我正在开发一个程序，它将用户的输入转换为一个保存在单独类中的数组，然后从文件中获取数据并输入到同一个数组中。 我遇到的问题是代码编译但是我在调用addStock时在main中收到了一个空指针错误以及addStock中的第一个while语句。 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());
}
}
stock.java
public class Stock{
private String d_name;
private String d_identifier;
private int d_quantity;
private double d_unitCost;
private double d_sellingPrice;
public Stock(String name, String identifier, int quantity,
double unitcost, double sellingprice){
d_name = name;
d_identifier = identifier;
d_quantity = quantity;
d_unitCost = unitcost;
d_sellingPrice = sellingprice;
}
public String getName(){
return d_name;
}
public String getIdentifier(){
return d_identifier;
}
public int getQuantity(){
return d_quantity;
}
public double getUnitCost(){
return d_unitCost;
}
public double getSellingPrice(){
return d_sellingPrice;
}
//sets the quantity of an item
public void setQuantity(int quantity){
d_quantity = quantity;
}
//returns calculation of total cost of one type of item
public double getTotalCost(){
return (d_quantity*d_unitCost);
}
//returns calculation of total sales value of one type of item
public double getTotalSellingPrice(){
return (d_quantity*d_sellingPrice);
}
public String toString(){
//this is the form the string must fit
//System.out.format("%-20s\t%9s\t%1d\t%9.2f\t%9.2f\t%9.2f\t%9.2f\n"
return "Product Name, Identifier, Quantity, Unit Cost, Selling Price, Total Cost, Total Selling Price";
}
public static void main(String[]args){
Stock stock = new Stock("movie", "0a1b2c3d4", 5, 10, 20);
System.out.println(stock);
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());
return;
}
答案 0 :(得分：2)
您正在将answer设置为null，然后再拨打电话
answer.equals("yes")
抛出空指针异常，因为answer为空。
您应该将其设置为非空状态，如空字符串或其他人注意到的，在比较之前请求输入，以便它绝对是非空的，或者使用Yoda风格的等于检查
"yes".equals(answer)
答案 1 :(得分：1)
您正在将answer设置为null，然后将其与某些String值进行比较。这没有意义。在将它与任何内容进行比较之前，您可能希望将某些输入（例如System.in）读入变量answer。
答案 2 :(得分：1)
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：您将答案设为空
第3行：您查询答案是否等于某事。
现在，关于java的一点注意事项。 Null是一种类型，就像String一样。但是，Null是任何其他类型的子集。这意味着当你问
answer.equals()
你真的对java，new String(answer).equals()说。
由于答案实际上是null类型，因此java会将您的代码解释为new Null().equals()
问题是，类型null没有方法。所以它抛出一个NullPointerException因为你永远不能在null类型的任何东西上调用方法。
答案 3 :(得分：0)
您正在明确设置String answer = null，然后立即尝试在while循环中调用方法（您执行answer.equals...）。
考虑到程序这一部分的逻辑，你应该安全地使用do...while循环。像这样：
do{
// get user input, set answer variable
while(answer.equals("yes") && !answer.equals("no"));
答案 4 :(得分：0)
在使用while循环时不要将answer声明为null，请先尝试读取输入或使用(" ")之类的无效内容初始化它，这些内容将无法识别但已初始化字符串{{1} }