Java 面向对象程序中的空指针错误

我正在开发一个程序，该程序将用户的输入输入保存在一个单独的类中的数组中，然后将文件中的数据输入到同一个数组中。 我遇到的问题是代码可以编译，但在调用addStock以及addStock中的第一个while语句时，我在main中收到了一个空指针错误。 谢谢你的帮助

INVENTORY.JAVA

    import java.io.File;
    import java.io.FileNotFoundException;
    import java.util.Scanner;

    //program skeleton is based on previous inventory.java(v 1.0) which was in turn based on        divider.java(v 1.0)
    //item data is now moved into a stock array using the stock.class,see stock.java notes   for info on its implementation.
    public class Inventory
    {
    private static int MAX_ITEMS = 100; //change based on total # of unique items
    private Stock[] d_list; //creation of Stock array object
    private int d_nextItem; //used to count total # of unique items

    //constructor for inventory
    public Inventory(){
       d_list = new Stock[MAX_ITEMS];
       d_nextItem = 0;
    }

    //user imputs item and info and is inputed into stock array d_list
    //not used
    public void addStock(String name, String identifier, int quantity,
                        double unitCost, double sellingPrice ){

      Scanner keyboard = new Scanner(System.in);

      name = null;
      identifier = null;
      quantity = 0;
      unitCost = 0.0;
      sellingPrice = 0.0;
      String answer = null;
      String cont = null;

      while(!answer.equals("yes") && !answer.equals("no")){
         System.out.println("do you want to input an additional item into the stock manually?(yes or no)");
         answer = keyboard.next();
         if(!answer.equals("yes") && !answer.equals("no")){
            System.out.println("you must enter yes or no");
         }
      }
      if(answer.equals("yes")){
         while(cont.equals("yes")){ //having an error here
            System.out.println("Enter the name of the item");
            name = keyboard.next();
            System.out.println("Enter the id tag");
            identifier = keyboard.next();
            System.out.println("Enter the quantity of "+name);
            quantity = keyboard.nextInt();
            System.out.println("Enter the Cost for the item");
            unitCost = keyboard.nextDouble();
            System.out.println("Enter the sales price of the item");
            sellingPrice = keyboard.nextDouble();
            System.out.println("do you want to enter info for a second item?");
            while(!cont.equals("yes") && !cont.equals("no")){
               cont = keyboard.next();
               System.out.println("you must enter yes or no");
            }
            d_list[d_nextItem] = new Stock(name, identifier, quantity, unitCost, sellingPrice);
            d_nextItem += 1;
         }
      }
      return;
    }

    public void loadInventory(String fileName)
    throws FileNotFoundException{
      if ( (fileName != null) && (!fileName.equals("")) ){
         Scanner ldInv = new Scanner(new File(fileName));
         String newLine = null;

         //initialization for variables
         String name = null;
         String identifier = null;
         int    quantity = 0;
         double unitCost = 0.0;
         double sellingPrice = 0.0;

         //reading of file into the Stock array
         while (ldInv.hasNextLine() && d_nextItem < MAX_ITEMS){
            if (ldInv.hasNextDouble()){
               System.err.println("Error:NO name of product detected!");
               System.exit(2);
            } else {
               name = ldInv.next();
            }
            if (ldInv.hasNextDouble()){
              System.err.println("Error:NO product identifier detected!");
              System.exit(2);
            } else {
               identifier = ldInv.next();
            }
            if (ldInv.hasNextInt()){
               quantity = ldInv.nextInt();
            } else {
               System.err.println("Error: Quantity of item is missing!");
               System.exit(2);
            }
            if (ldInv.hasNextDouble()){
               unitCost = ldInv.nextDouble();
            } else {
               System.err.println("Error: Price of Item is missing!");
               System.exit(2);
            }
             if (ldInv.hasNextDouble()){
               sellingPrice = ldInv.nextDouble();
            } else {
               System.err.println("Error: Sales price of Item is missing!");
               System.exit(2);
            }
            d_list[d_nextItem] = new Stock(name, identifier, quantity, unitCost, sellingPrice);
            newLine = ldInv.nextLine();
            d_nextItem += 1;
         }
      }
      if (d_nextItem == 0){
         System.err.println("There is no data in this file");
         System.exit(2);
      }
      return;
    }

