lambda concurrent List<Map> to Map

Object c = Stream.of(
        CompletableFuture.supplyAsync(() -> {
            Map m = new HashMap();
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("a order");
            m.put("a1", "a1");
            m.put("a2", "a2");
            return m;
        }),
        CompletableFuture.supplyAsync(() -> {
            Map m = new HashMap();
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            m.put("b1", "b1");
            m.put("b2", "b2");
            System.out.println("b order ");
            return m;
        }))
        .parallel()
        .flatMap(v -> ((Set<Map.Entry>) Optional.of(v.join()).orElse(new HashMap()).entrySet()).stream())
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (k1, k2) -> k1 == null ? k2 : k1));
System.out.println(c);