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Re: Some same exceptions used in a given file

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Merciadri Luca wrote:
> Thanks for the example. And now if another method of the same class
> throws also an IOException, but whose action needs not to be the same?
>
> For example, how would I convert
>
> ==
> public void foo
> {
>   try
>   {
>   // ...
>   }
>
>   catch (IOException problem1)
>   {
>
>   }
>
> }
>
> public void bar
> {
>   try
>   {
>   // ...
>   }
>
>   catch (IOException problem2)
>   {
>
>   }
>
> }
> ==
>
> ?
>
> I might do
>
> ==
> public void foo throws IOException
> {
>   // ...
> }
>
> public void bar throws IOException
> {
>   // ...
> }
> ==
>
> and then catch the exception in the caller, but how do I differentiate
> from problem1 to problem2? Sorry for this silly question, but it seems
> not that habitual regarding Google answers on this.

There are a few approaches.

Right now you're starting from the leaf level of how to implement try-catch 
and trying to glean an overall strategy.  A better approach is the exact 
opposite:  Decide on your overall *application-domain* strategy first.

As you code in various layers of your application, think of a type as part of 
the internal API for the application - a one-shot library.  Good O-O 
principles dictate that a compoment have a well-defined purpose and external 
contract for interaction with other components.  None of those strategic 
conversations involve exceptions as such - just conditions and desired 
responses and state transitions involving those conditions.

At the nitty-gritty detail level of implementation, OTOH, exceptions signal 
out-of-band issues - things that stop the action.  But their effect is local - 
not strategic, but specific to a particular action in service of that 
strategic effect.  Since the strategic conversation is exception-agnostic, the 
component must convert that exceptional condition into the appropriate 
response that accords with its contract for interaction.

That contract varies with the layer involved.  If the layer is a deeper one, 
say a data-access layer (DAL), it functions exactly like an API.  Again, 
define its contract from the outside - what should API clients experience? 
Sometimes that means that a particular API method has an 'Exception' subtype 
as part of its contract.  But that is dictated by the strategic purpose of the 
API, not by knee-jerk, thoughtless reactions to lower-level events.

That upward view often comprises an application-specific exception, one that 
tells the application "something lower broke", and then uses the 'cause' 
attribute to say what.

How fine-grained does the top level view have to be, for Pete's sake?  Why be 
so obsessive-compulsive?

Usually the 'cause' chain and stack trace contains everything needed for 
debugging, and nothing needed for the strategic upline viewpoint.  Usually.

So here's how you convert low-level exceptions to higher-level ones, if (and 
only if) that's what your strategy requires:

  public void actByContract( Foo stimulus ) throws LewsException
  {
    BufferedReader reader;
    try
    {
      reader = new BufferedReader( new FileReader( stimulus.getFile() ));
    }
    catch ( IOException exc )
    {
      final String msg = "actByContract() broke";
      logger.error( msg, exc );
      LewsException lewx = new LewsException( msg, exc );
      throw lewx;
    }
  )

Key points:

  - Log and act upon exceptions at the lowest point they occur, if they aren't 
one of your own wrapper exceptions like 'LewsException'.

  - Don't let exceptions walk all the way out of an application or lower-level 
exceptions out of a component - convert them to some strategically valid, 
contract-authorized interactions with outsiders.

  - Do determine the strategically and contractually valid interactions for 
the application and every component, first, before implementation.

  - Do allow only strategically and contractually valid interactions to occur 
and none other.

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

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Some same exceptions used in a given file Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-23 19:08 +0200
  Re: Some same exceptions used in a given file Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-23 14:40 -0400
    Re: Some same exceptions used in a given file Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-23 16:23 -0400
    Re: Some same exceptions used in a given file Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-04-23 17:23 -0300
    Re: Some same exceptions used in a given file Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-24 00:00 +0200
      Re: Some same exceptions used in a given file Lew <noone@lewscanon.com> - 2011-04-23 18:22 -0400
      Re: Some same exceptions used in a given file Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-23 18:31 -0400
        Re: Some same exceptions used in a given file Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-24 01:20 +0200
          Re: Some same exceptions used in a given file Lew <noone@lewscanon.com> - 2011-04-23 20:19 -0400
      Re: Some same exceptions used in a given file Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-04-23 22:25 -0300
        Re: Some same exceptions used in a given file Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-24 09:24 +0200
  Re: Some same exceptions used in a given file Patricia Shanahan <pats@acm.org> - 2011-04-23 13:48 -0700
    Re: Some same exceptions used in a given file Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-24 09:34 +0200
      Re: Some same exceptions used in a given file Lew <noone@lewscanon.com> - 2011-04-24 09:40 -0400
      Re: Some same exceptions used in a given file Patricia Shanahan <pats@acm.org> - 2011-04-24 06:53 -0700
  Re: Some same exceptions used in a given file Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-24 01:28 +0200

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