Union or intersection of Java Sets

Question

What is the simplest way to make a union or an intersection of Sets in Java? I've seen some strange solutions to this simple problem (e.g. manually iterating the two sets).

Answer 1

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:

var result = new HashSet<>(set1);          // In Java 10 and above Set<Integer> result = new HashSet<>(set1); // In Java < 10 

result.addAll(set2); // Union 

result.retainAll(set2); // Intersection 

Answer 2

While guava for sure is neater and pretty much standard, here's a non destructive way to do union and intersect using only standard Java

Set s1 = Set.of(1,2,3); Set s2 = Set.of(3,4,5);       Set union = Stream.concat(s1.stream(),s2.stream()).collect(Collectors.toSet());  Set intersect = s1.stream().filter(s2::contains).collect(Collectors.toSet());