private transient HashMap<E,Object> map;

// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();

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

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

    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    public boolean contains(Object o) {
        return map.containsKey(o);
    }