Java : How to find intersection of two Sets
This tutorial explains you two ways to find intersection of two Sets, i.e., using Java 8 Stream feature and Set’s retainAll() method.
Example: Find Intersection of two Sets
Let’s create two sets of strings as shown below and find the intersection between these two sets.
package com.sneppets.example;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;
public class IntersectionOfTwoSetsExample {
public static void main (String[] args) {
Set<String> set1 = new HashSet<>();
set1.add("a");
set1.add("b");
set1.add("c");
set1.add("d");
set1.add("e");
System.out.println("Set 1 : " + set1);
Set<String> set2 = new HashSet<>();
set2.add("b");
set2.add("d");
set2.add("f");
set2.add("g");
System.out.println("Set 2 : " + set2);
//Find intersect of sets using Stream API (Java 8)
System.out.println("Intersection of Set 1 & Set 2 Method 1: " + getIntersectOfSets1(set1, set2));
//Find intersect of sets using retainAll() method
System.out.println("Intersection of Set 1 & Set 2 Method 2: " + getIntersectOfSets2(set1, set2));
}
private static Set<String> getIntersectOfSets1(Set<String> set1, Set<String> set2) {
Set<String> intersectElements = set1.stream()
.filter(set2 :: contains)
.collect(Collectors.toSet());
if(!intersectElements.isEmpty()) {
return intersectElements;
}else {
return Collections.emptySet();
}
}
private static Set<String> getIntersectOfSets2(Set<String> set1, Set<String> set2) {
set1.retainAll(set2);
return set1;
}
}
Output
Set 1 : [a, b, c, d, e] Set 2 : [b, d, f, g] Intersection of Set 1 & Set 2 Method 1: [b, d] Intersection of Set 1 & Set 2 Method 2: [b, d]
Method 1 : Java 8 Stream API
Set<String> intersectElements = set1.stream() .filter(set2 :: contains) .collect(Collectors.toSet());
From Java 8 you can use sequential stream on Set 1 (bigger one) and filter it to produce a stream containing matching elements of specified collection (Set2) and use Collectors.toSet() to accumulate the intersects in to new Set.
Method 2: retainAll() method
set1.retainAll(set2);
The above code removes the elements from Set1 that are not contained in the specified collection (Set2).
Further Reading
- Get time units hours and minutes from Date in String format.
- Calculate Leap Year and Number of days in a Year
- Find first element from a collection (List or Set)
- How to add hours to unix timestamp or epochmilli in java?