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

If you are dealing with arrays, you can use System.arraycopy() for that.

 int[] a = {1,2,3,4,5};

 int[] b = new int[2];
 int[] c = new int[3];

 System.arraycopy(a, 0, b, 0, 2); // b will be {1,2}
 System.arraycopy(a, 2, c, 0, 3); // c will be {3,4,5}

Answer 2

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 <T> List<T>[] split(List<T> 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 <T> List<T>[] split(List<T> 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);
}