PHP：TypeError：传递给...的参数1必须是[...]

Question

嗨，我正在测试一些php代码，并且我从PHPunit得到了这个错误。

  

TypeError：传递给Checkout \ Offer \ ThreeForTwo :: applicable（）的参数1必须是Chechout \ Cart \ Line的实例，是Checkout \ Cart \ Line的实例

ThreeForTwo类的来源：

<?php
namespace Checkout\Offer;

use Checkout\Offer;
use Chechout\Cart\Line;

class ThreeForTwo implements Offer
{

    /**
     * @return ThreeForTwo
     */
    public static function createOffer()
    {
        return new self();
    }

    /**
     * @return boolean
     */
    public function applicable(Line $line)
    {
        $db = Database::getIsntance();
        if($line->quantity >= 3 && $db->hasThreeForTwo($line->item)) 
        {
            return true;
        }
        return false;
    }

    /**
     * @return float
     */
    public function calculate(Line $line)
    {
        return $line->item->getPrice() * $line->quantity - $line->item->getPrice() * (intVal($line->quantity/3));
    }
}

以及Line类的Source调用方法：

<?php

namespace Checkout\Cart;

use Checkout\Item;
use Checkout\Offer\ThreeForTwo;
use Checkout\Offer\Discount;


class Line
{
    /** @var  int */
    private $quantity;

    /** @var  Item */
    private $item;

    /** @var array */
    private $offers;

    /**
     * Line constructor
     * @param Item 
     * @param int
     */
    public function __construct($item, $quantity)
    {
        $this->item = $item;
        $this->quantity = $quantity;
        $this->offers = array(
            ThreeForTwo::createOffer(),
            Discount::createOffer()
        );
    }

    /**
     * @return int
     */
    public function getQuantity()
    {
        return $this->quantity;
    }

    /**
     * @return Item
     */
    public function getItem()
    {
        return $this->item;
    }

    /**
     * @return float
     */
    public function calculate()
    {
        foreach($this->offers as $offer)
        {
            if($offer->applicable($this))
            {
                return $offer->calculate($this);
            }
            else 
            {
                return $this->quantity * $this->item->getPrice();
            }
        }
    }

}

我认为我可能在我的班级课程中遗漏了一些东西...

感谢您的帮助