我在调用库存对象的所有方法中遇到此问题。但是，该对象不应该为null，因为我要求用户输入我存储在库存数组（对象数组）中的输入。我假设数据永远不会被存储，因为我在包含此库存数组的所有方法中都有此NPE。我相信我的方法代码还可以。
这是我的代码
public class HouseHoldGoods {
private static int numberOfGoods = 0;
private String type;
private String description;
private int priceRiksdaler;
private int priceSkilling;
private int priceRunstycken;
//Constructor to initialize instance variables to 0 and null
public HouseHoldGoods(){
this.description = null;
this.priceRiksdaler = 0;
this.priceSkilling = 0;
this.priceRunstycken = 0;
numberOfGoods++;
}
//Constructor taking 3 arguments and initializes them to the given values.
public HouseHoldGoods(String type, String description, OldSwedishCurrency currency){
this.type = type;
this.description = description;
/*currency = new OldSwedishCurrency()*/;//NOT SURE OF THIS ONE
}
//Accessors
public String getType(){
return type;
}
public String getDescription(){
return description;
}
public int getPriceRiksdaler(){
return priceRiksdaler;
}
public int getPriceSkilling(){
return priceSkilling;
}
public int getPriceRunstycken(){
return priceRunstycken;
}
public static int getNumberOfGoods(){
return numberOfGoods;
}
//Mutators
public void setType(String type){
this.type = type;
}
public void setDescription(String description){
this.description = description;
}
public void setPriceRiksdaler(int priceRiksdaler){
this.priceRiksdaler = priceRiksdaler;
}
public void setPriceSkilling(int priceSkilling){
this.priceSkilling = priceSkilling;
}
public void setPriceRunstycken(int priceRunstycken){
this.priceRunstycken = priceRunstycken;
}
//Equals method checking if 2 objects are the same (same price, type and description)
public boolean equals(HouseHoldGoods good){
return(this.getType().equalsIgnoreCase(good.getType()) && this.getDescription().equalsIgnoreCase(good.getDescription()) && this.getPriceRiksdaler() == good.getPriceRiksdaler() && this.getPriceSkilling() == good.getPriceSkilling() && this.getPriceRunstycken() == good.getPriceRunstycken());
}
//To string method which will return the content of the object of type HouseHoldGoods with the appropriate labels
public String toString(){
return ("Description: " + description + "\n" + "Type: " + type + "\n" + "Price: " + priceRiksdaler + " riksdaler, " + priceSkilling + " skilling, " + priceRunstycken + " runstycken.");
}
}
DRIVER
import java.util.Scanner;
public class HouseHoldGoodsDriver {
static Scanner keyboard = new Scanner(System.in);
static final String PASSWORD = "comp249"; //Password
static String passwordInput = " "; //Password input by the user
static int numItem = 0; //Number of items user wants to input in inventory
public static HouseHoldGoods[] inventory;//Array that will store the number of items
static int max = 5; //the maximum number of items that can be stored in inventory
static int itemRemain = 0; //Remaining number of item that are possible to store in inventory
public static void main(String[] args) {
System.out.println(" ********** Welcome to Nancy's HouseHold goods store! **********");
System.out.println();
int choice = 0; //Choice Nancy will be making
do{
//Displaying the menu for Nancy to choose
System.out.println("What would you like to do?");
displayMenu();
choice = keyboard.nextInt();
System.out.println();
//Declaring the inventory
inventory = new HouseHoldGoods[max];
switch (choice){
//Option 1
case 1:
if(askPassword()){
enterItem();
}
break;
case 2:
if(askPassword()){
askUpdateItem();
}
break;
case 3:
itemOfType();
break;
case 4:
itemPriceLowerThan();
break;
case 5:
int choiceStats = 0; //choice selected by user in the statistics menu
do{
System.out.println("What information would you like to access?");
displayStatsMenu();
choiceStats = keyboard.nextInt();
if(choiceStats == 1){
System.out.println(inventory[cheapestItem()]);
System.out.println("Item #" + (cheapestItem() + 1) + " is the cheapest.");
}
else if(choiceStats == 2){
System.out.println(inventory[mostCostlyItem()]);
System.out.println("Item #" + (mostCostlyItem() + 1) + " is the most costly.");
}
else if(choiceStats == 3){
itemOfType();
}
else if (choiceStats == 4){
averageCost();
}
//Handling the case where the input of the user does not appear in the stats menu
while(choiceStats != 1 && choiceStats != 2 && choiceStats !=3 && choiceStats != 4 && choice != 5){
displayStatsMenu();
choiceStats = keyboard.nextInt();
}
//When user inputs 5, we quit the stats menu and display the main menu
}while(choiceStats != 5);
case 6:
System.out.println("Come back soon!");
System.exit(0);
}
}
while(choice!=6);
}
//A method that will display the menu
