Comparator.java

Comparator<List<Integer>> cp = new Comparator<List<Integer>>(){
            public int compare(List<Integer> l1 , List<Integer> l2){
                return l1.size()-l2.size();
            }
        } ;
Collections.sort(res,cp);

//With JDK 8 the syntax is much simpler.

List<CustomObject> list = new ArrayList<CustomObject>();

Collections.sort(list, (left, right) -> left.getId() - right.getId());
System.out.println(list);