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).
4 Answers
The simplest one-line solution is this:
set1.addAll(set2); // Unionset1.retainAll(set2); // IntersectionThe 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 < 10result.addAll(set2); // Unionresult.retainAll(set2); // IntersectionWhile 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());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
In order to add guava library to your project, You can see
2 import java.util.*;
public class sets { public static void swap(int array[], int a, int b) { // Swap function for sorting int temp = array[a]; array[a] = array[b]; array[b] = temp; } public static int[] sort(int array[]) { // sort function for binary search (Selection sort) int minIndex; int j; for (int i = 0; i < array.length; i++) { minIndex = i; for (j = i + 1; j < array.length; j++) { if (array[minIndex] > array[j]) minIndex = j; } swap(array, minIndex, i); } return array; } public static boolean search(int array[], int search) { // Binary search for intersection and difference int l = array.length; int mid = 0; int lowerLimit = 0, upperLimit = l - 1; while (lowerLimit <= upperLimit) { mid = (lowerLimit + upperLimit) / 2; if (array[mid] == search) { return true; } else if (array[mid] > search) upperLimit = mid - 1; else if (array[mid] < search) lowerLimit = mid + 1; } return false; } public static int[] append(int array[], int add) { // To add elements int newArray[] = new int[array.length + 1]; for (int i = 0; i < array.length; i++) { newArray[i] = array[i]; } newArray[array.length] = add; newArray = sort(newArray); return newArray; } public static int[] remove(int array[], int index) { // To remove duplicates int anotherArray[] = new int[array.length - 1]; int k = 0; if (array == null || index < 0 || index > array.length) { return array; } for (int i = 0; i < array.length; i++) { if (index == i) { continue; } anotherArray[k++] = array[i]; } return anotherArray; } public static void Union(int A[], int B[]) { // Union of a set int union[] = new int[A.length + B.length]; for (int i = 0; i < A.length; i++) { union[i] = A[i]; } for (int j = A.length, i = 0; i < B.length || j < union.length; i++, j++) { union[j] = B[i]; } for (int i = 0; i < union.length; i++) { for (int j = 0; j < union.length; j++) { if (union[i] == union[j] && j != i) { union = remove(union, j); // Removing duplicates } } } union = sort(union); System.out.print("A U B = {"); // Printing for (int i = 0; i < union.length; i++) { if (i != union.length - 1) System.out.print(union[i] + ", "); else System.out.print(union[i] + "}"); } } public static void Intersection(int A[], int B[]) { int greater = (A.length > B.length) ? (A.length) : (B.length); int intersect[] = new int[1]; int G[] = (A.length > B.length) ? A : B; int L[] = (A.length < B.length) ? A : B; for (int i = 0; i < greater; i++) { if (search(L, G[i]) == true) { // Common elements intersect = append(intersect, G[i]); } } for (int i = 0; i < intersect.length; i++) { for (int j = 0; j < intersect.length; j++) { if (intersect[i] == intersect[j] && j != i) { intersect = remove(intersect, j); // Removing duplicates } } } System.out.print("A ∩ B = {"); // Printing for (int i = 1; i < intersect.length; i++) { if (i != intersect.length - 1) System.out.print(intersect[i] + ", "); else System.out.print(intersect[i] + "}"); } } public static void difference(int A[], int B[]) { int diff[] = new int[1]; int G[] = (A.length > B.length) ? A : B; int L[] = (A.length < B.length) ? A : B; int greater = G.length; for (int i = 0; i < greater; i++) { if (search(L, G[i]) == false) { diff = append(diff, G[i]); // Elements not in common } } for (int i = 0; i < diff.length; i++) { for (int j = 0; j < diff.length; j++) { if (diff[i] == diff[j] && j != i) { diff = remove(diff, j); // Removing duplicates } } } System.out.println("Where A is the larger set, and B is the smaller set."); System.out.print("A - B = {"); // Printing for (int i = 1; i < diff.length; i++) { if (i != diff.length - 1) System.out.print(diff[i] + ", "); else System.out.print(diff[i] + "}"); } } public static void main(String[] args) { Scanner sc = new Scanner(System.in); System.out.println("Enter the operation"); String operation = sc.next().toLowerCase(); System.out.println("Enter the length of the first set."); int l1 = sc.nextInt(); System.out.println("Enter the length of the second set."); int l2 = sc.nextInt(); int A[] = new int[l1]; int B[] = new int[l2]; System.out.println("Enter the elements of the first set."); System.out.print("A = "); for (int i = 0; i < l1; i++) { A[i] = sc.nextInt(); } System.out.println("Enter the elements of the second set."); System.out.print("B = "); for (int i = 0; i < l2; i++) { B[i] = sc.nextInt(); } A = sort(A); // Sorting the sets before passing B = sort(B); sc.close(); switch (operation) { case "union": Union(A, B); break; case "intersection": Intersection(A, B); break; case "difference": difference(B, A); break; default: System.out.println("Invalid Operation"); } }
}