public static void displayMenu() {
System.out.println(" 1. Enter a new item in inventory (password required)");
System.out.println(" 2. Change information of an item in inventory (password required)");
System.out.println(" 3. Display all items of a specific type");
System.out.println(" 4. Display all items under a certain price");
System.out.println(" 5. Statistics on your inventory");
System.out.println(" 6. Quit");
System.out.println("Please enter your choice >");
}
// Asking for password
public static boolean askPassword() {
int count = 0;
//Asking the password at least once
do {
System.out.println("Please enter your password");
passwordInput = keyboard.next();
if (passwordInput.equals(PASSWORD))
return (passwordInput.equals(PASSWORD));
count++; //Keeping track of the number of times it loops
} while (!passwordInput.equals(PASSWORD) && count < 3);//When the count is greater than 3 and password is wrong, stop looping
return false;
}
//Method to store items in the inventory
public static void enterItem() {
System.out.println("How many items would you like to enter?");
numItem = keyboard.nextInt();
itemRemain = max - HouseHoldGoods.getNumberOfGoods();
int numOfItems = 0;
/* If the user inputs a number of item bigger than the remaining
number of items possible*/
if (numItem > itemRemain) {
numOfItems = itemRemain - HouseHoldGoods.getNumberOfGoods();
System.out.println("You only have enough space for " + itemRemain + " item(s).");
for (int i = HouseHoldGoods.getNumberOfGoods(); i < max; i++) {
inventory[i] = createItem();
}
}
else {
numOfItems = HouseHoldGoods.getNumberOfGoods() + numItem;
for (int i = HouseHoldGoods.getNumberOfGoods(); i < numOfItems; i++) {
inventory[i] = createItem();
}
}
}
// Creating the items
public static HouseHoldGoods createItem() {
HouseHoldGoods good = new HouseHoldGoods();
System.out.println("Please enter the description of the item");
good.setDescription(keyboard.next());
System.out.println("Please enter the type of the item");
good.setType(keyboard.next());
System.out.println("Please enter the price (riksdaler, skilling, runstycken)");
good.setPriceRiksdaler(keyboard.nextInt());
good.setPriceSkilling(keyboard.nextInt());
good.setPriceRunstycken(keyboard.nextInt());
return good;
}
//Method to update an item
public static void askUpdateItem() {
String answerToUpdate = " ";
int itemNumber = 0;
int updateChoice = 0;
String answer = " ";
do {
System.out.println("Enter the number of the item you would like to update.");
itemNumber = keyboard.nextInt();
if (inventory[itemNumber - 1] == null) {
System.out.println("There is no item at this specific number.");
System.out.println("Would you like to enter another number? (yes/no)");
answer = keyboard.next();
if (answer.equalsIgnoreCase("yes")) {
askUpdateItem();
}
if (answer.equalsIgnoreCase("no")) {
displayMenu();
}
} else {
System.out.println("Item #" + itemNumber);
System.out.println(inventory[itemNumber - 1]);
do {
displayUpdateMenu();
updateChoice = keyboard.nextInt();
if (updateChoice == 1) {
System.out.println("Enter the new type");
inventory[itemNumber - 1].setType(keyboard.next());
} else if (updateChoice == 2) {
System.out.println("Enter the new description");
inventory[itemNumber - 1].setDescription(keyboard.next());
} else if (updateChoice == 3) {
System.out.println("Enter the new price (riksdaler, skilling, ");
inventory[itemNumber - 1].setPriceRiksdaler(keyboard.nextInt());
inventory[itemNumber - 1].setPriceSkilling(keyboard.nextInt());
inventory[itemNumber - 1].setPriceRunstycken(keyboard.nextInt());
}
// While loop to handle the case where the user inputs a
// number that is not within the choice
while (updateChoice != 1 && updateChoice != 2 && updateChoice != 3){
displayUpdateMenu();
updateChoice = keyboard.nextInt();
}
} while (updateChoice != 4);
}
System.out.println("Do you want to update another Item?(yes/no)");
answerToUpdate = keyboard.next();
} while (answerToUpdate.equalsIgnoreCase("yes"));
}
// Display menu to update item
public static void displayUpdateMenu() {
System.out.println("What would you like to update?");
System.out.println(" 1. Type " + "\n" + " 2. Description" + "\n" + " 3. Price"
+ "\n" + " 4. Quit");
}
//Method to display any item of a certain type
public static void itemOfType(){ /*******THIS DOESNT WORK*/
System.out.println("Please enter the type of the item(s) you would like to display.");
String typeInput = keyboard.next();
int count = 0; //To keep track of the number of item of the specific type
