Is there a common Java utility to break a list into batches?

Check out Lists.partition(java.util.List, int) from Google Guava:

Returns consecutive sublists of a list, each of the same size (the final list may be smaller). For example, partitioning a list containing [a, b, c, d, e] with a partition size of 3 yields [[a, b, c], [d, e]] -- an outer list containing two inner lists of three and two elements, all in the original order.


In case you want to produce a Java-8 stream of batches, you can try the following code:

public static <T> Stream<List<T>> batches(List<T> source, int length) {
    if (length <= 0)
        throw new IllegalArgumentException("length = " + length);
    int size = source.size();
    if (size <= 0)
        return Stream.empty();
    int fullChunks = (size - 1) / length;
    return IntStream.range(0, fullChunks + 1).mapToObj(
        n -> source.subList(n * length, n == fullChunks ? size : (n + 1) * length));
}

public static void main(String[] args) {
    List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14);

    System.out.println("By 3:");
    batches(list, 3).forEach(System.out::println);
    
    System.out.println("By 4:");
    batches(list, 4).forEach(System.out::println);
}

Output:

By 3:
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
[10, 11, 12]
[13, 14]
By 4:
[1, 2, 3, 4]
[5, 6, 7, 8]
[9, 10, 11, 12]
[13, 14]

Use Apache Commons ListUtils.partition.

org.apache.commons.collections4.ListUtils.partition(final List<T> list, final int size)

Another approach is to use Collectors.groupingBy of indices and then map the grouped indices to the actual elements:

    final List<Integer> numbers = range(1, 12)
            .boxed()
            .collect(toList());
    System.out.println(numbers);

    final List<List<Integer>> groups = range(0, numbers.size())
            .boxed()
            .collect(groupingBy(index -> index / 4))
            .values()
            .stream()
            .map(indices -> indices
                    .stream()
                    .map(numbers::get)
                    .collect(toList()))
            .collect(toList());
    System.out.println(groups);

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11]]