public synchronized List<Turnstile> init() {
        if (turnstiles == null) {
            turnstiles = new MyArrayList<Turnstile>(NUM_TURNSTILES);
            for (int i = 0; i < NUM_TURNSTILES; ++i) {
                Turnstile turnstile = new Turnstile();
                turnstile.connect(this);
                turnstiles.add(turnstile);
            }
        }

        return turnstiles;
    }