The simplest one-line solution is this:
set1.addAll(set2); // Union
set1.retainAll(set2); // Intersection
The above solution is destructive, meaning that contents of the original set1 my change.
If you don’t want to touch your existing sets, create a new set:
Set<E> result = new HashSet<>(set1);
// └─ your specific type
result.addAll(set2); // Union
result.retainAll(set2); // Intersection
