Set Operations (Union, Intersection, Difference)

This lesson covers computing a union, intersection, and difference in Java using Set, so you understand the basics of set operations. It's for anyone searching "Java Set operations tutorial."

Java's Set has addAll, retainAll, and removeAll methods, usable for computing a "union," "intersection," and "difference," respectively.

The example applies these methods to two TreeSets, {1, 2, 3, 4} and {3, 4, 5, 6}, and checks the union, intersection, and difference results. Using TreeSet automatically keeps the results sorted.

A common early mistake is not realizing these methods modify the calling Set directly. If you want to keep the original data intact, you need to make a copy first before performing the operation.

This is genuinely useful for real-world data processing too, like finding "users common to both groups" — and the ideas behind intersection and difference connect directly to understanding SQL joins in a database.

📖 Reference code

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Set<Integer> a = new TreeSet<>(Arrays.asList(1, 2, 3, 4));
        Set<Integer> b = new TreeSet<>(Arrays.asList(3, 4, 5, 6));

        Set<Integer> union = new TreeSet<>(a);
        union.addAll(b);

        Set<Integer> intersection = new TreeSet<>(a);
        intersection.retainAll(b);

        Set<Integer> difference = new TreeSet<>(a);
        difference.removeAll(b);

        System.out.println(union);
        System.out.println(intersection);
        System.out.println(difference);
    }
}

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🧪 This site can't compile or run Java directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).