Get intersection of two sets

Using 'retainAll' method, we can get intersection of two sets.

Example

Set<Integer> multiplesOf2 = new HashSet<>();

Set<Integer> multiplesOf3 = new HashSet<>();

multiplesOf2.retainAll(multiplesOf3);

One problem with above example is, after executing 'retainAll' statement, multiplesOf2 contain only the intersection of elements. To preserve the set data, create a new set and call retainAll on that set.

public static Set<Integer> getIntersection_1(Set<Integer> set1, Set<Integer> set2) {

         Set<Integer> newSet = new HashSet<>(set1);

         newSet.retainAll(set2);

         return newSet;

}

You can even use Streams to find the intersection of sets.

public static Set<Integer> getIntersection_2(Set<Integer> set1, Set<Integer> set2) {

         return set1.stream().filter(set2::contains).collect(Collectors.toSet());

}

Find the below working application.

App.java

package com.sample.app;

import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;

public class App {

 public static Set<Integer> getIntersection_1(Set<Integer> set1, Set<Integer> set2) {
  Set<Integer> newSet = new HashSet<>(set1);
  newSet.retainAll(set2);

  return newSet;
 }

 public static Set<Integer> getIntersection_2(Set<Integer> set1, Set<Integer> set2) {
  return set1.stream().filter(set2::contains).collect(Collectors.toSet());
 }

 public static void main(String args[]) {

  Set<Integer> multiplesOf2 = new HashSet<>();
  Set<Integer> multiplesOf3 = new HashSet<>();

  multiplesOf2.add(2);
  multiplesOf2.add(4);
  multiplesOf2.add(6);
  multiplesOf2.add(8);
  multiplesOf2.add(10);
  multiplesOf2.add(12);

  multiplesOf3.add(3);
  multiplesOf3.add(6);
  multiplesOf3.add(9);
  multiplesOf3.add(12);

  System.out.println("multiplesOf2 : " + multiplesOf2);
  System.out.println("multiplesOf3 : " + multiplesOf3);

  System.out.println("Calculating intersection of sets");
  Set<Integer> intersection1 = getIntersection_1(multiplesOf2, multiplesOf3);
  Set<Integer> intersection2 = getIntersection_2(multiplesOf2, multiplesOf3);

  System.out.println("multiplesOf2 : " + multiplesOf2);
  System.out.println("multiplesOf3 : " + multiplesOf3);
  System.out.println("intersection1 : " + intersection1);
  System.out.println("intersection2 : " + intersection2);

 }

}

Output

multiplesOf2 : [2, 4, 6, 8, 10, 12]

multiplesOf3 : [3, 6, 9, 12]

Calculating intersection of sets

multiplesOf2 : [2, 4, 6, 8, 10, 12]

multiplesOf3 : [3, 6, 9, 12]

intersection1 : [6, 12]

intersection2 : [6, 12]

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