Aggregate List from equal objects in multiple Lists

Question

I'm trying to create a List<Object> aggregated based on List<Object> pool1 and List<Object> pool2. The main concern here is obviously performance since we have to compare every object in pool1 to every object in pool2. Is List necessary the best option?

Bad code version of the above:

List<Object> pool1 = new ArrayList<>();
List<Object> pool2 = new ArrayList<>();
List<Object> matchingObjects = new ArrayList<Object>();
for (Object p1 : pool1) {
    for (Object p2 : pool2) {
        if (p1.equals(p2)) {
            matchingObjects.add(p1);
        }
    }
}

Might work, but is not equivalent to the code posted in the question. `retainAll()` will modify `list1`. — ci_, Mar 23 '15 at 14:00

Bhargav Modi · Accepted Answer · 2015-03-23T14:07:42.707

0

retainAll will give a common elements between two lists

  pool1.retainAll(pool2);
  //pool1 contains elements common between two lists

if you want a saparate list of common elements than follow below process

List<Object> matchingObjects =  (ArrayList<Object>)pool1.clone();
matchingObjects.retainAll(pool2); // now matchingObjects will contain all the common elements

edited Mar 23 '15 at 14:07

answered Mar 23 '15 at 13:55

Bhargav Modi

2,605
3
29
49

I get `incompatible types: boolean cannot be converted to List – Noobcanon Mar 23 '15 at 13:56
@Noobcanon change to `List` from `List – Bhargav Modi Mar 23 '15 at 13:59
That's bad advice `retainAll()` returns a boolean, _not_ a new list. – ci_ Mar 23 '15 at 14:00
@Noobcanon what kind of object contained in your both lists is it boolean types than change it from – Bhargav Modi Mar 23 '15 at 14:00
@BhargavModi you have edited the answer, your first version was clearly wrong – ci_ Mar 23 '15 at 14:04
@ci_ ya thats why I edited my answer after your guidance – Bhargav Modi Mar 23 '15 at 14:05
This will add all objects to `matchingObjects`, not all equal objects, no? – Noobcanon Mar 23 '15 at 14:24
@Noobcanon matchingObjects after retainAll() will contain list of all common objects – Bhargav Modi Mar 23 '15 at 14:29
@Noobcanon if it helped atleast upvote it and mark it as correct :) – Bhargav Modi Mar 23 '15 at 14:33

Aggregate List from equal objects in multiple Lists

1 Answers1