Saturday, June 02, 2018

Java 10: Collecting a Stream into an Unmodifiable Collection

Java 10 introduces several new methods to facilitate the creation of unmodifiable collections.

The List.copyOf, Set.copyOf, and Map.copyOf methods create new collection instances from existing instances. For example:

List<String> modifiable = Arrays.asList("foo", "bar");
List<String> unmodifiableCopy = List.copyOf(list);

// Note that since Java 9, you can also use "of" to create
// unmodifiable collections
List<String> unmodifiable = List.of("foo", "bar");

There are also new collector methods, toUnmodifiableList, toUnmodifiableSet, and toUnmodifiableMap, to allow the elements of a stream to be collected into an unmodifiable collection. For example:

// Java 10
Stream.of("foo", "bar").collect(toUnmodifiableList());

// before Java 10
Stream.of("foo", "bar").collect(
    collectingAndThen(toList(), Collections::unmodifiableList));