Why my collector method is not processing data parallely?

Question

Suppose, however, that the result container used in this reduction was a concurrently modifiable collection -- such as a ConcurrentHashMap. In that case, the

Answer 1

Your Collector does not know that you use a concurrent collection provided by Supplier, just add the characteristic and see that it is executed the way you want to; for example:

String s = Stream.of(1, 2, 3, 4).parallel()
            .unordered()
            .collect(
                    Collector.of(
                            () -> new ConcurrentLinkedQueue<>(),
                            (c, e) -> c.add(e.toString()),
                            (c1, c2) -> {
                                c1.addAll(c2);
                                return c1;
                            },
                            Characteristics.CONCURRENT))