CODEWITHSUNDEEP
javaindexoutofboundsexception
Cruncher
Cruncher

Reputation: 7812

IndexOutOfBoundsException: Index: 15, Size: 19

So I was combing through some logs from our web server the other day (looking for something else), when something peculiar caught my eye.

java.lang.IndexOutOfBoundsException: Index: 15, Size: 19
    at java.util.ArrayList.rangeCheck(Unknown Source)
    at java.util.ArrayList.get(Unknown Source)

This seemed impossible to me.

I looked up the source code for ArrayList#rangeCheck and it blew my mind

private void rangeCheck(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

Based on the the message that I received should never have happened.

Unfortunately I don't know what the contents were exactly at this time (this is called from code that's used for a lot of things). And I have been unable to reproduce it. I don't even know where I'd start.

The question: Should this be able to happen? Is this a glitch in Java? Or just a freak accident.

I recognize that this may be off topic. I wish I had more information about what happened, but I don't.

Upvotes: 1

Views: 1842

Answers (2)

Peter Lawrey
Peter Lawrey

Reputation: 533442

Should this be able to happen? Is this a glitch in Java? Or just a freak accident.

All the above.

Let us image you are adding to the ArrayList in one thread and accessing it in another.

Lets say you have a size of 14, but you access index 15

T1: if (index >= size) // is true
T2: for(int i=0;i<5;i++) list.add(N); // so now we have 19.
T1: outOfBoundsMsg(index); // Index: 15, Size: 19

Upvotes: 3

Patricia Shanahan
Patricia Shanahan

Reputation: 26175

ArrayList is not multithread safe. If it were being modified by one thread at about the time of an access by another thread, index >= size could have been true when the test was run, but false by the time the message was built.

Upvotes: 5

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