Java 24

Stream Gatherers

Enhances the Stream API to support custom intermediate operations.

Java provides some gatherers in the java.util.stream.Gatherers, such as:

windowFixed: A fixed window gatherer processes elements in non-overlapping groups of a fixed size.

public static List<String> titles = Arrays.asList("1", "2", "3", "4", "5", "6","7", "8","9");

titles.stream()
          .gather(Gatherers.windowFixed(3))
          .forEach(System.out::println);

[1, 2, 3]
[4, 5, 6]
[7, 8, 9]

windowSliding: A sliding window moves through the elements one at a time.

titles.stream()
        .gather(Gatherers.windowSliding(2))
        .forEach(System.out::println);

[1, 2]
[2, 3]
[3, 4]
[4, 5]
[5, 6]
[6, 7]
[7, 8]
[8, 9]

fold: The fold operation accumulates results in order.

titles.stream()
        .gather(Gatherers.fold(
          () -> "List of numbers: ",
          (result, num) -> result + num + ", ")) (1)
        .forEach(System.out::println);

1	Doesn’t force any type

List of numbers: 1, 2, 3, 4, 5, 6, 7, 8, 9,

scan: Similar to fold but emits intermediate results.

titles.stream()
        .gather(Gatherers.scan(
          () -> "List of numbers: ",
          (result, num) -> result + num + ", ")) (1)
        .forEach(System.out::println);

List of numbers: 1,
List of numbers: 1, 2,
List of numbers: 1, 2, 3,
List of numbers: 1, 2, 3, 4,
List of numbers: 1, 2, 3, 4, 5,
List of numbers: 1, 2, 3, 4, 5, 6,
List of numbers: 1, 2, 3, 4, 5, 6, 7,
List of numbers: 1, 2, 3, 4, 5, 6, 7, 8,
List of numbers: 1, 2, 3, 4, 5, 6, 7, 8, 9,

mapConcurrent: An operation which executes a function concurrently with a configured level of max concurrency, using virtual threads.

titles.stream()
        .gather(Gatherers.mapConcurrent(2,
           word -> "- " + word
          )
        )
        .forEach(System.out::println);

- 1
- 2
- 3
- 4
- 5

These are the built-in gathers, you can implement custom gatherers using Gatherer.of and Gatherer.ofSequential methods.