Java Stream: is there a way to iterate taking two elements a time instead of one?

Question

Let\'s say we have this stream

Stream.of(\"a\", \"b\", \"err1\", \"c\", \"d\", \"err2\", \"e\", \"f\", \"g\", \"h\", \"err3\", \"i\", \"j\");

Answer 1

Other approach with Collector.of and List> as structure to collect pairs. First collect to List>:

List> collect = Stream.of("a", "b", "err1", "c", "d", "err2", "e", "f", "g", "h", "err3", "i", "j")
        .collect(
                Collector.of(
                        LinkedList::new,
                        (a, b) -> {
                            if (b.startsWith("err"))
                                a.add(new ArrayList<>(List.of(b)));
                            else if (!a.isEmpty() && a.getLast().size() == 1)
                                a.getLast().add(b);
                        },
                        (a, b) -> { throw new UnsupportedOperationException(); }
                )
        );

Then it can be transform to map

Map toMap = collect.stream().filter(l -> l.size() == 2)
        .collect(Collectors.toMap(
                e -> e.get(0),
                e -> e.get(1))
        );

Or all in one with Collectors.collectingAndThen

Map toMap = Stream.of("a", "b", "err1", "c", "d", "err2", "e", "f", "g", "h", "err3", "i", "j")
        .collect(Collectors.collectingAndThen(
                Collector.of(
                        LinkedList>::new,
                        (a, b) -> {
                            if (b.startsWith("err"))
                                a.add(new ArrayList<>(List.of(b)));
                            else if (!a.isEmpty() && a.getLast().size() == 1)
                                a.getLast().add(b);
                        },
                        (a, b) -> { throw new UnsupportedOperationException(); }
                ), (x) -> x.stream().filter(l -> l.size() == 2)
                        .collect(Collectors.toMap(
                                e -> e.get(0),
                                e -> e.get(1))
                        )
        ));