public class Combinations {
    public static void main(String[] args) throws IOException {
        LinkedList<String> list = new LinkedList<>();
        list.add("a");
        list.add("b");
        list.add("c");

        ArrayList<ArrayList<String>> result = new ArrayList<>();
        result.add(new ArrayList<String>());

        for (int i = 0; i < list.size(); i++) {
            ArrayList<ArrayList<String>> current = new ArrayList<>();

            Iterator<ArrayList<String>> resultIter = result.iterator();
            while (resultIter.hasNext()){
                ArrayList<String> l = resultIter.next();
                for (int j = 0; j < l.size()+1; j++) {
                    l.add(j, list.get(i));

                    ArrayList<String> temp = new ArrayList<>(l);
                    current.add(temp);
                    l.remove(j);
                }
            }
            result =new ArrayList<>(current);
        }
    }
}