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From Leon Widdershoven <>
Subject Re: [RT] Checked exceptions considered harmful
Date Thu, 15 Apr 2004 07:50:47 GMT

Bertrand Delacretaz wrote:
> Le 14 avr. 04, à 20:00, Leon Widdershoven a écrit :
>> ...I don't think throws Exception in all declarations is a plus; a 
>> function which operates on files could have an IOException, a function 
>> which operates on numbers a NumberException
>> or something akin; in essence, the root of the expected Exception 
>> hierarchy (for interfaces and abstratc classes)....
> I like the idea, similar to what Berin suggests: using base exceptions 
> which tell us what the problem is about, without being too specific.
> -Bertrand

Similar but *not* the same. And it holds only for interfaces. I think the
actual classes should be a bit more specific (not just Exception - that 
doesn't tell anything).

But to give an example, I define a virtual DataSource (not a cocoon/avalon one):

interface DataSource {
// ...
	public String getData() throws IOException;

In does not matter why a datasource could not get the data. Whether
a file is not found, could not be read, a netwerk address is not available,
a database does not exist, an SQLException occurred - for this function
it only matters that no input could be obtained.

An implementation should then of course re-wrap the exception to an IO
exception. With an appropriate message (I think cocoon has good exception
messages btw).

I think it is important in such a case to be generic since it is extremely 
hard for software to automatically correct any problem in such a case.
You might want to access a backup data source, but for that you only need to
know that the operation has failed. Not exactly why (it should be in the
logs though). If you/the software knew a file does not exist, could it
automatically create one with the requested data? Normally not - if you
had the data you would not need to ask it.

That is why I think overly specific exceptions are not always the best
way - the exceptions you specify as a library developer should not just
image the exceptions thrown by used functions, but be of use to the calling
developer. And (s)he want's to know whether the action has failed.


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