How to iterate nested lists with lambda streams?

Question

I\'m trying to refactor the following code to lambda expressions with `stream, especially the nested foreach loops:

public static Result match (Response rsp) {
          


        

Answer 1

You could use that fact that StreamSupport provides a stream method that takes a Spliterator and Iterable has a spliterator method.

You then just need a mechanism to flatten your structure into an Iterable - something like this.

class IterableIterable implements Iterable {

    private final Iterable> i;

    public IterableIterable(Iterable> i) {
        this.i = i;
    }

    @Override
    public Iterator iterator() {
        return new IIT();
    }

    private class IIT implements Iterator {

        // Pull an iterator.
        final Iterator> iit = i.iterator();
        // The current Iterator
        Iterator it = null;
        // The current T.
        T next = null;

        @Override
        public boolean hasNext() {
            boolean finished = false;
            while (next == null && !finished) {
                if (it == null || !it.hasNext()) {
                    if (iit.hasNext()) {
                        it = iit.next().iterator();
                    } else {
                        finished = true;
                    }
                }
                if (it != null && it.hasNext()) {
                    next = it.next();
                }
            }
            return next != null;
        }

        @Override
        public T next() {
            T n = next;
            next = null;
            return n;
        }
    }

}

public void test() {
    List> list = new ArrayList<>();
    List first = new ArrayList<>();
    first.add("First One");
    first.add("First Two");
    List second = new ArrayList<>();
    second.add("Second One");
    second.add("Second Two");
    list.add(first);
    list.add(second);
    // Check it works.
    IterableIterable l = new IterableIterable<>(list);
    for (String s : l) {
        System.out.println(s);
    }
    // Stream it like this.
    Stream stream = StreamSupport.stream(l.spliterator(), false);
}

You can now stream directly from your Iterable.

Initial research suggests that this should be done with flatMap but whatever.