'Java get not containing Objects of List a in List b

Is there a better way of doing this - its such a boilderplate code. I use Java 8 and I would do this with streams - but I would need some help doing this. I have tried it with ...removeIf() but it did not work.

final List<CalendarEventUserConnection> calendarEventUserConnectionsToDelete = new ArrayList<>();
    for (final CalendarEventUserConnection existingCalendarEventUserConnection : existingCalendarEventUserConnections) {
        boolean contains = false;
        for (final CalendarEventUserConnection newCalendarEventUserConnection : calendarEventUserConnections) {
            if (newCalendarEventUserConnection.getId() != null
                && newCalendarEventUserConnection.getId().equals(existingCalendarEventUserConnection.getId())) {
                contains = true;
            }
        }
        if (contains == false) {
            calendarEventUserConnectionsToDelete.add(existingCalendarEventUserConnection);
        }
    }
javalist


Solution 1:[1]

Your own search is O(NxM), where N is the number of elements in one list, and M in the other.

I suggest collecting all the IDs in the calendarEventUserConnections into a set.

Then you could collect all the elements in existingCalendarEventUserConnections whose ID is in that set into your deletion list.

Assuming your IDs are strings, this would be something like:

Set<String> idsToDelete = calendarEventUserConnections.stream()
                          .map( CalendarEventUserConnection::getId )
                          .filter( Objects::nonNull )
                          .collect(Collectors.toCollection(HashSet::new));
List<CalendarEventUserConnection> connectionsToDelete =
                          existingCalendarEventUserConnections.stream()
                          .filter( idsToDelete::contains )
                          .collect(Collectors.toList());

(Code untested)

Considering that you use a HashSet, this would reduce the complexity to O(M+N) instead of O(MxN)

Solution 2:[2]

If you want to obtain all Objects that are on listA and not on listB

public static <T> List<T> aNotB(List<T> listA, List<T> listB) {

    List<T> result = new ArrayList(listA);
    result.removeAll(listB);

    return result;
}

This only works if the equals method of T is correctly implemented...

Sources

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Source: Stack Overflow

Solution Source
Solution 1 RealSkeptic
Solution 2

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