Union or intersection of Java Sets

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

You can achieve this using Google's Guava library. The following explanation is given below with the help of an example:

    // Set a
    Set<String> a = new HashSet<String>();
    a.add("x");
    a.add("y");
    a.add("z");

    // Set b
    Set<String> b = new HashSet<String>();
    b.add("x");
    b.add("p");
    b.add("q");

Now, Calculating Intersection of two Set in Java:

Set<String> intersection = Sets.intersection(a, b);
System.out.printf("Intersection of two Set %s and %s in Java is %s %n",
                a.toString(), b.toString(), intersection.toString());

Output: Intersection of two Set [z, y, x] and [q, p, x] in Java is [x]

Similarly, Calculating Union of two Set in Java:

Set<String> union = Sets.union(a, b);
System.out.printf("Union of two Set %s and %s in Java is %s %n",
                a.toString(), b.toString(), union.toString());

Output: Union of two Set [z, y, x] and [q, p, x] in Java is [q, p, x, z, y]

You can read more about guava library at https://google.github.io/guava/releases/18.0/api/docs/

In order to add guava library to your project, You can see https://stackoverflow.com/a/4648947/8258942

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());