Java – Convert Iterable to Stream Using Java 8 JDK

iterablejavajava-8java-stream

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?

Best Answer

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.

Related Solutions

Java 8 Stream – How to Create a Stream from an Iterator

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

static <T> Stream<T> iteratorToInfiniteStream(final Iterator<T> iterator) {
    return Stream.generate(iterator::next);
}

Java – How to Convert an Iterator to a Stream

One way is to create a Spliterator from the Iterator and use that as a basis for your stream:

Iterator<String> sourceIterator = Arrays.asList("A", "B", "C").iterator();
Stream<String> targetStream = StreamSupport.stream(
          Spliterators.spliteratorUnknownSize(sourceIterator, Spliterator.ORDERED),
          false);

A potentially more readable alternative is to use an Iterable - and creating an Iterable from an Iterator is very easy with lambdas because Iterable is a functional interface:

Iterator<String> sourceIterator = Arrays.asList("A", "B", "C").iterator();

Iterable<String> iterable = () -> sourceIterator;
Stream<String> targetStream = StreamSupport.stream(iterable.spliterator(), false);

Best Answer

Related Solutions

Java 8 Stream – How to Create a Stream from an Iterator

Java – How to Convert an Iterator to a Stream

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