java.lang.NullPointerException, но мой объект не равен нулю?

Question

У меня есть эта проблема со всеми моими методами, когда я вызываю объект инвентаря. Тем не менее, объект не должен быть пустым, потому что я прошу пользователя ввести данные, которые хранятся в массиве инвентаризации (массив объектов). Я предполагаю, что данные никогда не сохраняются, так как у меня есть этот NPE во всех методах, которые содержат этот массив инвентаря. Я считаю, что мой код для методов в порядке.

• Я посмотрел другие ответы на подобные вопросы
• Я связался с моим репетитором
• Я рассмотрел мой код несколько раз
• Я гугле мой вопрос и получили более сложных ответов для моего уровня в Java.

Вот мой код

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."); 
    } 
} 

ВОДИТЕЛЯ

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"); 
     } 



} 

Исключение:

Exception in thread "main" java.lang.NullPointerException 
at HouseHoldGoodsDriver.itemOfType(HouseHoldGoodsDriver.java:231) 
at HouseHoldGoodsDriver.main(HouseHoldGoodsDriver.java:41) 

Answer 1

На каждом цикле вашего меню "", вы создаете новый экземпляр inventory = new HouseHoldGoods[max]; отбросив все, что было ранее введено в него, и leavi нг она полна null элементов

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; 

Перемещение создание массива перед do-loop начинается