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

javajava-stream

Is it possible to create a Stream from an Iterator, in which the sequence of objects is the same as that generated by calling the iterator's next() method repeatedly? The specific case I am thinking of concerns the use of the iterator returned by TreeSet.descendingIterator(), but I can imagine other circumstances in which an iterator, but not the collection it references, is available.

For example, for a TreeSet<T> tset we can write tset.stream()... and get a stream of the objects in that set, in the set's sort order, but what if we want them in a different order, such as that available through using descendingIterator()? I am imagining something like tset.descendingIterator().stream()... or stream( tset.descendingIterator() )..., though neither of these forms are valid.

Best Answer

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);
}

Related Solutions

Java – Convert Iterable to Stream Using Java 8 JDK

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.

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 – Convert Iterable to Stream Using Java 8 JDK

Java – How to Convert an Iterator to a Stream

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