How can I convert a Stream of Strings to Stream of String pairs?

Question

I want to take a stream of strings and turn it into a stream of word pairs. eg:

I have: { "A", "Apple", "B", "Banana", "C", "Carrot" }

I want: { ("A", "Apple"), ("Apple", "B"), ("B", "Banana"), ("Banana", "C") }.

This is nearly the same as Zipping, as outlined at Zipping streams using JDK8 with lambda (java.util.stream.Streams.zip)

However, that produces: { (A, Apple), (B, Banana), (C, Carrot) }

The following code works, but is clearly the wrong way to do it (not thread safe etc etc):

static String buffered = null;

static void output(String s) {
    String result = null;
    if (buffered != null) {
        result = buffered + "," + s;
    } else {
        result = null;
    }

    buffered = s;
    System.out.println(result);
}

// ***** 

Stream<String> testing = Stream.of("A", "Apple", "B", "Banana", "C", "Carrot");
testing.forEach(s -> {output(s);});

Answer 1

If you:

1. Don't like the idea of creating a list with all strings from your stream
2. Don't want to use external libraries
3. Like to get your hands dirty

Then you can create a method to group elements from a stream using Java 8 low-level stream builders StreamSupport and Spliterator:

class StreamUtils {
    public static<T> Stream<List<T>> sliding(int size, Stream<T> stream) {
        return sliding(size, 1, stream);
    }

    public static<T> Stream<List<T>> sliding(int size, int step, Stream<T> stream) {
        Spliterator<T> spliterator = stream.spliterator();
        long estimateSize;

        if (!spliterator.hasCharacteristics(Spliterator.SIZED)) {
            estimateSize = Long.MAX_VALUE;
        } else if (size > spliterator.estimateSize()) {
            estimateSize = 0;
        } else {
            estimateSize = (spliterator.estimateSize() - size) / step + 1;
        }

        return StreamSupport.stream(
                new Spliterators.AbstractSpliterator<List<T>>(estimateSize, spliterator.characteristics()) {
                    List<T> buffer = new ArrayList<>(size);

                    @Override
                    public boolean tryAdvance(Consumer<? super List<T>> consumer) {
                        while (buffer.size() < size && spliterator.tryAdvance(buffer::add)) {
                            // Nothing to do
                        }

                        if (buffer.size() == size) {
                            List<T> keep = new ArrayList<>(buffer.subList(step, size));
                            consumer.accept(buffer);
                            buffer = keep;
                            return true;
                        }
                        return false;
                    }
                }, stream.isParallel());
    }
}

Methods and parameters naming was inspired in their Scala counterparts.

Let's test it:

Stream<String> testing = Stream.of("A", "Apple", "B", "Banana", "C", "Carrot");
System.out.println(StreamUtils.sliding(2, testing).collect(Collectors.toList()));

[[A, Apple], [Apple, B], [B, Banana], [Banana, C], [C, Carrot]]

What about not repeating elements:

Stream<String> testing = Stream.of("A", "Apple", "B", "Banana", "C", "Carrot");
System.out.println(StreamUtils.sliding(2, 2, testing).collect(Collectors.toList()));

[[A, Apple], [B, Banana], [C, Carrot]]

And now with an infinite Stream:

StreamUtils.sliding(5, Stream.iterate(0, n -> n + 1))
        .limit(5)
        .forEach(System.out::println);

[0, 1, 2, 3, 4]
[1, 2, 3, 4, 5]
[2, 3, 4, 5, 6]
[3, 4, 5, 6, 7]
[4, 5, 6, 7, 8]