@Override
    public String toString() {
        StringBuilder sb = new StringBuilder("[");
        for (int i = 0; i < size(); i++) {
            if (i != 0) {
                sb.append(", ");
            }
            sb.append(get(i) == null ? "null" : get(i).toString());
        }
        sb.append("]");
        return sb.toString();
    }