Collection issues

Question

Hi,
Collection P has elements A,B,C,D and another Collection Q has C,D,E,F. I want a third Collection which will contain the elements A,B,E,F. i.e. the common elements of P and Q should be removed. Can you please give code for this?
Thanks
Srini

Answer 1

Here are two implementations you can have a look at. One is based on ArrayLists only. But because both contains() and remove() are used implicitly it gets very slow for large collections. So there's a second version based on LikedList/HashMap. It requires more temporary memory but is much faster for collections over say 5.000 members.
ArrayList differentObjects(ArrayList a1,ArrayList a2) { // LikedList/HashMap version
    LinkedList l1 = new LinkedList(a1);
    LinkedList l2 = new LinkedList(a2);
    HashSet h = new HashSet(l1);
    l1.removeAll(new HashSet(l2));
    l2.removeAll(h);
    h = null;
    l1.addAll(l2);
    return new ArrayList(l1);
ArrayList differentObjects1(ArrayList a,ArrayList b) { // ArrayList version
    ArrayList alessb = new ArrayList(a);
    alessb.removeAll(b);
    ArrayList blessa = new ArrayList(b);
    blessa.removeAll(a);
    alessb.addAll(blessa);
    return alessb;
void printArrayList(ArrayList a) {
    for (int i=0; i<a.size(); i++) {
        String s = (String) a.get(i);
        System.out.print(s);
    System.out.println();
void test() {
    ArrayList a1 = new ArrayList();
    ArrayList a2 = new ArrayList();
    a1.add(new String("A"));
    a1.add(new String("B"));
    a1.add(new String("C"));
    a1.add(new String("D"));
    a2.add(new String("C"));
    a2.add(new String("D"));
    a2.add(new String("E"));
    a2.add(new String("F"));
//    ArrayList a = differentObjects(a1,a2);
    ArrayList a = differentObjects1(a1,a2);
    printArrayList(a1);
    printArrayList(a2);
    printArrayList(a);