List<Integer> l1 = new ArrayList<Integer>();

l1.add(1);
l1.add(2);
l1.add(3);

List<Integer> l2= new ArrayList<Integer>();
l2.add(4);
l2.add(2);
l2.add(3);

List<Integer> l3 = new ArrayList<Integer>(l2);
l3.retainAll(l1);

listA.retainAll(listB);
// listA now contains only the elements which are also contained in listB.

List<Integer> common = new ArrayList<Integer>(listA);
common.retainAll(listB);
// common now contains only the elements which are contained in listA and listB.