Efficient way to divide a list into lists of n size

Question

I have an ArrayList, which I want to divide into smaller Lists of n size, and perform an operation on each. My current method of doing this is

implemented with Array

Answer 1

Here is a way to partition a List into an array of sublists, which ensures all but the last sublist have equal number of elements:

static  List[] split(List source, int numPartitions) {
    if (numPartitions < 2)
        return new List[]{source};

    final int sourceSize = source.size(),
        partitions = numPartitions > sourceSize ? sourceSize: numPartitions,
        increments = sourceSize / partitions;

    return IntStream.rangeClosed(0, partitions)
        .mapToObj(i -> source.subList(i*increments, Math.min((i+1)*increments, sourceSize)))
        .toArray(List[]::new);
}

If you want to guarantee numPartitions array size then you want:

static  List[] split(List source, int numPartitions) {
    if (numPartitions < 2)
        return new List[]{source};

    final int sourceSize = source.size(),
        partitions = numPartitions > sourceSize ? sourceSize: numPartitions,
        increments = sourceSize / partitions;

    return IntStream.range(0, partitions)
        .mapToObj(i -> source.subList(i*increments, i == partitions-1 ? sourceSize : (i+1)*increments))
        .toArray(List[]::new);
}