Why does this result in an ArrayIndexOutOfBoundsException at run-time?

Question

This is the code:

public class Test {
    public static void main(String[] args) {
        int[] a= new int[3];
        System.out.print(a[a.length]);
    }
}

Why does this result in an ArrayIndexOutOfBoundsExceptionwill at run-time?

because the index **start from 0 to 2** not **from 1 to 3** :) — Youcef LAIDANI, Oct 23 '18 at 08:05

score 3 · Answer 1 · answered Oct 23 '18 at 08:06

3

a.length return the number of elements in the array, here it's 3.

Array index starts at 0. With 3 elements, it goes 0,1,2.

There is no index 3, hence the exception.

answered Oct 23 '18 at 08:06

jhamon

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score 0 · Answer 2 · edited Oct 23 '18 at 08:22

0

Index starts from 0 (not form 1). So, in your case a has the index 0, 1 and 2.

But you are trying to access index 3 (length or size).

Use System.out.print(a[a.length]-1); instead.

edited Oct 23 '18 at 08:22

Rene Knop

1,788
3
15
27

answered Oct 23 '18 at 08:07

Sai Prasad

286
1
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score 0 · Answer 3 · edited Oct 23 '18 at 09:01

0

You should use that:

public class Test{

     public static void main(String []args){
        int[] a= new int[3];
        System.out.print(a[a.length-1]);
    }
}

Explanation:

a.length will return the length and that is 3 (3 existing fields). But the index couting for a[3] starts with 0 and goes up to 2. Reducing the length with -1 returns the last really existing index (2).

So a[a.length] (= a[3]) causes the Array Index Out Of Bounds Exception.

edited Oct 23 '18 at 09:01

Rene Knop

1,788
3
15
27

answered Oct 23 '18 at 08:08

Harsh Kumrawat

160
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1

code without explanation is usually not helpful to understand the issue – jhamon Oct 23 '18 at 08:08

score 0 · Answer 4 · answered Oct 23 '18 at 08:12

0

It starts from 0 so you must Change to [a.length - 1]

 public class Test{
       public static void main(String []args){
          int[] a= new int[3];
          System.out.print(a[a.length-1]);
       }
    }

answered Oct 23 '18 at 08:12

khaled_bhar

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score -1 · Answer 5 · edited Oct 23 '18 at 09:54

This is the hierarchy of IndexOutOfBoundsException:

 java.lang.Object
    java.lang.Throwable
        java.lang.Exception
            java.lang.RuntimeException
                java.lang.IndexOutOfBoundsException

It is sometimes harder to solve it. During those moments, you can use IDLE debugger. It will show you the value of each variable at each iteration in the graphical interface.

You can use Eclipse debugger, Netbeans, Visual Studio code, Atom extensions works as well.

Why does this result in an ArrayIndexOutOfBoundsException at run-time?

5 Answers5