Sets: unique values

1import java.util.*;
2 
3public class Main {
4    public static void main(String[] args) {
5        Set<Integer> seen = new HashSet<>();
6        seen.add(5);
7        seen.add(1);
8        seen.add(5);
9        System.out.println(seen.size() + " " + seen.contains(1) + " " + seen.contains(7));
10    }
11}