public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Reserva other = (Reserva) obj;
        if (this.libro != other.libro && (this.libro == null || !this.libro.equals(other.libro))) {
            return false;
        }
        if (this.usuario != other.usuario && (this.usuario == null || !this.usuario.equals(other.usuario))) {
            return false;
        }
        return true;
    }