HashSet: No Duplicates Allowed

A bag of unique things

A set stores values with no duplicates. Adding something that's already there does nothing:

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet<String> visited = new HashSet<>();
        System.out.println(visited.add("Goa"));
        System.out.println(visited.add("Kerala"));
        System.out.println(visited.add("Goa"));
        System.out.println("Places: " + visited.size());
        System.out.println(visited.contains("Kerala"));
    }
}

Output

true
true
false
Places: 2
true

add returns false when the value was already in the set.

Think of it like this: A guest list at a party: each name is written once, no matter how many times someone says it.

Removing duplicates, sorted

A TreeSet is a set that keeps its values sorted:

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        int[] rolls = {4, 2, 6, 2, 4, 1, 6, 6};
        TreeSet<Integer> unique = new TreeSet<>();
        for (int r : rolls) {
            unique.add(r);
        }
        System.out.println(unique);
        System.out.println("Smallest: " + unique.first() + ", largest: " + unique.last());
    }
}

Output

[1, 2, 4, 6]
Smallest: 1, largest: 6

Which collection should I use?