NullPointerException when accessing array elements

Question

I'm using BlueJ to create an array. The array is of 10 objects inherited from another class.

Now, my problem is that every time I try to find an object within the array (using the findInventoryItem, I get a java.lang.NullPointerException: null error. Now the "inventoryItem" object comes from another class. I am using inventoryItems as the actual array name. the objects that go into it are the inventoryItem of class InventoryItem

    import java.util.ArrayList;
    import java.util.Iterator;
    import java.util.Random;
    /**
    * THIS IS THE MODIFIED VERSION OF THE ARRAY LIST MANAGER CLASS
    * 
    * @author RAGEED A BASRAWI 
    * @version VERSION 1
    */
    public class HomeInventoryManagerARRAYClass
    {
    private InventoryItem inventoryItem;
    private InventoryItem[] inventoryItems;
    /**
 * Initialise the home inventory manager.
 */
public HomeInventoryManagerARRAYClass()
{
    InventoryItem inventoryItem;
    inventoryItems = new InventoryItem[10];
}

/**
 * Add an inventory item to the list.
 * 
 */
public void addInventoryItem(InventoryItem inventoryItem)
{
    Random random = new Random();
    inventoryItems[random.nextInt(9 - 0 + 1) + 0] = inventoryItem;
}

/**
 * Try to find an inventory item in the inventory list with the given partNumber.
 * 
 */
public InventoryItem findInventoryItem(int partNumber)
{
    for(int index = 0; index < 9; index ++)
    {
        if (inventoryItem.getPartNumber() == partNumber)
        {
            return inventoryItem;
        }
        if (inventoryItem.getPartNumber() != partNumber)
        {
            System.out.println("The entry " + partNumber + " does not exist. Pleast try again.");
        }
    }

    return inventoryItem;
}

/**
 * Locate an inventory item with the given partNumber, and return how
 * many of this item are in inventory. If the partNumber does not
 * match any item, return zero.
 */
public int numberInInventory(int partNumber)
{
    InventoryItem inventoryItems = findInventoryItem(partNumber);
    if(inventoryItems != null)//There aren't too many ways to write a statement to be NOT NULL. Its kind of annoying... -___-
    {
        return inventoryItems.getQuantity();
    }
    else
    {
        return 0;
    }
}

/**
 * Return the total number of items in inventory.
 * 
 */
public int numberInInventoryList()
{
    return inventoryItems.length;
}

/**
 * Print details of all the home inventory items.
 */
public void printInventoryList()
{
    int index = 0;
    while(index < inventoryItems.length)
    {
        System.out.println(inventoryItem = inventoryItems[index]);
        index ++;
    }
}

public int totalNumberInInventory()
{
    int index = 0;
    int absoluteInventory = 0;

    while (index < inventoryItems.length)
    {
        InventoryItem inventoryItem = inventoryItems[index];
        absoluteInventory += inventoryItem.getQuantity();
        index ++;
    }

    return absoluteInventory;
}

}

Answer 1

inventoryItems[random.nextInt(9 - 0 + 1) + 0] = inventoryItem;

if you initialize the array with the line above, how do you make sure that all elements of the array are initialized? It's generating a random number and doesn't guarantee all elements between 0 to 9 are initialized.

Remember that with this line:

  inventoryItems = new InventoryItem[10];

you are initializing your array, not its elements. When running functions again each array's element you should be sure that you have initialized the element itself.

For example :

   InventoryItem[i].getPartNumber()

this line of code should be executed before

   InventoryItem[i] = new InventoryItem();

Answer 2

As @Jahoy has mentioned, it's likely that the elements in the array that you are accesing are null. It is essential firstly to check if an element exists in the first place.

Your FindInventoryItem function should be like this I think:

public InventoryItem findInventoryItem(int partNumber)
{
    for(int index = 0; index < 9; index ++)
    {
        InventoryItem tempItem = inventoryItems[index]; // ADD THIS LINE HERE
        if(tempItem != null)
        {
           if (tempItem.getPartNumber() == partNumber)
           { 
              return tempItem;
           }
           if (tempItem.getPartNumber() != partNumber)
           {
              System.out.println("The entry " + partNumber + " does not exist. Please try again.");
           }
       }
    }
    // at this stage you've got no result, I dont really know what you want to return
    return inventoryItem;
}