Re: Exception Handling

Patricia Shanahan <>
Sun, 11 Mar 2012 17:51:12 -0700
On 3/11/2012 4:23 PM, Novice wrote:

Patricia Shanahan<> wrote in

On 3/11/2012 2:54 PM, Novice wrote:

Lew<> wrote in news:jjipli$np7$1


It is common and frequently useful to create an application-specific
checked exception. Again, and I've said this many times, think about
what will be useful when troubleshooting a problem. Many times, a
custom exception is useful. It says that the underlying exception has
been caught, logged and wrapped.

But how is a custom FooException better than a good ol'
IllegalArgumentException? I'm not quite getting that yet.

Generally, you should use different Exception subclasses for cases that
may need different handling, so that callers can set up appropriate
catch blocks.

Regardless of the conditions under which you choose to generate
IllegalArgumentException, many methods you will be calling use it to
indicate an illegal argument condition that has not yet been logged or
otherwise reported.

If you also use it to indicate exceptions that have already been


you risk either repeated logging of the same issue, or failure to log.

On the other hand, if you create an Exception subclass that you only
throw after logging the underlying problem, there is no ambiguity.
Callers have to deal with the consequences of the failure, but should
only generate log messages if they have something to add. The


problem has already been logged.

I'm sure this will seem like good advice when I understand it but it's
still vague to me.

It would really help if you could suggest a concrete example or two that
show when to use a custom exception and why it's better.

Using the same exception for "illegal argument" and "already logged and
wrapped" is unlikely to get you into much trouble in a short program
viewed for a few minutes.

The potential trouble comes when you have a large program, too large to
read the whole thing before making a change, and it is being modified by
a programmer who does not know, or does not remember, that the exception
has those two meanings.

That makes it difficult to illustrate in a small example.

Remember that a lot of what you are being told, both in the newsgroup
and in the articles you are reading, is based on experience with
maintaining large programs over many years.


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