class RandomSet {

    private Map<Object, Integer> indexes = new HashMap<Object, Integer>();
    private List<Object> list = new ArrayList<Object>();
    private SecureRandom random = new SecureRandom();

    public void add(Object obj) {
        list.add(obj);
        indexes.put(obj, list.size() -1);
    }

    public void remove(Object obj) {
        final int index = indexes.get(obj);
        indexes.remove(obj);

        final Object last = list.get(list.size() - 1);
        list.set(index, last);
        list.remove(list.size() -1);

        indexes.put(last, index);
    }

    public Object random() {
        final int index = random.nextInt(list.size());
        return list.get(index);
    }
}