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java.lang.NullPointerException

Submitted by Almasy on Sun, 10/18/2009 - 02:04.
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Hi there. I've tried to remove the java.lang.NullPointerException error for hours but to no avail. Would anyone please point out what causes the problem in my code? Here goes...

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

public class MyShop {
  private String shopName = "ITGrocers";       //name of the shop
  int capacity = 2;                            //maximum number of items that the shop can sell
  Item[] listOfItems;    //list of items in the shop   
  int numItems = 0;                            //number of items in the shop
 
  public String getShopName() {
    return shopName;
  }
 
  public void setShopName(String name) {
    shopName = name;
  }
 
  public int addItem(String itemName, double itemPrice) {
    boolean itemInShop = false;
    if (numItems != 0) {
      for(int i = 0; i < numItems; i++) {
        if (itemName.equalsIgnoreCase(listOfItems[i].getItemName()))
          itemInShop = true;
      }
    }
    if (itemInShop)
      return 0;
   
    if (numItems == capacity)
      return 1;
   
    else {
      listOfItems[numItems].setItemName(itemName);
      listOfItems[numItems].setItemPrice(itemPrice);
      numItems += 1;
      return 2;
    }
  }
 
  public boolean updateItemPrice(String itemName, double newItemPrice) {
    boolean itemInShop = false;
    for(int i = 0; i < numItems; i++) {
      if (listOfItems[i].getItemName().equalsIgnoreCase(itemName)) {
        listOfItems[i].setItemName(itemName);
        listOfItems[i].setItemPrice(newItemPrice);
        itemInShop = true;
      }
    }
    if (itemInShop)
      return true;
    else
      return false;
  }
 
  public void display() {
    System.out.println("Shop name: " + shopName + "\nMaximum capacity: " + capacity);
    if (numItems == 0)
      System.out.println("No item added yet!\n");
    else {
      System.out.println("Total: " + numItems + " item(s)");
      for (int i = 0; i < numItems; i++)
      System.out.println(listOfItems[i].toString() + "\n");
    }
  }
 
  public static void main (String[] args) {
    MyShop obj = new MyShop();
    Item[] listOfItems = new Item[obj.capacity];
    for (int i = 0; i < obj.capacity; i++) {
      listOfItems[i] = new Item();
    }
    boolean loopChoice = true;
    int choice;
    double itemPrice;
    String itemName;
    Scanner s = new Scanner(System.in);
    while (loopChoice) {
      loopChoice = false;
      System.out.println("--------------------Welcome to Shop Management Program--------------------");
      System.out.println("1.Add Item\n2.Update Item Price\n3.Display all items\n4.Exit");
      System.out.println("Enter choice:");
      choice = s.nextInt();
      s.nextLine();
     
      switch (choice) {
        case 1: {
          System.out.print("Enter item name:");
          itemName = s.nextLine();
          System.out.print("Enter item price:");
          itemPrice = s.nextDouble();
          if (obj.addItem(itemName,itemPrice) == 0)
            System.out.println("This item is already in the list.");
          else if (obj.addItem(itemName,itemPrice) == 1)
            System.out.println("Adding failed. The shop has already reached its maximum capacity.");
          else
            System.out.print("New item added successfully.");
          loopChoice = true;
          break;
        }
       
        case 2: {
          System.out.println("Enter item name:");
          itemName = s.nextLine();
          System.out.println("Enter item new price:");
          itemPrice = s.nextDouble();
          if (obj.updateItemPrice(itemName,itemPrice))
            System.out.println("Item updated successfully.");
          else
            System.out.println("The item does not exist.");
          loopChoice = true;
          break;
        }
       
        case 3: {
          obj.display();
          loopChoice = true;
          break;
        }
        
        case 4: {
          System.out.print("Thank you for using the program. Goodbye!");
          break;
        }
       
        default: {
          System.out.println("Invalid choice!");
          loopChoice = true;
        }
      }
    }
  }
}

class Item {
  private String itemName;
  private double itemPrice;
 
  public String getItemName() {
    return itemName;
  }
  public void setItemName(String n) {
    itemName = n;
  }
 
  public double getItemPrice() {
    return itemPrice;
  }
  public void setItemPrice(double p) {
    itemPrice = p;
  }
 
  public String toString() {
    return itemName + "\t$" + itemPrice;
  }
}

The error lies at line 36 and 94 which are as follows:

listOfItems[numItems].setItemName(itemName);

and

if (obj.addItem(itemName,itemPrice) == 0)

Explanations would be helpful. Thanks in advance:)