Java collection methods for union, Intersection and set difference

Question

I have written a program with 2 different collections of numbers and I was wondering how would I get the union, intersection and set difference from these two collections? I know that BitSet has methods for it but those doesn't work here.

public class Collections {

public static void main(String[] args) {

    // 1. Collection
    Set<Integer> grp1 = new HashSet<Integer>();

    grp1.add(1);
    grp1.add(2);
    grp1.add(3);
    grp1.add(4);
    grp1.add(5);

    // printing 1. collection
    System.out.println("1. collection: ");
    Iterator<Integer> i = grp1.iterator();
    while(i.hasNext()) {
        int numbers1 = i.next();
        System.out.print(numbers1 + " ");
    }
    System.out.println();   

    // 2. collection
    Set<Integer> grp2 = new HashSet<Integer>();

    grp2.add(8);
    grp2.add(7);
    grp2.add(6);
    grp2.add(5);
    grp2.add(4);

    // printing 2. collection
    System.out.println("2. collection: ");
    Iterator<Integer> y = grp2.iterator();
    while(y.hasNext()) {
        int numbers2 = y.next();
        System.out.print(numbers2 + " ");

    // Union


    // Intersection


    // Difference

         }
     }

}

Jean Logeart · Answer 1 · 2016-08-04T14:20:30.213

12

Union:

Set<Integer> union = new HashSet<>(grp1);
union.addAll(grp2);

Intersection:

Set<Integer> intersection = new HashSet<>(grp1);
intersection.retainAll(grp2);

Difference:

Set<Integer> diff = new HashSet<>(grp1);
diff.removeAll(grp2);

edited Aug 04 '16 at 14:20

answered Aug 04 '16 at 13:43

Jean Logeart

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I wish it worked that way, but all of those operations are actually destructive, i.e. they work in-place, on the existing collection, and they all return boolean. Your code does unfortunately not compile. I wish it did. – Sean Patrick Floyd Aug 04 '16 at 13:54
I already did try those but none of them work in Eclipse:'' Type mismatch: cannot convert from boolean to Set''. EDIT: like Sean said they return boolean values. – Daniel Aug 04 '16 at 14:01
Ah true!! I forgot about the signatures! Too much scala those days :) – Jean Logeart Aug 04 '16 at 14:21

score 7 · Answer 2 · answered Aug 04 '16 at 13:57

Guava has these operations in it's Sets class.

Set<Integer> union = Sets.union(set1, set2);
Set<Integer> intersection = Sets.intersection(set1, set2);
Set<Integer> difference = Sets.difference(set1, set2);

All of these return unmodifiable views, backed by the original Sets.

See Guava Explained -> Collection Utilities -> Sets

Java collection methods for union, Intersection and set difference

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