Set union and intersection using java streams

Ramesh K :

I currently have a java program that uses nested for loops to compute the union and intersection of a list of set of integers. How to do this using java parallel streams ? The code i have currently is as follows

for(Set<Integer> x : listA) {
  for (Set<Integer> y : listB) {
       Set u = Sets.union(x,y); // Uses Guava library
       Set i = Sets.intersection(x,y);
  }
}

I would like to make this fast as listA and listB are large.

Darshan Mehta :

You don't need streams for union, however, you can use it for intersection, e.g.:

Set<Integer> setA = new HashSet<>(Arrays.asList(1,2,3));
Set<Integer> setB = new HashSet<>(Arrays.asList(2,3,4));
Set<Integer> union = new HashSet<>();
union.addAll(setA);
union.addAll(setB);

Set<Integer> intersection = setA.parallelStream()
        .filter(setB::contains)
        .collect(Collectors.toSet());

System.out.println("Union : " + union);
System.out.println("Intersection : " +intersection);

Update

The above code finds intersection and union using Java's native libraries and streams. However, if you have a list of sets then you can wrap the above code in function and call it from stream that iterates two lists, e.g.:

private static void unionAndIntersection(Set<Integer> setA, Set<Integer> setB) {
    Set<Integer> union = new HashSet<>();
    union.addAll(setA);
    union.addAll(setB);

    Set<Integer> intersection = setA.parallelStream()
            .filter(setB::contains)
            .collect(Collectors.toSet());

    System.out.println("Union : " + union);
    System.out.println("Intersection : " +intersection);
}

public static void main(String[] args){ 
    List<Set<Integer>> listA = new ArrayList<>();
    List<Set<Integer>> listB = new ArrayList<>();
    listA.stream()
        .forEach(a -> {
            listB.stream()
            .forEach(b -> unionAndIntersection(a, b));
        });
}