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How can I compare two array lists for equality with a custom comparator?

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 an issue. I tried to use a comparator to sort them first and then check if they're equal using the equals() method of lists. But I don't want to override the equals method for SystemUserWithNameAndId and I was wondering if I could use the comparator I created for sorting or a similar one to check for equality without explicitly iterating through the 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 be able to say,

CollectionUtils.isEqualCollections(systemUsers1, systemUsers2, someCustomComparator);
like image 400
Srini Avatar asked May 07 '15 16:05

Srini


1 Answers

Just implement the method that iterates, and reuse it every time you need it:

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;
}
like image 177
JB Nizet Avatar answered Sep 22 '22 11:09

JB Nizet