Hi I'm having trouble with this program I am working on. It is to simulate a grocery store. The program will read in a file which is called inventory.dat
the file will have items in it like:

11012 gallon-milk 1.99 F
11014 butter 2.59 F

Then a customer will input the product number then the quantity they want
then they will type 0 0 to end their order. When that happen it suppose to ouput a reicpet.

Customer Input

11012 1
11014 1
0 0

Output Recipet

Customer 1

gallon-milk 1@1.99
butter 1@2.59

Subtotal:
Tax:
Total:

I keep getting the errors
Exception in thread "main" java.lang.NullPointerException
at GroceryItem.itemSearch(GroceryItem.java:64)
at GrocerySim.calcCustomerReciept(GrocerySim.java:98)
at GrocerySim.main(GrocerySim.java:138)
I've gotten lost in my code and I'm having a hard time trying to figure out what to do next.

Here is my code for it in two parts GroceryItem and GrocerySim

import java.util.*;
import java.io.*;


public class GrocerySim {
      
        public static final int MAXNUMBEROFITEMS = 100;    // the maximum number of items that can be bought.
        static GroceryItem[] dataList=new GroceryItem[MAXNUMBEROFITEMS];
        static int inventory=0;
        static int quantity=0;
        static int productNumber=0;
      
      
/* readIventory will read in the file "inventory.dat" */
      
      
    private static void readInventory(String filename)
    {
        Scanner fileInput;
        File inFile = new File("inventory.dat");
        try
         {
           fileInput =  new Scanner(inFile);
           while(fileInput.hasNext())
           {
                 GroceryItem.readItem(fileInput);
                 ++inventory;  //stick update for inventory
                 //System.out.print(inventory);
                 System.out.println("File was read successfully.");      // Debug to make sure file was read in correctly.
                   if(inventory>100)  
                   {
                                        {
                                                System.out.println("Item Number "+dataList[inventory].getProductNumber()+" has already been added.");
                                        }
                   }
              
                }

         }
          catch(FileNotFoundException e)
          {
            System.out.println(e);                                                
            System.exit(1);
          }
    } 
 /*readCustomerInput(Scanner keyboard) will get the user input for the product number and
  * quantity of that item they want.
  */
        public static void readCustomerInput(Scanner keyboard)
        {
              
                System.out.print("Enter Product Number and Quantity:"+'\n');
                while(keyboard.hasNext())
                {
                        if(productNumber == 0 && quantity == 0)
                        {
                        		productNumber=keyboard.nextInt();
                               quantity=keyboard.nextInt();
                        }
                        else break;
               
                }
              }   
                // System.out.println(productNumber+ " "+quantity); // debug to make sure productNumber and quantity are being read in.
                      
                    
               
       
      
/*calcCustomerReciept() will calculate the customers order displaying the subtotal, tax,
 * and final price.
 */
 public static void calcCustomerReciept()
 {
                int item=0;
                int numberOfCustomers=1;
                int nonFoodItem=0;                      // will determine if the item is a nonFoodItem.
                int foodItem=0;                         // will determine if the item is a foodItem.
                double foodTax = 0.2;           // food tax is 2%
                double nonFoodTax = 0.7;    // nonfood tax is 7%
              
                System.out.println('\n'+"Customer"+" "+numberOfCustomers+'\n');
              // System.out.println(productNumber+ " "+quantity);
         
                while(productNumber == 0 && quantity== 0);
                
                        {
                        	//System.out.println("stop");
                                item=GroceryItem.itemSearch(dataList,inventory,productNumber);
                                if(item!=-1)
                                {
                                double priceTimesQuantity = dataList[item].getPrice()*quantity*100.0f/100.0f;
                                if(dataList[item].getTax() == 'N')
                                {
                                        nonFoodItem += priceTimesQuantity;
                                }
                                else
                                {
                                        foodItem += priceTimesQuantity;
                                }
                                      
                                System.out.println(dataList[item].getDescription()+"\t"+
                                        quantity+" @ "+ "$ "+ (dataList[item].getPrice())*100.0f/100.0f+
                                " "+"\t"+"$ "+priceTimesQuantity+" "+dataList[item].getTax());           // displays the quantity followed by the price
                                }                                                                        // then quantity times price and finally the food tax.)
                                //{
                                //      System.out.println("Item number "+productNumber+" is not in located in the inventory.");
                        
                              //}
                       
                        float subtotal = nonFoodItem+foodItem*100.0f/100.0f;                                    // subtotal = the addition of both food and nonfood items
                        double tax = (foodItem*foodTax)+(nonFoodItem*nonFoodTax)*100.0f/100.0f; // tax is food items * 2% and non food * 7%
                      
