Efficient way to divide a list into lists of n size

java arraylist partitioning
Rowhawn picture Rowhawn · Apr 28, 2011 · Viewed 95.6k times · Source

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 ArrayLists in Java (any pseudocode will do)

    for (int i = 1; i <= Math.floor((A.size() / n)); i++) {
            ArrayList temp = subArray(A, ((i * n) - n),
                    (i * n) - 1);
            // do stuff with temp
        }

    private ArrayList<Comparable> subArray(ArrayList A, int start,
                int end) {
            ArrayList toReturn = new ArrayList();
            for (int i = start; i <= end; i++) {
                toReturn.add(A.get(i));
            }
            return toReturn;
        }

where A is the list, n is the size of the desired lists

I believe this way is taking too much time when working with considerably large lists (of up to 1 million in size) so I'm trying to figure out what would be more efficient.

Answer

ColinD picture ColinD · Apr 28, 2011

You'll want to do something that makes use of List.subList(int, int) views rather than copying each sublist. To do this really easily, use Guava's Lists.partition(List, int) method:

List<Foo> foos = ...
for (List<Foo> partition : Lists.partition(foos, n)) {
  // do something with partition
}

Note that this, like many things, isn't very efficient with a List that isn't RandomAccess (such as a LinkedList).

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