xml-dev - Re: SAX2 RFD: Inheritance vs. Modification vs. Amalgamation

[Arkin <arkin@trendline.co.il>]

Empty adpater class called ParserProp and an extended interface called
ParserEx.

The application will create a ParserProp instance by passing it a
reference to Parser. The application will then try to set a feature or a
property. If Parser implements ParserEx, it would work nicely. If Parser
does not implement ParserEx (an old version), then ParserProp will throw
an exception.

Thus:

  ParserProp  prop;

  prop = new ParserProp( myParser );
  try
  {
    prop.setFeature( "xyz" );
  }
  catch ( SAXException except )
  {
    // Do something about it, or do nothing about it.
  }

Arkin



David Megginson wrote:
> 
> (I'd like to hear from as many people as possible on this issue.)
> 
> For SAX2, we need to add at least four methods to the SAX 1.0 Parser
> interface:
> 
>   public void setFeature (String featureId, boolean state)
>     throws SAXException;
> 
>   public boolean getFeature (String featureId)
>     throws SAXException;
> 
>   public void setProperty (String propertyId, Object value)
>     throws SAXException;
> 
>   public Object getProperty (String propertyId)
>     throws SAXException;
> 
> There are three ways of doing this:
> 
> 1. Create a new interface, org.xml.sax.Parser2, that extends
>    org.xml.sax.Parser.
> 
>    PRO: - provides nice, two-way compatibility
>         - easy to write adapters for existing SAX 1.0 drivers.
>    CON: - subclassing always leads to crime (see the GOF book for
>           copious warnings).
> 
> 2. Add the methods to org.xml.sax.Parser, and require applications to
>    catch NoSuchMethodException when using the new methods, in case
>    they're concerned about what version they're dealing with.
> 
>    PRO: - doesn't limit options for subclassing in the future
>    CON: - very difficult to write adapters for existing SAX 1.0
>           drivers (slower acceptance and implementation of SAX2)
>         - can cause unexpected behaviour at deployment time, unless
>           the application designer knows to catch
>           NoSuchMethodException
> 
> 3. Create a separate interface org.xml.sax.ParserProps (or something
>    like that), and require SAX2 drivers to implement both interfaces.
> 
>    PRO: - easy to writer adapters for existing SAX 1.0 drivers
>         - fewer nasty deployment surprises
>    CON: - will require lots of casting
>         - conceptually ugly
> 
> Please, comment, comment, comment! I'm not smart enough to figure this
> out myself.
> 
> All the best,
> 
> David
> 
> --
> David Megginson                 david@megginson.com
>            http://www.megginson.com/
> 
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