Java Utililty Methods Collection Union

ArrayList list = new ArrayList();
Map mapa = getCardinalityMap(a);
Map mapb = getCardinalityMap(b);
Set elts = new LinkedHashSet(a);
elts.addAll(b);
Iterator it = elts.iterator();
while (it.hasNext()) {
    Object obj = it.next();
...

final List list = new ArrayList();
addAllUnique(list, c1);
addAllUnique(list, c2);
return list;

return () -> new Iterator() {
    int which = 0;
    Iterator it = null;
        while (which < sets.length && !it.hasNext()) {
            it = sets[which++].iterator();
    @Override
    public boolean hasNext() {
        return which < sets.length;
    @Override
    public Object next() {
        final Object result = it.next();
        while (which < sets.length && !it.hasNext()) {
            it = sets[which++].iterator();
        return result;
};

List<T> result = new ArrayList<T>(a);
result.addAll(b);
return result;

int index1 = 0;
int index2 = 0;
while (true) {
    if (index1 >= a.length) {
        copyRemainder(b, index2, collector);
        break;
    } else if (index2 >= b.length) {
        copyRemainder(a, index1, collector);
...