                        System.out.println('\n'+"\t"+"Subtotal:\t"+"$ "+subtotal);                                        //display the subtotal
                        System.out.println("\t"+"Tax:\t\t"+"$ "+tax+'\n');                                                                                              // display the tax
                        System.out.println("\t"+"Total:\t\t"+"$ "+(subtotal+tax)*100.0f/100.0f+'\n');                               // display the final total
                        numberOfCustomers++;    //begin new customer
                        
                }
 }

              

        public static void main( String [] args)
        {
                        Scanner keyboard = new Scanner(System.in);
                        readInventory("inventory.dat");
                        readCustomerInput(keyboard);
                        calcCustomerReciept();

                      
                      
        }

}
import java.util.*;

public class GroceryItem
{


                               
private static int productNumber;           // the product number for an item
private static String description;                      // the description of the item
private static double price;                // the price for the item
private static char tax;                        // the tax for the item

// accessor methods

  public  int getProductNumber() { return productNumber;}
  public String getDescription() { return description;}
  public double getPrice() { return price;}
  public char getTax() { return tax;}
      

 //*******************************************************************************************

        /* readItem(Scanner file) will read in the productNumber, description, price and tax from
         * the inventory data, it will then leave the position of the object on the next input line.
         */

    public static void readItem(Scanner file)
        {
                while(file.hasNext())
                {
                productNumber= file.nextInt();
                description=file.next();
                price=file.nextDouble();
                tax=(file.next()).charAt(0);
                //System.out.print(productNumber+"  "+ description+"  "+ price+"  "+ tax+"  "); // debug test
                file.hasNextLine();    //leaves at the next input line
                }
      
        }
  
    /* itemSearch(GroceryItem [] data, int elementsUsed, int productItem) searches the array of items
     * looking for the item that matches productItem. It if cannot find it wil return -1. 
     */
  
    public static int itemSearch(GroceryItem [] data, int elementsUsed , int productItem)
    {
    	
        int index = -1;
        for(int i = 0; i < elementsUsed; i++)
        {
        	System.out.print(productItem);
                if(data[i].getProductNumber() == productItem)
                {
                        index = i;
                        break;
                }                     
        }
        return index; 
    }

    /* printIventory(GroceryItem[] data, int elementsUsed) prints all the inventory that is
     * from i to i<elementsUsed.
     */
    public static void printInventory(GroceryItem[] data, int elementsUsed)
        {
                for(int i=0; i<elementsUsed; i++)
                {
                        System.out.printf("\t"+data[i].getProductNumber()+"\t"+data[i].getDescription()+"\t"
                                        +"\t"+data[i].getPrice()+"\t"+data[i].getTax()+'\n');
                }
        }
      
}

it's a bit hard to debug is just looking at the code and I can't really be bothered compiling it myself. But since it is a nullpointer I think that one of your objects in itemSearch is not initiliased properly.

this program needs a lot of improvements
you dont have any customers array
or even shop list for a single customer included in the program
i fixed some major bugs so now it works fine at least for one item entered

you obviously need to read more about oo programming

iv included some comments in the code below

import java.util.*;
import java.io.*;


public class GrocerySim {
        
        // consider to use array list instead of table
        public static final int MAXNUMBEROFITEMS = 100;    // the maximum number of items that can be bought.
        static GroceryItem[] dataList=new GroceryItem[MAXNUMBEROFITEMS];
        static int inventory=0;
        static int quantity=0;
        static int productNumber=0;


/* readIventory will read in the file "inventory.dat" */


    private static void readInventory(String filename)
    {
        Scanner fileInput;
        File inFile = new File("inventory.dat");
        try
         {
           fileInput =  new Scanner(inFile);
           while(fileInput.hasNext())
           {
                 //You havent got items stored!!!
                 GroceryItem tempItem = new GroceryItem(); // creating new item
                 tempItem.readItem(fileInput); // filling it with data
                 dataList[inventory] = tempItem; // putting it in table
                 

                 ++inventory;  //stick update for inventory
                 //System.out.print(inventory);
                 
                   if(inventory>100)
                   {
                                        {
                                                System.out.println("Item Number "+dataList[inventory].getProductNumber()+" has already been added.");
                                        }
                   }

                }

         }
          catch(FileNotFoundException e)
          {
            System.out.println(e);
            System.exit(1);
          }
    }
 /*readCustomerInput(Scanner keyboard) will get the user input for the product number and
  * quantity of that item they want.
  */
        public static void readCustomerInput(Scanner keyboard)
        {

                System.out.print("Enter Product Number and Quantity:"+'\n');
                while(keyboard.hasNext())
                {
                        if(productNumber == 0 && quantity == 0)
                        {
                        	   productNumber=keyboard.nextInt();
                               quantity=keyboard.nextInt();
                               // the values are not stored anywhere,
                               // so you only have one item for 1 customer

                        }
                        else break;

                }
              }
                // System.out.println(productNumber+ " "+quantity); // debug to make sure productNumber and quantity are being read in.





