我怎样才能打印程序？ 尝试访问.toString（）时收到NullPointerException

Question

When I run my program it gives me this error. The errors are located in the MergeInventories class as well as the CombInv class. I have labeled them in the classes provided below.

java.lang.NullPointerException
    at CombInv.print(CombInv.java:19)
    at MergeInventories.main(MergeInventories.java:17)

I am simply trying to print out the data. My program essentially takes two arrays and MergeSorts them.

Here are the multiple classes of my program.

import java.io.*;

public class MergeInventories
{
    public static void main()
    {
        header();
        try
        {
            Inventory store1 = new Inventory(new File("F:\\M359 AP Computer Science\\JAVA\\Merge Inventories\\Store1.txt"));
            Inventory store2 = new Inventory(new File("F:\\M359 AP Computer Science\\JAVA\\Merge Inventories\\Store2.txt"));

            CombInv mergedInventory = mergeInventories(store1,store2);

            //store1.print();
            //store2.print();
            mergedInventory.print(); //********THIS IS LINE 17 OF THE ERROR*************
        }
        catch(Exception e)
        {
            e.printStackTrace();
            //System.out.print("Error.");
        }

    }

    private static CombInv mergeInventories(Inventory a, Inventory b)
    {
        CombInv inv = new CombInv();
        int i=0,j=0,k=0;

        while(i<a.size()&&j<b.size())
        {
            if(a.getProduct(i).getID() == b.getProduct(j).getID())
            {
                inv.add(new Product(a.getProduct(i).getID(),a.getProduct(i).getQty()+b.getProduct(j).getQty(),a.getProduct(i).getUP()));
                i++;
                j++;
            }
            else if(a.getProduct(i).getID() > b.getProduct(j).getID())
            {
                inv.add(b.getProduct(j));
                j++;
            }
            else
            {
                inv.add(a.getProduct(i));
                i++;
            }
            k++;
        }

        if(j<b.size())
        {
            for(j=j;j<b.size();j++)
            {
                inv.add(b.getProduct(j));
            }
        }
        else if(i<a.size())
        {
            for(i=i;i<a.size();i++)
            {
                inv.add(a.getProduct(i));
            }
        }
        return inv;
    }

    private static void header()
    {
        System.out.println("Varun Pinto \n8th Hour \nM359 AP Comp Sci \n");
    }
}

Then we have the Inventory class

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

public class Inventory
{
    protected int increment = -1;
    protected int size = 0;
    protected Product[] inventory = new Product[11];
    protected String header;
    protected static String CATEGORIES = "ID\tQTY\tU\\P";

    public Inventory()
    {
        header = "TOTAL INVENTORY";
    }

    public Inventory(File file)throws FileNotFoundException
    {
        Scanner fileScan = new Scanner(file);
        header = fileScan.nextLine();
        String throwaway = fileScan.nextLine();
        while(fileScan.hasNextLine())
        {
            Scanner lineScan = new Scanner(fileScan.nextLine());
            Product p = new Product(lineScan.nextInt(), lineScan.nextInt(), lineScan.nextDouble());
            increment+=1;
            inventory[increment]=p;
        }
    }

    public void add(Product p)
    {
        increment +=1;
        inventory[increment]=p;
        size +=1;
    }

    public Product getProduct(int i)
    {
        return inventory[i];    
    }

    public int size()
    {
        return size;
    }

    public void print()
    {
        System.out.println(header);
        System.out.println(CATEGORIES);
        for(Product p: inventory)
        {
            System.out.println(p.toString());
        }
        System.out.println();
    }
}

Next I have the CombInv class where one of the errors is located.

import java.text.*;

public class CombInv extends Inventory
{ 
    public void print(){

        System.out.println(header);
        System.out.println(CATEGORIES + "\tVALUE");

        double total = 0;

        NumberFormat f = NumberFormat.getNumberInstance();
        f.setMinimumFractionDigits(2);
        f.setMaximumFractionDigits(2);

        for(Product p: inventory)
        {
            System.out.println(p.toString() + "\t" + f.format(p.getQty() * p.getUP()));
//******THERE IS AN ERROR ON THE LINE ABOVE**************************
            total += p.getQty() * p.getUP();
        }

        System.out.println("Total Value of Stock:\t$" + total);
        System.out.println();
    }
}

Finally we have the Product class which is a basic superclass where I am trying to access .toString() but cannot.

import java.text.*;

public class Product
{
    private int idNum;
    private int quantity;
    private double unitPrice;

    public Product()
    {
        idNum = 0;
        quantity = 0;
        unitPrice = 0;
    }

    public Product(int id, int q, double price)
    {
        idNum = id;
        quantity = q;
        unitPrice = price;
    }

    public int getID()
    {
        return idNum;
    }

    public int getQty()
    {
        return quantity;
    }

    public double getUP()
    {
        return unitPrice;
    }

    public String toString()
    {
        NumberFormat f = NumberFormat.getNumberInstance();
        f.setMinimumFractionDigits(2);
        f.setMaximumFractionDigits(2);

        return idNum + "\t" + quantity + "\t" + f.format(unitPrice);
    }
}

Please be aware that the use of ArrayLists in the inventory do make this program work, however as this is a project I am NOT ALLOWED TO USE ARRAYLISTS only Arrays. Also I MUST use a MergeSort to sort the data.

Here are the two text files from which i read the data: Store1.txt

STORE 1 INVENTORY
ID  QTY U/P       
362 5   12.98
471 2   6.70
792 13  25.00
901 7   2.98

and Store2.txt

STORE 2 INVENTORY
ID  QTY U/P
163 4   7.20
209 5   11.98
471 5   6.70
608 4   5.90
627 2   9.99
792 1   25.00
812 4   6.00

Here is what I expect to be printed

My Name 
8th Hour 
M359 AP Comp Sci 

TOTAL INVENTORY                         //It only prints out
ID      QTY     U\P VALUE               //these two lines.
163     4       7.20    28.80
209     5       11.98   59.90
362     5       12.98   64.90
471     7       6.70    46.90
608     4       5.90    23.60
627     2       9.99    19.98
792     14      25.00   350.00
812     4       6.00    24.00
901     7       2.98    20.86
Total Value of Stock:   $638.94

Answer 1

It took me while to answer this because I essentially put it off for the longest time. The error is a NullPointerException and i fixed it with a simple addition of:

    if(p != null)

before all the instances in which I called the p.toString(). The answer is so simple I can't believe I lost my mind over it, all I had to do was check that the value wasn't a null and it would continue looping till it found actual values. I knew my Merge Sort worked. In the end it was a really simple fix and didn't require me to change the bulk my code at all.

Also, I had to increment size again in the while loop of the constructor method of the Inventory class. I forgot to do this, but my code worked, despite what people said about rewriting the MergeSorter!

Answer 2

When you merge two sorted lists, perhaps you should consider using the following strategy:

• Consider how large the resulting sorted list will be
• Make your loop for the size of the resulting merged list
• at each step consider which of the input lists elements should be used next
• Make sure you handle an input list "running out" of elements

[Homework .. so no code supplied]