Java - array's index out of bounds when its not?

Question

I have a problem in the code below:

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone {
    public static void main (String[] args) throws java.lang.Exception {
        int[] map = new int[100 * 100];
        System.out.println("Works : " + map[10 * 100 + 5]);
        System.out.println("Works? : " + map[99 * 100 + 99]);
        System.out.println("Works?! : " + map[20 * 100 + 100]);
        //System.out.println("And this?? : " + map[99 * 100 + 100]);
    }
}

As you can see the last line (the comment) doesn't work; it throws an ArrayIndexOutOfBoundsException.

But I dont get it; it should be in the array's bounds. The array's size is 100*100 so 99 * 100 + 100 = 100*100 so index 100*100 is in the array.

I've already fixed the problem; I would just like to know why I get an exception on the above line.

Link to code: http://ideone.com/ydgU9s

Christian Tapia · Accepted Answer · 2013-12-27 22:30:24Z

4

It throws that exception because the index of an array are from 0 to length -1.

So if you declared it to have a length of 100*100, the indexes are from 0 to 100*100 -1.

And what you tried:

map[99 * 100 + 100]

is equal to

map[100*100]

which cannot be accesed.

answered Dec 27, 2013 at 22:30

Christian Tapia

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1 Comment

julian Over a year ago

ok too many comments.. yeah that's what i thought, just wanted to be sure, thanks and accepted :D in 9 minutes

Vivek Vermani · Accepted Answer · 2013-12-27 22:32:55Z

1

You are trying to access Map[10000]. Array index starts with 0 , so the array with size 10,000 means 0 to 9999.

answered Dec 27, 2013 at 22:32

Vivek Vermani

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Comments

Alexis C. · Accepted Answer · 2013-12-27 22:38:37Z

1

Arrays are 0 base indexed.

So you can access the elements in map from index 0 to map.length - 1.

I.e : map[0] to map[100*100-1]

Or 99 * 100 + 100 = 10000 > 9999, that's why you got an IOOBE.

enter image description here

edited Dec 27, 2013 at 22:38

answered Dec 27, 2013 at 22:30

Alexis C.

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