Process Java Collection in batches

The following snippets shows how to process a given Java Collection (List / Set) in batches and process each batch in sequence or parallel.

Split a given collection into batches

int batchSize = 2;
List<String> input = List.of(
  "apple",
  "banana",
  "orange",
  "mango",
  "peach"
);
List<List<String>> batches = IntStream.range(0, input.size())
  .boxed()
  .collect(Collectors.groupingBy(index -> index / batchSize))
  .values()
  .stream()
  .map(indices -> indices.stream().map(input::get).toList())
  .toList();

batches.forEach(values -> System.out.println("values = " + values));

Parallel process collection in batches using Java's ForkJoin.commonPool()

int batchSize = 2;
List<String> input = List.of(
  "apple",
  "banana",
  "orange",
  "mango",
  "peach"
);
List<List<String>> batches = IntStream.range(0, input.size())
  .boxed()
  .collect(Collectors.groupingBy(index -> index / batchSize))
  .values()
  .stream()
  .map(indices -> indices.stream().map(input::get).toList())
  .toList();

batches
  .stream()
  .parallel()
  .forEach(values -> System.out.println("values = " + values));

Parallel process collection in batches using Java's Virtual Threads

int batchSize = 2;
List<String> input = List.of(
        "apple",
        "banana",
        "orange",
        "mango",
        "peach"
);
List<List<String>> batches = IntStream.range(0, input.size())
        .boxed()
        .collect(Collectors.groupingBy(index -> index / batchSize))
        .values()
        .stream()
        .map(indices -> indices.stream().map(input::get).toList())
        .toList();

try (ExecutorService executorService = Executors.newVirtualThreadPerTaskExecutor()) {
    List<CompletableFuture<Void>> futures = batches.stream()
            .map(values -> CompletableFuture
                    .runAsync(() -> System.out.println("values = " + values), executorService))
            // since map() is a lazy operation, we need to use toList() to create the Future
            .toList();
    // join() method blocks the main thread until all Futures are completed
    futures.forEach(CompletableFuture::join);
}

Parallel process collection in batches and return result using Java's Virtual Threads

int batchSize = 2;
List<String> input = List.of(
        "apple",
        "banana",
        "orange",
        "mango",
        "peach"
);
List<List<String>> batches = IntStream.range(0, input.size())
        .boxed()
        .collect(Collectors.groupingBy(index -> index / batchSize))
        .values()
        .stream()
        .map(indices -> indices.stream().map(input::get).toList())
        .toList();

try (ExecutorService executorService = Executors.newVirtualThreadPerTaskExecutor()) {
    List<CompletableFuture<List<Integer>>> futures = batches.stream()
            .map(values -> CompletableFuture
                    .supplyAsync(() -> values.stream().map(String::length).toList(), executorService))
            // since map() is a lazy operation, we need to use toList() to create the Future
            .toList();
    List<List<Integer>> result = futures.stream()
            // join() method blocks the main thread until all Futures are completed and returns the result
            .map(CompletableFuture::join)
            .toList();
    result.forEach(integers -> System.out.println("result = " + integers));
}
IntStream essentials:
// end value excluded
IntStream.range(0, 2)
  .boxed()
  .forEach(integer -> System.out.println("integer = " + integer));

// end value included
IntStream.rangeClosed(0, 2)
  .boxed()
  .forEach(integer -> System.out.println("integer = " + integer));

// ascending iteration
IntStream.iterate(0, value -> value <= 2, operand -> operand + 1)
  .boxed()
  .forEach(integer -> System.out.println("integer = " + integer));

// ascending iteration with step of 2
IntStream.iterate(0, value -> value <= 5, operand -> operand + 2)
  .boxed()
  .forEach(integer -> System.out.println("integer = " + integer));

// descending iteration
IntStream.iterate(2, value -> value >= 0, operand -> operand - 1)
  .boxed()
  .forEach(integer -> System.out.println("integer = " + integer));

// descending iteration with step of 2
IntStream.iterate(5, value -> value >= 0, operand -> operand - 2)
  .boxed()
  .forEach(integer -> System.out.println("integer = " + integer));