CODEWITHSUNDEEP
javaarrays
Ken
Ken

Reputation: 343

Don't know what's causing my ArrayIndexOutOfBoundsException error

I have written a piece of code which keeps giving me an ArrayIndexOutOfBoundsException error and I don't really know why. I think I've set the sizes of the arrays correctly but apparently that isn't true. Even if I set the sizes of the arrays to 100 I still get the error. Below the code you can find the data input.

import java.util.Scanner;

public class GameOfLife {

public static void main(String []args) {

    Scanner scanner = new Scanner(System.in);

    int length = scanner.nextInt();
    int width = scanner.nextInt();
    int generations = scanner.nextInt();
    Boolean[][] cellsInput = new Boolean[length - 1][width - 1];

    System.out.println();   
    int count = 0;
    int y = 0;
    while (scanner.hasNext()) {
        count++;
        if (count <= length) {
            if (scanner.next().equals(".")){
                cellsInput[y++][count] = false;
            } else if (scanner.next().equals("*")) {
                cellsInput[y++][count] = true;
            }
        }
        else {
            count = 0;
            y++;
            if (scanner.next().equals(".")){
                cellsInput[y++][count] = false;
            } else if (scanner.next().equals("*")) {
                cellsInput[y++][count] = true;
            }   
        }
    }

}

}

Input (for example):

15 15 3
. . . . . . . . . . . . . * .
. . . . . . . . . . . . * . .
. . . . . . . . . . . . * * *
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
* * * * * * * * . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .

Upvotes: 0

Views: 57

Answers (2)

mdewitt
mdewitt

Reputation: 2534

The problem is here:

if (count <= length) {

Eventually, this is going to try to reference

cellsInput[y++][length]

Where length is the length of that second array. However, the last index in the second array is actually at length - 1.

The problem here occurs because all arrays in Java start with 0. So you always want to do 

if (count < length) {

whenever length is the length is the length of the array.

Length is always the number of objects in the array, which starts counting at 1.

Example:

Array arr1 = [a, b, c, d]
Length of arr1 = 4, it has 4 elements

Element   |   Index
--------------------
   a      |    0
   b      |    1
   c      |    2
   d      |    3

As you can see index 4 is out of bounds. So when you try to reference arr1[arr1.length] you get an IndexOutOfBoundsException

Upvotes: 2

Peter Pei Guo
Peter Pei Guo

Reputation: 7870

For example the following line is wrong:

if (count <= length) {

Since you use count as index, when count equals length, it exceeds the largest index length - 1 - thus ArrayIndexOutOfBoundsException. It should be:

if (count < length) {

Upvotes: 4

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