Efficient way to divide a list into lists of n size

Question

I have an ArrayList, which I want to divide into smaller List objects of n size, and perform an operation on each. My current method of doing this is implemented with ArrayList objects 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 and 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 1

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).