    //prints onto screen data taken from file in a format to align with headings
    public void printInventory(){
      System.out.println();
      System.out.println(d_list[0]);
      for (int i = 0; i < d_nextItem; i++){
         Stock stock = d_list[i];
          System.out.format("%-20s\t%9s\t%1d\t%9.2f\t%9.2f\t%9.2f\t%9.2f\n",
stock.getName(),
                           stock.getIdentifier(), stock.getQuantity(), stock.getUnitCost(),
                           stock.getSellingPrice(), stock.getTotalCost(), stock.getTotalSellingPrice());
      /*
         System.out.println(stock.getName() + "\t" + stock.getIdentifier() + "\t" +
                            stock.getQuantity() + "\t" + stock.getUnitCost() + "\t" +
                            stock.getSellingPrice() + "\t" + stock.getTotalCost() +
                            "\t" + stock.getTotalSellingPrice());
      */

      }
      return;
    }
    //calculates total value of all items from the file
    public double getTotalSalesPrice(){
      double totalSP = 0.0;
      for (int i = 0; i < d_nextItem; i++){
         Stock stock = d_list[i];
         totalSP += stock.getTotalSellingPrice();
      }

      return totalSP;
    }
    //calculates total cost of all items from the file
    public double getTotalCost(){
      double totalV = 0.0;
      for (int i = 0; i < d_nextItem; i++){
         Stock stock = d_list[i];
         totalV += stock.getTotalCost();
      }

      return totalV;
    }
    /*
     //user inputs name returns info from stock
    //not used
    public Stock getStock(String name){
    }
    */
    public static void main(String[] args) throws FileNotFoundException{
      String name = null;
      String identifier = null;
      int quantity = 0;
      double unitCost = 0.0;
      double sellingPrice = 0.0;
      if (args.length!=1){
             System.err.println("Usage:java Inventory <input file name>");
             System.exit(1);
          }
      Inventory inventory = new Inventory();
      inventory.addStock(name, identifier, quantity, unitCost, sellingPrice);
      inventory.loadInventory(args[0]);
      inventory.printInventory();
      System.out.format("\nTotal potential sales from inventory = %6.3f\n",
                        inventory.getTotalSalesPrice());
      System.out.format("\nTotal store cost of inventory = %6.3f\n",
                        inventory.getTotalCost());

     }
    }

---

您正在将

answer

设置为

null

，然后将其与一些

String

值进行比较。那没有道理。在将变量

answer

与任何对象进行比较之前，您可能希望从

System.in

中读取一些输入。

将

answer

设置为

null

，然后调用

answer.equals("yes")

由于

answer

为空，因此引发空指针异常

您应该将其设置为非null的值，例如空字符串，或者如其他人所指出的，在比较之前请求输入，以便它肯定是非null的，或者使用Yoda样式的equals检查

"yes".equals(answer)

---

让我们从您的代码块开始

String answer = null;
String cont = null;

while(!answer.equals("yes") && !answer.equals("no")){

第1行：您将答案设置为空

第3行：查询答案，看看它是否等于某个值

现在，请注意java

Null

是一种类型，就像

String

一样<但是，code>Null是任何其他类型的子集。这意味着当你问

answer.equals（）

您实际上是在对java说，

新字符串（答案）.equals（）

。 因为答案实际上是

null

类型，所以java将您的代码解释为

newnull（）.equals（）

---

问题是，类型

null

没有方法。因此，它抛出一个

NullPointerException

，因为您永远不能对任何

null

类型的对象调用方法。

您正在显式设置

String answer=null

，然后立即尝试在while循环中调用该对象上的方法（在该循环中，

answer.equals…

）

考虑到程序这一部分的逻辑，您应该可以安全地使用

do…while

循环。像这样：

do{
    // get user input, set answer variable
while(answer.equals("yes") && !answer.equals("no"));

---

使用while循环时，不要将

answer

声明为null，请先尝试读取输入，或使用类似

（“”）

之类的无效内容对其进行初始化，该内容无法识别，但已初始化字符串

answer
您正在显式设置
字符串answer=null。为什么要填充它？作为一个侧节点，我不明白为什么要传递
名称
，
标识符
，
数量
，
单位成本
和
销售价格
作为
addStock（）
的参数，只是为了在函数开始时重置它们。
do{
    // get user input, set answer variable
while(answer.equals("yes") && !answer.equals("no"));