Re: Design Questions about static factory classes

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 26 May 2010 18:27:00 -0700
Message-ID:
<i9SdnQv77pr6V2DWnZ2dnUVZ_oSdnZ2d@earthlink.com>
Arne Vajh?j wrote:

On 26-05-2010 14:54, Rhino wrote:

Arne Vajh?j<arne@vajhoej.dk> wrote in
news:4bfc7690$0$278$14726298@news.sunsite.dk:

On 25-05-2010 12:02, Rhino wrote:

Arne Vajh?j<arne@vajhoej.dk> wrote in

In most cases I don't think the exception text would
tell the end user anything.

Let us say that you have developed an accounting program and
the code throws an exception telling that it could not
load the JDBC driver.

No point in telling an accountant that. He don't know what
JDBC is.

You tell the accountant that there were a configuration error
and that he should contact his IT department.

You write in the log file what the problem is and when IT
support look at the log file, then they know what the
problem is.

Of course there are cases where the message text from the
exception can be used in the end user error text. Just don't
display class names, stack traces etc..


I completely agree with your argument in this scenario.

I was thinking more of cases where user input is faulty and could be
corrected by the user. For instance, I have a little GUI-based "color
converter" class: it has an input field where I type the hex
representation of a Color, e.g. FFCC00, and it gives me the RGB
representation, e.g. 25, 204, 0, in another field when I click on the
Convert button. Now, if the user mistypes the hex reprsentation of
the color, say FFCCZZ, the method that does the conversion detects
that ZZ is not a valid hex representation of the Blue component of a
Color, throws an IllegalArgumentExpection with text to that effect
(the message is retrieved from a ResourceBundle to allow for
internationalization) and then the try/catch block in the
ColorConverter class displays that message in a JOptionPane so that
the user knows what to fix.

I don't log any of this and certainly don't show the user anything
like a stacktrace; just the message from the ResourceBundle.

That's a reasonable approach, isn't it?


It is an OK approach.

A possible alternative could be to throw a custom exception
NotHexColorStringException and let the exception handling
code read the localized text.


Okay, I'll give that some thought.

But it sounds like you are saying that this is not the routine or best
way to handle input errors so maybe I need to imitate the better
approaches. If someone could point me to examples of good input error
handling, maybe I could finally see how I'm supposed to be doing it....


I am just reluctant to rely too much on the message of an Exception.


I'm particularly dubious about this in connection with
internationalization, because in some cases logging should be in the
developers' working language, but GUI display in a language chosen by
the end user. The same Exception might trigger either or both.

I prefer to rely on the type of the Exception.


Also remember that you can add your own fields to an Exception subclass.

I don't know whether this makes sense, but for a String conversion
exception it might be helpful to include the index in the String of the
first error. I'm not sure whether it would ever make sense to add an
enum for more detailed classification - one could always subclass instead.

Patricia

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