调试器说我收到一个空指针异常。 为什么？

Question

I'm writing a simple program that sorts objects by a specific way. However, runtime stops after

hardware[i].setItemID(itemID[i]); 

As indicated in eclipse's debugger. Here's the full code:

public class TestItem
{
    public static void main(String [] args)
    {
        Item[] hardware = new Item[6];
        String[] itemID = {"1011", "1034", "1101", "1600", "1500", "1201"};
        String[] itemName = {"Air Filters", "Door Knobs", "Hammers", "Levels", "Ceiling Fans", "Wrench Sets"}; 
    int[] inStore = {200, 60, 90, 80, 100, 55};
    double[] price = {10.5, 21.5, 9.99, 19.99, 59, 80}; //80 dollars for a wrench set? Really? Shoulda gone to Sears... 
    for(int i = 0; i <= 5; i++)
    {
        hardware[i].setItemID(itemID[i]);
        hardware[i].setItemName(itemName[i]);
        hardware[i].setInStore(inStore[i]);
        hardware[i].setPrice(price[i]);
    }
    printInventory(hardware);
sortID(hardware);
sortInStore(hardware);
sortName(hardware);
sortPrice(hardware);

}
public static void printInventory(Item[] items)
{
for(int i = 0; i < items.length; i++)
 {
    System.out.println(items[i].toString());
 }
}
public static void sortName(Item[] items)
{
for(int i = 1; i < items.length; i++)
    {
    int j = i;
    int compareMeForWhile = items[i].getItemName().compareTo(items[i-1].getItemName());

    while(j>0 && compareMeForWhile == 1)
        {
        compareMeForWhile = items[i].getItemName().compareTo(items[i-1].getItemName());
        String temp1 = items[i].getItemName();
        String temp2 = items[i-1].getItemName();
        items[i].setItemName(temp2);
        items[i-1].setItemName(temp1);
        j--;
        }
    }
for(int i = 0; i < items.length-1; i++)
    System.out.println(items[i].toString());
}
public static void sortID(Item[] items)
{
for(int i = 1; i < items.length; i++)
    {
    int j = i;          
    while(j>0 && items[i].getItemID().compareTo(items[i-1].getItemID()) < 0)
        {

        String temp1 = items[i].getItemID();
        String temp2 = items[i-1].getItemID();
        items[i].setItemID(temp2);
        items[i-1].setItemID(temp1);
        j--;
        }
    }
for(int i = 0; i < items.length-1; i++)
    System.out.println(items[i].toString());
}
public static void sortInStore(Item[] items)
{
for(int i = 1; i < items.length; i++)
    {
    int j = i;          
    while(j>0 && items[i].getInStore() > items[i-1].getInStore())
        {

        int temp1 = items[i].getInStore();
        int temp2 = items[i-1].getInStore();
        items[i].setInStore(temp2);
        items[i-1].setInStore(temp1);
        j--;
        }
    }
    for(int i = 0; i < items.length-1; i++)
        System.out.println(items[i].toString());
}
public static void sortPrice(Item[] items)
{
for(int i = 1; i < items.length; i++)
    {
    int j = i;          
    while(j>0 && items[i].getPrice() > items[i-1].getPrice())
        {

        double temp1 = items[i].getPrice();
        double temp2 = items[i-1].getPrice();
        items[i].setPrice(temp2);
        items[i-1].setPrice(temp1);
        j--;
        }
    }
for(int i = 0; i < items.length-1; i++)
    System.out.println(items[i].toString());
}

//Sort ID, Name, In store, and price. Print results.



//for i ↠1 to length(A)
//j ↠i
//while j > 0 and A[j-1] > A[j]
//    swap A[j] and A[j-1]
//    j ↠j - 1
}

Answer 1

Item[] hardware = new Item[6];

by this you initialized an array with 6 references, all references are still null you need to initialize each element like

hardware[i] = new Item();

before accessing it

Answer 2

Every Item is null in Item[] hardware = new Item[6]; that's why you are getting NPE.

Initialize each array element with new Item object before accessing.

Answer 3

您为数组Item []保留了内存，但没有为其中的每个元素保留内存。

Answer 4

Initializing an array ( Item[] hardware = new Item[6]; ) just creates an array of the specified size with null s. If you with to set properties on these items, you must first initialize them:

for(int i = 0; i <= 5; i++)
{
    hardware[i] = new Item(); // Missing from your snippet
    hardware[i].setItemID(itemID[i]);
    hardware[i].setItemName(itemName[i]);
    hardware[i].setInStore(inStore[i]);
    hardware[i].setPrice(price[i]);
}

Answer 5

All the element in hardware[] are initialize to null. You will need to store the reference of an object in your case Item() and then use the setter method.

   for(int i = 0; i <= 5; i++)
    {
        hardware[i]=new Item();
        hardware[i].setItemID(itemID[i]);
        hardware[i].setItemName(itemName[i]);
        hardware[i].setInStore(inStore[i]);
        hardware[i].setPrice(price[i]);
}