为什么我得到“ArrayIndexOutOfBoundsException”？

Question

I'm pretty new to programming and working on an assignment for class. Now, I'm not asking for anyone to write my code for me but I'm stuck with a runtime error. In the assignment we need to read a file, use the first line, "15", to initialize the size of an array, and proceed to fill the array with the information from each line.

edit: I didn't want to post all of the code because I thought it would look too long but because of the downvotes for being vague, here it goes.

File:

15
produce,3554,broccoli,5.99,1
produce,3554,broccoli,5.99,1
produce,3555,carrots,2.23,0.25
produce,3555,carrots,2.23,0.25
produce,3555,carrots,2.23,0.25
cleaning,2345,windex,5.99,1 unit
cleaning,2345,windex,5.99,1 unit
cleaning,2345,windex,5.99,1 unit
cleaning,2345,windex,5.99,1 unit
cleaning,2346,toilet paper,12.99,4 rolls
cleaning,2346,toilet paper,12.99,4 rolls
cleaning,2335,windex,2.25,1 mini sprayer
cleaning,1342,wipes,3.99,10 units
cleaning,1342,wipes,3.99,10 units
produce,3546,lettuce,2.99,0.5

My Error:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 15
    at Inventory.readFile(Inventory.java:45)
    at Inventory.<init>(Inventory.java:12)
    at Supermarket.main(Supermarket.java:3)

Class with the Line 45 in Question (line 45 is commented, scroll to the right)"

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;

public class Inventory{
    Product[] list;
    String[] invData;
    private int i = 0;
    public int count;

    public Inventory (String f){
        readFile(f);
    }

    public int indexOfProduct(int code){        
        for(i=0; i<list.length; i++){ 
            if (list[i] != null)
                if (list[i].getCode() == code)
                    return i;

        }
        return -1;
    }


    public Product delete(int pos){
        Product temp = new Product();
        temp = list[pos];
        list[pos] = null;
        return temp;
    }

    public void readFile(String fileName){
        try{
            File invList = new File (fileName);
            Scanner s = new Scanner(invList);
            int itemCount = s.nextInt();
            list = new Product[itemCount];
            count = itemCount;
            while (s.hasNext()){
                String line = s.nextLine();
                invData = line.split(",");
                if (invData[0].equals("produce")){
                    list[i] = new Produce(invData[1], invData[2], invData[3], invData[4]); // This is Line 45, Where the error occurs
                } else if(invData[0].equals("cleaning")){
                    list[i] = new Cleaning(invData[1], invData[2], invData[3], invData[4]);
                }
                i++;
            }//end of while loop
        } catch (FileNotFoundException Abra) {
            String error = Abra.getMessage();
            System.out.println(error);
            } 
    } // end of method

    public Product findCode(int c){
        for(int i=0; i<list.length;i++)
            if(list[1].getCode() == c)
                return list[i];
        return null;
    }//end of method
}//end of class

Why did I get an "ArrayIndexOutOfBoundsException"? I hope someone can point out the flaw in my logic so I don't repeat it again.

Answer 1

Your issue is clearly with the use of i , as that is the only variable index on that line, and the out of range index is "15", which is just past the end of your 15-item array. So, couple of issues, all surrounding the use of i :

As nhellwig mentioned, be sure that i is actually initialized to 0 before calling this function.

Additionally, you're putting a lot of faith in the consistency of the item number in the file and the actual number of items. You should either produce a warning and stop trying to store items in the array if i >= itemCount , or use a container like an ArrayList that can grow to accommodate new items instead of a fixed size array.

Edit: Also, I should point out that you increment i whether you read an item or not, which means even blank lines will increment i , causing gaps in your list or array overruns. Since itemCount is the number if items, you should stick to that and only increment i if you read an actual item.

In that same spirit, you should verify that invData.length == 5 after you call split(), because a misplaced comma, etc. in your file may also end up with an OOB error. Granted, for your project, it's probably OK to make assumptions about the number of elements in a line that starts with "produce" or "cleaning", but in general it's important to be cautious with data coming from a user-created file.

Answer 2

I found the answer to be that I needed an "s.nextLine();"

Because I used "s.nextInt();" the pointer was just hangin around at the end of "15" in my file. Then, when the first line in the While loop "String line = s.nextLine();" executed the pointer moved from the end of 15 to before the p in produce in the 2nd line of the list file.

The working method looks like this:

public void readFile(String fileName){
    try{
        File invList = new File (fileName);
        Scanner s = new Scanner(invList);
        int itemCount = s.nextInt();
        s.nextLine(); // This is the new line that made it work
        list = new Product[itemCount];
        count = itemCount;
        while (s.hasNext()){
            String line = s.nextLine(); //moves file pointer over one
            invData = line.split(",");
            if (invData[0].equals("produce")){
                list[i] = new Produce(invData[1], invData[2], invData[3], invData[4]);
            } else if(invData[0].equals("cleaning")){
                list[i] = new Cleaning(invData[1], invData[2], invData[3], invData[4]);
            }
            i++;
        }//end of while loop
    } catch (FileNotFoundException Abra) {
        String error = Abra.getMessage();
        System.out.println(error);
        } 
} // end of method

Answer 3

How many times do you call readFile? You should have i = 0; at the beginning of the function.

Answer 4

“i”不应该是全局值，而应该是方法局部变量，初始化为零。