Pack 3: Collections, Streams & Optional

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

class Key {
    String id;

    Key(String id) {
        this.id = id;
    }

    @Override
    public int hashCode() {
        return id.hashCode();
    }

    @Override
    public boolean equals(Object o) {
        return o instanceof Key k && id.equals(k.id);
    }
}

public class Main {
    public static void main(String[] args) {
        Set<Key> set = new HashSet<>();
        Key k = new Key("x");
        set.add(k);
        k.id = "y";
        System.out.println(set.contains(new Key("y")));
    }
}