XOM API for Processing XML with Java

[Janek Bogucki <janekdb AT yahoo.co.uk>]

 --- Elliotte Harold <elharo AT metalab.unc.edu> wrote:
> Currently in CanonicalXMLSerializer methods such as setLineSeparator() 
> and setMaxLength() ignore their arguments because the canonicalization 
> algorithm sets certain required values for these properties. For 
> example, in caonical XML the line break must be a linefeed. Therefore 
> the code for setLineSeparator() looks like this:
> 
>    public final void setLineSeparator(String lineSeparator) {
>        super.setLineSeparator("\n");    }
> 
> I have an open question as to whether I should throw an exception if 
> someone passes in something else such as "\r". Is it better to silently 
> do the right and required thing here? (current behavior) or should I 
> warn clients when they attempt to do the wrong thing by throwing an 
> IllegalArgumentException?  I'm leaning toward throwing the exception, 
> but I'm open to discussion and argument about this. I'd also be curious 
> if anyone has references to suggested programming practices (e.g. Bloch 
> item #s) that are squarely on point here.
> 
> Hmm, one more point that argues in favor of throwing the exception: the 
> superclass (Serializer) variants of these methods to throw exceptions 
> when illegal values such as NEL are passed to these methods. So it's not 
> out of the question or unexpected for these methods to throw an 
> IllegalArgumentException. On the other hand throwing the exception for 
> different values the superclass will accept means I'd be tightening the 
> preconditions on the subclass implementation. I think Bertrand Meyer 
> (Object Oriented Software Construction) claims I'm not allowed to do that.
> 
> -- 
> Elliotte
> 

One way of ensuring that CanonicalXMLSerializer has less surprising behaviour 
and remains a
subtype of Serializer is to move the properties mutators that should not be 
used out of Serializer
into a subclass called 'ConfigurableSerializer':

   void setIndent(int)
   void setLineSeparator(String)
   void setMaxLength(int)
   void preserveBaseURI(boolean)

Then you get this heirarchy:

Serializer
  |
  + ConfigurableSerializer
  |
  + CanonicalXMLSerializer

This is an improvement as the inappropriate setter methods do not exist on 
CanonicalXMLSerializer
and any method expecting a Serializer will not be surprised by 
IllegalArgumentExceptions.

An alternative would be to make Serializer and CanonicalXMLSerializer 
immutable by setting all
configuration via the constructor, possibly using a parameter object to keep 
the API uncluttered.

(How about changing 'void preserveBaseURI(boolean)' to 'void 
setPreserveBaseURI(boolean)'. Then
Serializer becomes more JavaBeanie.)

-Janek





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