Question

我使用selectItem（）方法进行堆叠。如果我运行它我得到这个错误“java.lang.NullPointerException”。绝对是arrayList的一个问题，但我无法找到答案。请帮忙。或者更好地在Dispenser类中创建ArrayList？
如果您需要更多信息我可以给它。感谢。

    public class Dispenser
    {
       private String name; // Item name.
       private int price; //Item cost.
        private int stock; // Total quantity in stock.

    /**
    * Create a new dispenser with the given details.
    * At the start it has 5 items in stock.
    */
    public Dispenser(String itemName, int itemPrice)
    {
        name = itemName;
        price = itemPrice;
        stock = 5; 
    }

    /**
    * Return an item name.
    * @return Item name.
    */ 
    public String getName()
    {
        return name;
    }


import java.util.*;

    /**
    * Main class for a vending mashine. 
    */ 
    public class VendingMachine
    {   
       private Dispenser disp; 
       private CoinBox box;
       private int credit;
       private ArrayList<Dispenser> items = new ArrayList<Dispenser>();


        /**
        * Constructor.
        */
        public VendingMachine()
        {   

            credit = 0;
            items.add(new Dispenser("Snickers", 55));
            items.add(new Dispenser("Mars", 50));
            items.add(new Dispenser("Bounty", 75)); 
     }
        public void selectItem(String itemName)
          {

         for(Dispenser product : items){
            if(disp.getName() == itemName){
               System.out.println("Get Your " + itemName);
         }
     }
         if((disp.getName() != itemName)){
               System.out.println( itemName + " is Out of stock" );
        }

     }

Answer 1

在public void selectItem(String itemName)函数中，我想（猜猜在这里）你使用disp引用而不是product引用循环内的变量。

         for(Dispenser product : items){
            if(product.getName() == itemName){//<--here
               System.out.println("Get Your " + itemName);
         }

方法selectProduct（）获取“java.lang.NullPointerException”。错误

1 个答案: