Convert Iterable to Stream using Java 8 JDK

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?

There's a much better answer than using spliteratorUnknownSize directly, which is both easier and gets a better result. Iterable has a spliterator() method, so you should just use that to get your spliterator. 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:

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

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

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

Streams.stream(iterable)

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
	);
}

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.

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
  );
}

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).

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>.

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

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