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Re: When to throw exceptions and when to use System.err?

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David Lamb wrote:
> Lew wrote:

>> David Lamb wrote:
>>> - You need to figure out an appropriate exception to use for "number
>>> not in the right range". If you want to go whole hog you could define your own

>> You could also subtype 'IllegalArgumentException' or
>> 'NumberFormatException' to distinguish the range problem:

> Thanks for pointing that out -- I'd certainly have looked for something to
> subclass. I'd want to think about what parent class to use, though. Neither of
> those seems quite right for an incorrect user input, if your method is popping
> up a dialog box itself. If you're talking about a string passed to your method
> that happened to come from user input -- well, there's no way for your method
> to know that, so it's an illegal argument (there's nothing wrong with the
> format).

Sorry, I can't agree that using exceptions to process user input is a good idea.

Runtime exceptions are to catch and repair programming errors, not user input.

There is no such thing as "bad" input.  Therefore no input should ever thrown 
an exception - not from the business perspective.  Yes, exceptions happen low 
down in the code - presumably only checked exceptions, but that's another 
debate.  But the code that invokes such a method, especially because the 
checked exception requires it, will catch and deal with the exception.

As touted in /Effective Java/ by Joshua Bloch, Chapter 9, [1] you should favor 
prevention of exceptions over trapping them.  To process input, sometimes you 
have to cheat and use an exception, but by and large you want to validate 
input prior to submitting it to business logic.  The cheat with an exception 
is a highly-localized idiom to substitute for a more proper conditional, 
wrapped to pretend to higher layers that it really was a conditional, used 
only at pressing need.

Therefore, no exception, much less the ones I mentioned, is at all "right for 
an incorrect user input".  Runtime exceptions are for things like incorrect 
arguments to methods or improper data fed to range-bound methods (a kind of 
incorrect argument).  So I figure that in the great majority of applications 
I'd use 'IllegalArgumentException' for such a thing.

[1]
<http://java.sun.com/docs/books/effective/toc.html>

Item 57: Use exceptions only for exceptional conditions
Item 58: Use checked exceptions for recoverable conditions and runtime 
exceptions for programming errors
Item 60: Favor the use of standard exceptions

*Item 62: Document all exceptions thrown by each method*

*Item 65: Don't ignore exceptions*

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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Re: When to throw exceptions and when to use System.err? David Lamb <dalamb@cs.queensu.ca> - 2011-03-29 07:29 -0400
  Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-03-29 11:21 -0400
    Re: When to throw exceptions and when to use System.err? David Lamb <dalamb@cs.queensu.ca> - 2011-03-29 18:38 -0400
      Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-03-29 18:53 -0400
        Re: When to throw exceptions and when to use System.err? Eric <e.d.programmer@gmail.com> - 2011-03-30 11:48 -0700
          Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-03-30 23:49 -0400
            Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-03-30 23:52 -0400
            Re: When to throw exceptions and when to use System.err? Eric <e.d.programmer@gmail.com> - 2011-03-31 06:55 -0700
              Re: When to throw exceptions and when to use System.err? Nigel Wade <nmw-news@ion.le.ac.uk> - 2011-03-31 15:58 +0100
                Re: When to throw exceptions and when to use System.err? Eric <e.d.programmer@gmail.com> - 2011-03-31 09:02 -0700
                Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-03-31 19:57 -0400
              Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-03-31 19:50 -0400
                Re: When to throw exceptions and when to use System.err? Eric <e.d.programmer@gmail.com> - 2011-04-01 10:44 -0700
                Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-04-01 19:52 -0400
                Re: When to throw exceptions and when to use System.err? Eric <e.d.programmer@gmail.com> - 2011-04-04 08:47 -0700
                Re: When to throw exceptions and when to use System.err? Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-04-04 13:40 -0400
                Re: When to throw exceptions and when to use System.err? Eric <e.d.programmer@gmail.com> - 2011-04-04 11:56 -0700
                Re: When to throw exceptions and when to use System.err? Lew <noone@lewscanon.com> - 2011-04-06 12:40 -0400

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