Converting Spring Page<T> to Stream<T>

PageStreamer.java

package com.github.chakrit;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.domain.Pageable;

import java.util.function.Function;
import java.util.stream.Stream;

/**
 * PageStreamer provides convenience methods for working with Page-inated result by converting them to Streams instead.
 *
 * This helps in situations where we cannot return Streams from JPA directly such as when we want to use Specification<T>
 * but the API is more convenient to process in a stream fashion such as when generating reports or calculating complex
 * statistics.
 *
 * Usage:
 *
 *     PaymentRepository paymentRepository;
 *
 *     // generate sales report by category, streaming, concurrently.
 *     var stream = PageStreamer.stream(paymentRepository::findAll)
 *         .collect(Collectors.groupingByConcurrent(Payment::getCategory),
 *              Collectors.reducing(0, Payment::getAmount, (a, b) -> a + b));
 */
public final class PageStreamer {
    public static final int DEFAULT_BATCH_SIZE = 100;

    public static <E> Stream<E> stream(Function<Pageable, Page<E>> pageSupplier) {
        var pageable = PageRequest.of(0, DEFAULT_BATCH_SIZE);
        var page = pageSupplier.apply(pageable);
        if (page == null || !page.hasContent()) {
            return Stream.empty();
        }

        return Stream.iterate(page, p -> !p.isEmpty(), p -> p.hasNext() ? pageSupplier.apply(p.nextPageable()) : Page.empty())
                .flatMap(p -> p.getContent().stream());
    }
}