Question

我对代码的其他部分没有太大问题，但如果有，请向我强调！我得到的主要问题是当我进入

时

3
Fish n Chips
Chicken Chop
Grilled Salmon
9
Order 1 2 1 3

我收到此错误。

Exception in thread "main" java.lang.NullPointerException
        at ListOrder.addNewOrder(QuickEat.java:30)
        at QuickEat.main(QuickEat.java:75)

我已经检查过，它与dishID-1没有任何关系，但提供方法但问题仍然存在

人们告诉我，我已经实例化了dishQueues，这是队列整数的一个arraylist但是我从未在数组列表的每个添加中实例化任何队列整数。我试过了，但它仍然给了我 NPE 。

我该如何正确地做到这一点？

  1 // This program helps staff manage customers'
  2 // orders and decide who should be given a ready dish.
  3 
  4 import java.util.*;
  5 
  6 // This class represents all orders of customers
  7 class ListOrder {
  8 
  9     // Data member
 10     private int numDishes;
 11     // All dishes which the restaurant offers
 12     private String[] dishes;
 13     // Each dish has a queue of customers who ordered this dish
 14     // All such queues are put inside an ArrayList called dishQueues
 15     private ArrayList<Queue<Integer>> dishQueues;
 16 
 17     // Constructor
 18     public ListOrder(int numDishes, Scanner sc) {
 19         this.numDishes = numDishes;
 20         dishes = new String[this.numDishes];
 21         ArrayList<Queue<Integer>> dishQueues = new ArrayList<Queue<Integer>>();
 22         for(int i=0;i<this.numDishes;i++)   {
 23             dishes[i] = new String(sc.nextLine());
 24             dishQueues.add(new LinkedList<Integer>());
 25         }
 26     }
 27 
 28     // Add new order to the list
 29     public void addNewOrder(int dishID, int tag) {
 30         dishQueues.get(dishID-1).offer(new Integer(tag));
 31     }   
 32     
 33     // Process food when it is ready
 34     // Return the customer who currently ordered for the dish
 35     // if there is no customer order for this dish return -1
 36     public int processReadyFood(int dishID) {
 37         int ready=0;
 38         if (dishQueues.get(dishID-1).peek() == null)
 39             return -1;
 40         else    
 41             ready = dishQueues.get(dishID-1).poll();
 42             
 43         return ready;
 44     }   
 45     
 46     // Get dish's name
 47     public String getDishName(int dishID) {
 48         return dishes[dishID-1];
 49     }   
 50     
 51 }
 52 
 53 public class QuickEat {
 54 
 55     public static void main(String [] args) {
 56     
 57         Scanner sc = new Scanner(System.in);
 58         int numDishes = sc.nextInt();
 59         String command; 
 60         sc.nextLine();
 61 
 62         // Create the list order of food
 63         ListOrder listOrder = new ListOrder(numDishes, sc);
 64 
 65         int noOfCommands = sc.nextInt();
 66         sc.nextLine();
 67 
 68         // Process commands
 69         for(int i=0; i<noOfCommands;i++)    {
 70             command = sc.next();
 71             if(command.equals("Order")) {
 72                 int tag = sc.nextInt();
 73                 int numOfDishes = sc.nextInt();
 74                 for(i=0;i<numOfDishes;i++)
 75                     listOrder.addNewOrder(sc.nextInt(),tag);
 76             }
 77             else if(command.equals("Ready"))    {
 78                 int dishID = sc.nextInt();
 79                 int customer = listOrder.processReadyFood(dishID);
 80                 if(customer == -1)
 81                     System.out.println("Throw away " + listOrder.getDishName(dishID));
 82                 else
 83                     System.out.println(listOrder.getDishName(dishID) + " ready to be served to Tag " + customer);
 84             }
 85         }
 86     }
 87 }

Answer 1

ArrayList<Queue<Integer>> dishQueues = new ArrayList<Queue<Integer>>();

使用以下代码替换构造函数中的上一行：

dishQueues = new ArrayList<Queue<Integer>>();

您已将dishQueues ArrayList声明为实例变量。您必须在构造函数中初始化它。

链表队列输入中的nullpointerexception

1 个答案: