java.util. HashSet

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    @java.io.Serial
    static final long serialVersionUID = -5024744406713321676L;

    transient HashMap<E,Object> map;

    static final Object PRESENT = new Object();

    public HashSet() {
        map = new HashMap<>();
    }

    public HashSet(Collection<? extends E> c) {
        map = HashMap.newHashMap(Math.max(c.size(), 12));
        addAll(c);
    }

    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }