StreamGatherersBuiltin.java

import static java.util.stream.Collectors.toList;
import java.util.stream.Stream;
import java.util.stream.Gatherer;
import java.util.stream.Gatherers;

/*
 * To run: `java --enable-preview --source 22 StreamGatherersBuiltin.java`
 */
public class StreamGatherersBuiltin {
	public static void main(String[] args) {
		System.out.println("=== Peek ===");
		peek();
		System.out.println("=== Fold ===");
		fold();
		System.out.println("=== Scan ===");
		scan();
		System.out.println("=== Window Fixed ===");
		windowFixed();
		System.out.println("=== Window Sliding ===");
		windowSliding();
	}

	static void peek() {
		System.out.print("Elements: ");
		var elements = generate(10).collect(toList());
		System.out.println(elements);
		/*
		* Shows in [Javadoc](https://cr.openjdk.org/~vklang/gatherers/api/java.base/java/util/stream/Gatherers.html#peek(java.util.function.Consumer))
		* but seems not implemented yet.
		generate(10)
			.gather(
				Gatherers.peek(elem -> System.out.printf("%s "))
			)
			.collect(toList());
		System.out.println();
		*/
	}

	static void fold() {
		var sum = generate(10)
				.gather(
					Gatherers.fold(() -> 0, (acc, elem) -> acc + elem)
				)
				.findFirst()
				.get();
		System.out.println("The sum of all elements: " + sum);
	}

	static void scan() {
		var sums = generate(10)
			.gather(
				Gatherers.scan(() -> 0, (acc, elem) -> acc + elem)
			)
			.collect(toList());
		System.out.println("The sum of each element with its previous: " + sums);
	}

	static void windowFixed() {
		var pairs = generate(10)
				.gather(
					Gatherers.windowFixed(2)
				)
				.collect(toList());
		System.out.println("Pair of elements: " + pairs);
	}

	static void windowSliding() {
		var neighbors = generate(10)
				.gather(
					Gatherers.windowSliding(2)
				)
				.collect(toList());
		System.out.println("Elements that are neighbor: " + neighbors);
	}

	static Stream<Integer> generate(int size) {
		return Stream.iterate(1, acc -> acc + 1).limit(size);
	}
}