/*calcCustomerReciept() will calculate the customers order displaying the subtotal, tax,
 * and final price.
 */
 public static void calcCustomerReciept()
 {
                int item=0;
                int numberOfCustomers=1;
                int nonFoodItem=0;                      // will determine if the item is a nonFoodItem.
                int foodItem=0;                         // will determine if the item is a foodItem.
                double foodTax = 0.2;           // food tax is 2%
                double nonFoodTax = 0.7;    // nonfood tax is 7%

                System.out.println('\n'+"Customer"+" "+numberOfCustomers+'\n');
                System.out.println("debug: "+productNumber+ " "+quantity);

                // this loop makes mo sense at all 
                // while(productNumber == 0 && quantity== 0)

                                               	//System.out.println("stop");
                                item=GroceryItem.itemSearch(dataList,inventory,productNumber);
                                
                                if(item!=-1)
                                {
                                double priceTimesQuantity = dataList[item].getPrice()*quantity*100.0f/100.0f;
                                if(dataList[item].getTax() == 'N')
                                {
                                        nonFoodItem += priceTimesQuantity;
                                }
                                else
                                {
                                        foodItem += priceTimesQuantity;
                                }

                                System.out.println(dataList[item].getDescription()+"\t"+
                                        quantity+" @ "+ "$ "+ (dataList[item].getPrice())*100.0f/100.0f+
                                " "+"\t"+"$ "+priceTimesQuantity+" "+dataList[item].getTax());           // displays the quantity followed by the price
                                }                                                                        // then quantity times price and finally the food tax.)
                                //{
                                //      System.out.println("Item number "+productNumber+" is not in located in the inventory.");

                              //}

                        float subtotal = nonFoodItem+foodItem*100.0f/100.0f;                                    // subtotal = the addition of both food and nonfood items
                        double tax = (foodItem*foodTax)+(nonFoodItem*nonFoodTax)*100.0f/100.0f; // tax is food items * 2% and non food * 7%

                        System.out.println('\n'+"\t"+"Subtotal:\t"+"$ "+subtotal);                                        //display the subtotal
                        System.out.println("\t"+"Tax:\t\t"+"$ "+tax+'\n');                                                                                              // display the tax
                        System.out.println("\t"+"Total:\t\t"+"$ "+(subtotal+tax)*100.0f/100.0f+'\n');                               // display the final total
                        numberOfCustomers++;    //begin new customer

                
 }



        public static void main( String [] args)
        {
                        Scanner keyboard = new Scanner(System.in);
                        readInventory("inventory.dat");
                        readCustomerInput(keyboard);
                        calcCustomerReciept();



        }

}
import java.util.*;

public class GroceryItem
{
// static means that you have only one variable per class!!!


private int productNumber;           // the product number for an item
private String description;                      // the description of the item
private double price;                // the price for the item
private char tax;                        // the tax for the item

// accessor methods

  public  int getProductNumber() { return productNumber;}
  public String getDescription() { return description;}
  public double getPrice() { return price;}
  public char getTax() { return tax;}


 //*******************************************************************************************

        /* readItem(Scanner file) will read in the productNumber, description, price and tax from
         * the inventory data, it will then leave the position of the object on the next input line.
         */

    public GroceryItem readItem(Scanner file) // it should return new object containing data
        {
                // while(file.hasNext())
                //   you don't need that loop here it reads trough all file
                //   only most recent line in the object and leves
                //{

                productNumber= file.nextInt();
                description=file.next();
                price=file.nextDouble();
                tax=(file.next()).charAt(0);
                //System.out.print(productNumber+"  "+ description+"  "+ price+"  "+ tax+"  "); // debug test
                file.hasNextLine();    //leaves at the next input line
                //}
                return this;

        }

    /* itemSearch(GroceryItem [] data, int elementsUsed, int productItem) searches the array of items
     * looking for the item that matches productItem. It if cannot find it wil return -1.
     */

    public static int itemSearch(GroceryItem [] data, int elementsUsed , int productItem)
    {

        int index = -1;
        for(int i = 0; i < elementsUsed; i++)
        {
        	System.out.print("debug: "+data[i].getProductNumber());
                if(data[i].getProductNumber() == productItem)
                {
                        index = i;
                        break;
                }
        }
        return index;
    }

    /* printIventory(GroceryItem[] data, int elementsUsed) prints all the inventory that is
     * from i to i<elementsUsed.
     */
    public static void printInventory(GroceryItem[] data, int elementsUsed)
        {
                for(int i=0; i<elementsUsed; i++)
                {
                        System.out.printf("\t"+data[i].getProductNumber()+"\t"+data[i].getDescription()+"\t"
                                        +"\t"+data[i].getPrice()+"\t"+data[i].getTax()+'\n');
                }
        }

}
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