How to generate the power-set of a given List?

Question

I\'m trying to generate a collection of all 2^N - 1 possible combinations of a given List of length N. The collection will map the number of elements in a combination to an

Answer 1

I test the code proposed by Elist and I found errors.

Here is a proposed correction : (in the last else of the function getPermutation(), I made two changes)

public class OrderedPowerSet {
private ArrayList inputList;
public int N;
private Map>> map = 
        new HashMap>>();

public OrderedPowerSet(ArrayList list) {
    inputList = list;
    N = list.size();
}

public List> getPermutationsList(int elementCount) {
    if (elementCount < 1 || elementCount > N) {
        throw new IndexOutOfBoundsException(
                "Can only generate permutations for a count between 1 to " + N);
    }
    if (map.containsKey(elementCount)) {
        return map.get(elementCount);
    }

    ArrayList> list = new ArrayList>();

    if (elementCount == N) {
        list.add(new HashSet(inputList));
    } else if (elementCount == 1) {
        for (int i = 0 ; i < N ; i++) {
            Set set = new HashSet();
            set.add(inputList.get(i));
            list.add(set);
        }
    } else {
        for (int i = 0 ; i < N-elementCount ; i++) {
            @SuppressWarnings("unchecked")
            ArrayList subList = (ArrayList)inputList.clone(); // one change
            subList.remove(0);
            OrderedPowerSet subPowerSet = 
                    new OrderedPowerSet(subList);
            for (Set s : subPowerSet.getPermutationsList(elementCount-1)) {
                Set set = new HashSet();
                set.add(inputList.get(i));
                set.addAll(s);
                list.add(set); // second change
            }

        }
    }

    map.put(elementCount, list);

    return map.get(elementCount);
}

}