for(int i = 0; i < HouseHoldGoods.getNumberOfGoods(); i++){
if(inventory[i].getType().equalsIgnoreCase(typeInput)){
System.out.println("Item #" + (i+1));
System.out.println(inventory[i]);
count++;
}
System.out.println("There are " + count + " items of type " + typeInput + ".");
}
}
//Method to display item of specific price
public static void itemOfPrice(){
System.out.println("Please enter the price of the item(s) you would like to display. (riksdaler, skilling, runstycken)");
int riksdalerInput = keyboard.nextInt();
int skillingInput = keyboard.nextInt();
int runstyckenInput = keyboard.nextInt();
int count = 0;
for(int i = 0; i < HouseHoldGoods.getNumberOfGoods(); i++){
if(inventory[i].getPriceRiksdaler() == riksdalerInput && inventory[i].getPriceSkilling() == skillingInput && inventory[i].getPriceRunstycken() == runstyckenInput){
System.out.println("Iten #" + (i+1));
System.out.println(inventory[i]);
count++;
}
System.out.println("There are " + count + " items of price " + riksdalerInput + " riksdaler, " + skillingInput + " skilling, " + runstyckenInput + " runstycken.");
}
}
//Method to display the items with a lower price than the entered item
public static void itemPriceLowerThan(){
System.out.println("Please enter the price you would like to compare your items to. (riksdaler, skilling, runstycken");
int priceInputRiksdaler = 0;
int priceInputSkilling = 0;
int priceInputRunstycken = 0;
priceInputRiksdaler = keyboard.nextInt();
priceInputSkilling = keyboard.nextInt();
priceInputRunstycken = keyboard.nextInt();
for(int i = 0; i < inventory.length; i++){
if(inventory[i].getPriceRiksdaler() < priceInputRiksdaler && inventory[i].getPriceSkilling() < priceInputSkilling && inventory[i].getPriceRunstycken() < priceInputRunstycken){
System.out.println("Item #" + (i+1));
System.out.println(inventory[i]);
}
}
}
//Method to display the statistics menu
public static void displayStatsMenu() {
System.out.println(
" 1. Cost and details of cheapest item " + "\n" + " 2. Cost and details of most costly iten" + "\n"
+ " 3. Number of items of each type" + "\n" + " 4. Average cost of items in inventory"
+ "\n" + " 5. Quit" + "\n" + "Enter your choice");
}
//Method to find the cheapest item
public static int cheapestItem(){
int lowestCostIndex = 0;
for (int i = 0; i < HouseHoldGoods.getNumberOfGoods(); i++) {
if (inventory[i].getPriceRiksdaler() < inventory[lowestCostIndex].getPriceRiksdaler() && inventory[i].getPriceSkilling() < inventory[lowestCostIndex].getPriceSkilling() && inventory[i].getPriceRunstycken() < inventory[lowestCostIndex].getPriceRunstycken())
lowestCostIndex = i;
}
return (lowestCostIndex);
}
//Method to find the most costly item
public static int mostCostlyItem(){
int highestCostIndex = 0;
for(int i = 0; i < HouseHoldGoods.getNumberOfGoods(); i++){
if(inventory[i].getPriceRiksdaler() > inventory[highestCostIndex].getPriceRiksdaler() && inventory[i].getPriceSkilling() > inventory[highestCostIndex].getPriceSkilling() && inventory[i].getPriceRunstycken() > inventory[highestCostIndex].getPriceRunstycken())
highestCostIndex = i;
}
return(highestCostIndex);
}
//Method to calculate the average cost
public static void averageCost(){
int sumRiksdaler = 0;
int sumSkilling = 0;
int sumRunstycken = 0;
int averageRiksdaler = 0;
int averageSkilling = 0;
int averageRunstycken = 0;
for(int i = 0; i < inventory.length; i++){
sumRiksdaler += inventory[i].getPriceRiksdaler();
sumSkilling += inventory[i].getPriceSkilling();
sumRunstycken += inventory[i].getPriceRunstycken();
}
averageRiksdaler = sumRiksdaler/HouseHoldGoods.getNumberOfGoods();
averageSkilling = sumSkilling/HouseHoldGoods.getNumberOfGoods();
averageRunstycken = sumRunstycken/HouseHoldGoods.getNumberOfGoods();
System.out.println("The average cost is " + averageRiksdaler + " riksdaler, " + averageSkilling + " skilling, " + averageRunstycken + " runstycken");
}
}
例外：
Exception in thread "main" java.lang.NullPointerException
at HouseHoldGoodsDriver.itemOfType(HouseHoldGoodsDriver.java:231)
at HouseHoldGoodsDriver.main(HouseHoldGoodsDriver.java:41)
答案 0 :(得分：1)
在＆＃34;菜单＆＃34;的每个循环中，您创建一个inventory = new HouseHoldGoods[max];的新实例，丢弃先前输入的所有内容，并使其中包含null个元素
int choice = 0; //Choice Nancy will be making
do {
//Displaying the menu for Nancy to choose
System.out.println("What would you like to do?");
displayMenu();
choice = keyboard.nextInt();
System.out.println();
// *** This be bad ***//
//Declaring the inventory
inventory = new HouseHoldGoods[max];
switch (choice) {
//Option 1
case 1:
if (askPassword()) {
enterItem();
}
break;
将数组的创建移至do-loop启动之前