Identify duplicates in a List

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The method add of Set returns a boolean whether a value already exists (true if it does not exist, false if it already exists, see Set documentation).

So just iterate through all the values:

public Set<Integer> findDuplicates(List<Integer> listContainingDuplicates) { 
    final Set<Integer> setToReturn = new HashSet<>(); 
    final Set<Integer> set1 = new HashSet<>();
         
    for (Integer yourInt : listContainingDuplicates) {
        if (!set1.add(yourInt)) {
            setToReturn.add(yourInt);
        }
    }
    return setToReturn;
}

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