How can I make Cartesian product with Java 8 streams?

Question

I have the following collection type:

Map> map;

I would like to create unique combinations of each o

Answer 1

I wrote a class implementing Iterable, and holding only the current item in memory. The Iterable as well as the Iterator can be converted to a Stream if desired.

class CartesianProduct implements Iterable> {
    private final Iterable> factors;

    public CartesianProduct(final Iterable> factors) {
        this.factors = factors;
    }

    @Override
    public Iterator> iterator() {
        return new CartesianProductIterator<>(factors);
    }
}

class CartesianProductIterator implements Iterator> {
    private final List> factors;
    private final Stack> iterators;
    private final Stack current;
    private List next;
    private int index = 0;

    private void computeNext() {
        while (true) {
            if (iterators.get(index).hasNext()) {
                current.add(iterators.get(index).next());
                if (index == factors.size() - 1) {
                    next = new ArrayList<>(current);
                    current.pop();
                    return;
                }
                index++;
                iterators.add(factors.get(index).iterator());
            } else {
                index--;
                if (index < 0) {
                    return;
                }
                iterators.pop();
                current.pop();
            }
        }
    }

    public CartesianProductIterator(final Iterable> factors) {
        this.factors = StreamSupport.stream(factors.spliterator(), false)
                .collect(Collectors.toList());
        if (this.factors.size() == 0) {
            index = -1;
        }
        iterators = new Stack<>();
        iterators.add(this.factors.get(0).iterator());
        current = new Stack<>();
        computeNext();
    }

    @Override
    public boolean hasNext() {
        if (next == null && index >= 0) {
            computeNext();
        }
        return next != null;
    }

    @Override
    public List next() {
        if (!hasNext()) {
            throw new IllegalStateException();
        }
        var result = next;
        next = null;
        return result;
    }
}