public function setQuantity( $quantity )
    {
        if (!is_int( $quantity )) {
            throw new InvalidArgumentException( sprintf( 'Invalid quantity', $quantity ) );
        }

        $this->quantity = $quantity;

        return $this;
    }
public function setQuantity( $quantity )
    {
        if (!is_int( $quantity )) {
            throw new InvalidArgumentException( sprintf( 'Invalid quantity', $quantity ) );
        }

        $this->quantity = $quantity;

        return $this;
    }
@param  string $quantity

@return $this