Set

A Set is a collection of elements that does not allow duplicates. Sets that come with Java implement the java.util.Set interface.

interface Set<E> {
    boolean contains(Object o);
    boolean add(E e);
    boolean remove(Object o);
    // and more
}

HashSet provides an implementation of Set that works via the same mechanics - .hashCode and .equals - as a HashMap.

import java.util.Set;
import java.util.HashSet;

class Main {
    void main() {
        Set<String> agents = new HashSet<>();
        agents.add("Syril");
        agents.add("Dedra");

        IO.println(agents);

        // If you try to add a duplicate to a set it won't
        // crash, but there won't be two in the collection
        agents.add("Syril");

        IO.println(agents);
    }
}

Checking membership in a large set is generally fast for the same reasons adding a new element to a large map is fast.