Question

Is there a tool or library to find duplicate entries in a Collection according to specific criteria that can be implemented?

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To make myself clear: I want to compare the entries to each other according to specific criteria. So I think a Predicate returning just true or false isn’t enough.

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I can’t use equals.

Answer 1

I’ve created a new interface akin to the IEqualityComparer<T> interface in .NET.

Such a EqualityComparator<T> I then pass to the following method which detects duplicates.

public static <T> boolean hasDuplicates(Collection<T> collection,
        EqualsComparator<T> equalsComparator) {
    List<T> list = new ArrayList<>(collection);
    for (int i = 0; i < list.size(); i++) {
        T object1 = list.get(i);
        for (int j = (i + 1); j < list.size(); j++) {
            T object2 = list.get(j);
            if (object1 == object2
                    || equalsComparator.equals(object1, object2)) {
                return true;
            }
        }
    }
    return false;
}

This way I can customise the comparison to my needs.