Collectors.toUnmodifiableList in java-10

Question

How do you create an Unmodifiable List/Set/Map with Collectors.toList/toSet/toMap, since toList (and the like) are document as :

There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned

Before java-10 you have to provide a Function with Collectors.collectingAndThen, for example:

 List<Integer> result = Arrays.asList(1, 2, 3, 4)
            .stream()
            .collect(Collectors.collectingAndThen(
                    Collectors.toList(),
                    x -> Collections.unmodifiableList(x)));

Answer 1

With Java 10, this is much easier and a lot more readable:

List<Integer> result = Arrays.asList(1, 2, 3, 4)
            .stream()
            .collect(Collectors.toUnmodifiableList());

Internally, it's the same thing as Collectors.collectingAndThen, but returns an instance of unmodifiable List that was added in Java 9.

Answer 2

Additionally to clear out a documented difference between the two(collectingAndThen vs toUnmodifiableList) implementations :

The Collectors.toUnmodifiableList would return a Collector that disallows null values and will throw NullPointerException if it is presented with a null value.

static void additionsToCollector() {
    // this works fine unless you try and operate on the null element
    var previous = Stream.of(1, 2, 3, 4, null)
            .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList));

    // next up ready to face an NPE
    var current = Stream.of(1, 2, 3, 4, null).collect(Collectors.toUnmodifiableList());
}

and furthermore, that's owing to the fact that the former constructs an instance of Collections.UnmodifiableRandomAccessList while the latter constructs an instance of ImmutableCollections.ListN which adds to the list of attributes brought to the table with static factory methods.