Identify duplicates in a List

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;
}

I needed a solution to this as well. I used leifg's solution and made it generic.

private <T> Set<T> findDuplicates(Collection<T> collection) {

    Set<T> duplicates = new LinkedHashSet<>();
    Set<T> uniques = new HashSet<>();

    for(T t : collection) {
        if(!uniques.add(t)) {
            duplicates.add(t);
        }
    }

    return duplicates;
}