code: public function setQuantity($quantity)
      {
  
          if (!is_null($quantity) && ($quantity <= 0)) {
              throw new \InvalidArgumentException('invalid value for $quantity when calling OrderItem., must be bigger than 0.');
          }
  
          $this->container['quantity'] = $quantity;
  
          return $this;
      }
docs: Sets quantity
  @param float $quantity
  @return $this