Re: ArrayIndexOutOfBoundsException

Knute Johnson <>
Thu, 25 Feb 2010 09:11:33 -0800
On 2/25/2010 8:18 AM, Eric Sosman wrote:

On 2/25/2010 10:04 AM, rossum wrote:

On Thu, 25 Feb 2010 05:07:02 -0800 (PST), dendeezen
<> wrote:


With the following code I get an ArrayIndexOutOfBoundException, and I
can't figer out why.
(This is a really newby question, I know)

Eric's answer should have been sufficient, there is a great deal of
information contained in the information Java gives you after it
detects an error. That information should have been sifficient for
you to determine the cause of your error.

Exactly. But he wanted to know "why" -- and since the
proximate cause was so blindingly obvious (and even if it
weren't, Java helpfully pointed it out to him), I couldn't
imagine what sort of a "why" he wanted.

In "The Cuckoo's Egg," Clifford Stoll tells of facing
a panel of professors at his thesis orals. They grilled
him in turn, and when it came time for the last professor's
question, he asked Stoll "Why is the sky blue?" Stoll almost
fainted with relief at getting such a simple question, and
said something about scattering angles and wave lengths. The
professor asked for more detail. Stoll gave more detail, and
the professor asked for more yet. Within a fairly brief time,
Stoll was sweating bullets and covering the blackboard with
partial differential equations that he hoped he remembered
correctly ...

"Why" can be a simple question, or can be a deep one.
Since the O.P.'s simple answer had already been handed to
him by Java, I couldn't tell what deeper meaning he had in
mind. But in view of his politeness, I don't care any more.

The obvious isn't always.

I can remember when I first started using the newsgroups to help me
write code, that I was so clueless, telling me to look at what had
frustrated me enough to post on the newsgroup just pissed me off and
made me go away. I think in most cases it would be much better for the
helped to have the helper provide a minimal direct clue. In this case
there was plenty of opportunity to do just that without completely
explaining the problem.

I'm not a trained programmer but I was a professional instructor. This
instruction was expensive and often it was much more efficient to
provide an immediate clue or example rather than spend the student's
time and money waiting for an epiphany. That is probably the reason I
often post SSCCEs in addition to explanations. Asking and answering a
question on a newsgroup doesn't cost other than the posters' time.

I often feel that a question is poorly constructed or incompletely
thought out and tailor my response accordingly. That is certainly
appropriate. Asking questions effectively is a learned experience as
well and maybe that is where the emphasis of the answer should often lie.


Knute Johnson
email s/nospam/knute2010/

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