:45Z

5

A very simple work-around for this issue is to create a Streamable<T> interface extending Iterable<T> that holds a default <T> stream() method.

interface Streamable<T> extends Iterable<T> {
    default Stream<T> stream() {
        return StreamSupport.stream(spliterator(), false);
    }
}

Now any of your Iterable<T>s can be trivially made streamable just by declaring them implements Streamable<T> instead of Iterable<T>.

edited Aug 9, 2017 at 10:27

answered Apr 10, 2017 at 11:31

OldCurmudgeon

66k18 gold badges126 silver badges220 bronze badges

Comments

Grzegorz Piwowarek · Accepted Answer · 2017-07-25 17:30:53Z

0

If you happen to use Vavr(formerly known as Javaslang), this can be as easy as:

Iterable i = //...
Stream.ofAll(i);

answered Jul 25, 2017 at 17:30

Grzegorz Piwowarek

14.2k8 gold badges73 silver badges97 bronze badges

Collectives™ on Stack Overflow

Convert Iterable to Stream using Java 8 JDK

8 Answers 8

7 Comments

2 Comments

3 Comments

1 Comment

Comments

Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

7 Comments

2 Comments

3 Comments

1 Comment

Comments

Comments

Comments

Comments

Linked

Related