How to compare two lists of arrays for equality with a custom comparator?

Question

To be specific, I have two lists:

List<SystemUserWithNameAndId> list1;
List<SystemUserWithNameAndId> list2;

I want to check if they contain the same system users, and ordering is not a problem. I tried to use a comparator to sort them first, and then check if they are equal using the equals () method of the lists. But I don’t want to redefine the equals method for SystemUserWithNameAndId, and I was wondering if I could use a matchr created for sorting or similar, to check for equality without explicitly repeating lists after sorting.

Comparator<SystemUserWithNameAndId> systemUserComparator = new Comparator<SystemUserWithNameAndId>()
    {

        @Override
        public int compare(SystemUserWithNameAndId systemUser1, SystemUserWithNameAndId systemUser2)
        {
            final int systemUserId1 = systemUser1.getSystemUserId();
            final int systemUserId2 = systemUser2.getSystemUserId();

            return systemUserId1 == systemUserId2 
                    ? 0
                    : systemUserId1 - systemUserId2;
        }
    };

    Collections.sort(systemUsers1, systemUserComparator);
    Collections.sort(systemUsers2, systemUserComparator);

    return systemUsers1.equals(systemUsers2);

Ideally, I want to say

CollectionUtils.isEqualCollections(systemUsers1, systemUsers2, someCustomComparator);

Answer 1

, , , :

public static <T> boolean areEqualIgnoringOrder(List<T> list1, List<T> list2, Comparator<? super T> comparator) {

    // if not the same size, lists are not equal
    if (list1.size() != list2.size()) {
        return false;
    }

    // create sorted copies to avoid modifying the original lists
    List<T> copy1 = new ArrayList<>(list1);
    List<T> copy2 = new ArrayList<>(list2);

    Collections.sort(copy1, comparator);
    Collections.sort(copy2, comparator);

    // iterate through the elements and compare them one by one using
    // the provided comparator.
    Iterator<T> it1 = copy1.iterator();
    Iterator<T> it2 = copy2.iterator();
    while (it1.hasNext()) {
        T t1 = it1.next();
        T t2 = it2.next();
        if (comparator.compare(t1, t2) != 0) {
            // as soon as a difference is found, stop looping
            return false;
        }
    }
    return true;
}

Answer 2

Java 8 . , :

List<SystemUserWithNameAndId> list1 = ... ;
List<SystemUserWithNameAndId> list2 = ... ;

if (list1.size() != list2.size()) {
    return false;
}

, . , - , , . .

Comparator<SystemUserWithNameAndId> comp =
    Comparator.comparingInt(SystemUserWithNameAndId::getSystemUserId);

. , , , . ( LinkedList ?), ArrayList .

list1.sort(comp);
list2.sort(comp);

, . 0, . , .

return IntStream.range(0, list1.size())
    .allMatch(i -> comp.compare(list1.get(i), list2.get(i)) == 0);