Remove duplicates from ArrayLists

Question

I have an ArrayList of custom objects. I want to remove duplicate entries.

The objects have three fields: title, subtitle, and id. If a su

Answer 1

The solution depends on circumstances.

If you don't have much data then go with a Set Set unique = new HashSet<>(yourList); (use LinkedHashSet if you care about the order. It creates a new collection, but usually it's not a problem.

When you want to modify existing list and don't want to/can't create a new collection, you can remove duplicates like here:

List numbers =
    new ArrayList<>(asList(1, 1, 2, 1, 2, 3, 5));

System.out.println("Numbers: " + numbers);
ListIterator it = numbers.listIterator();
while (it.hasNext()) {
    int i = it.nextIndex();
    Integer current = it.next();
    for (int j = 0; j < i; ++j) {
        if (current.equals(numbers.get(j))) {
            it.remove();
            break;
        }
    }
}
System.out.println("Unique: " + numbers);

It works in O(n^2), but it works. Similar implementation, but simpler, is when the list is sorted - works in O(n) time. Both implementations are explained at Farenda: remove duplicates from list - various implementations.