静态 ArrayLists 和转换类

Question

我在尝试使用多态性的 Java 项目中遇到了一些问题。我有以下课程，也可以共享指令、代码类和 UML 图。提前致谢。

                   GroceryItem abstract Class

          Perishable Class             NonPerishable Class

meat Class extends Perishables-----CannedSoup Class extends NonPerishables
fruit Class extends Perishables----Cereal Class extends NonPerishables

我附上下面的代码，我的问题是：

1. 如何从外部方法访问主方法中的数组列表？在项目的描述中，它说我使用静态数组列表，但是当我尝试这样做时，它给了我错误。

2.我如何知道我的方法中使用的易腐烂物品是水果还是肉类？

3.为什么要求我按照说明对数组列表中的项目进行排序和添加？什么意思？？

import java.util.ArrayList;

public class ShoppingCart {
public static void main(String args[]) {//main method

    ArrayList<Perishable> perishables = new ArrayList<Perishable>();
    ArrayList<NonPerishable> nonPerishables = new ArrayList<NonPerishable>();

    Cereal cereal = new Cereal();
    cereal.setBrandName("Kellogs");
    cereal.setManufacturer("Sereal Manufacturing Co.");
    cereal.setUnitPrice(12.79);
    //nonPerishables.add(cereal);//need to use sortItems method according to the instruction

    CannedSoup soup = new CannedSoup();
    soup.setSoupMaker("TastySoups Co.");
    soup.setDescription("Mushroom soup with natural flavors...");
    soup.setUnitPrice(2.39);
    //nonPerishables.add(soup); //need to use sortItems method according to the instruction


    Meat meat = new Meat();
    meat.setWeightInPounds(1.29);
    meat.setDescription("T-Bone Steak");
    meat.setPricePerPound(10.89);
    //perishables.add(meat); //need to use sortItems method according to the instruction

    Fruit fruit = new Fruit();
    fruit.setDescription("Bananas imported from Brazil.");
    fruit.setPricePerBag(6.99);
    //perishables.add(fruit); //need to use sortItems method according to the instruction

    System.out.println("Price of " + meat.getDescription() + " is:" + meat.getTotalPrice());
    System.out.println("Price of " + fruit.getDescription() + " is:" + fruit.getTotalPrice());

    sortItems(cereal);//adding the object to list through this method
    sortItems(soup);
    sortItems(meat);
    sortItems(fruit);

    displayItems();//print all the items 

}


/*
 * sortItems: This function will be to take the item passed in and sort and add the item into one of the of Perishable 
 * or NonPerishable ArrayList. Use casting and/or the instanceof keyword to determine which list the item should be added to.
 * I dont know what did they mean by sort items...
 */
static void sortItems(GroceryItem item){//This method needs to take 
    if(item instanceof Perishable) 
        perishables.add(item);//error here because I dont have access to arraylist

    if(item instanceof NonPerishable) 
        nonPerishables.add(item);//error here because I dont have access to arraylist
}


static void displayItems() {//display all the items from arraylist
    //if(item instanceof Perishable)

    System.out.println("Here is the list of your Shopping Cart:\n--------------------------");
    System.out.println("Perishables:\n----");

        for(Perishable item : perishables) //print all the perishables-How can I acces to arraylist in main method
            System.out.println(item.getDescription() + ", total price:" + item.getTotalPrice());

    System.out.println("NonPerishables:\n----");

    for(NonPerishable item : nonPerishables) {//how do I access to array list at main method
        if(item.getSoupMaker)//how do I check if it is soup or not without adding extra method to class
            System.out.println(item.getDescription + " " + item.getSoupMaker() + ", unitPrice:" + item.getUnitPrice() + 
                                        " totalPrice:" + item.getTotalPrice());
        else//if it is not soup it should be cereal
            System.out.println(item.getBrand() + " " + item.getManufacturer() + ", unitPrice:" + item.getUnitPrice() +
                                    " totalPrice:" + item.getTotalPrice());
    }
}


}

Answer 1

您的ArrayLists 应该是类变量，因此必须在main() 之外声明它们，以便类的其余部分可以访问它们。由于您处于静态上下文中，因此必须将它们声明为 static ArrayList<...>。