xml-dev - Re: AttributeMap (was Re: Announcement: SAX 1998-01-12 Draft)

[James Clark <jjc@jclark.com>]

Matthew Gertner wrote:
> 
> James Clark wrote:
> 
> >AttributeMap seems way too complicated.
> >
> >I don't think using Enumeration to get all the attributes is a good
> >idea.  JDK 1.2 replaces Enumeration by Iterator.  The method names in
> >Enumeration are a real disaster in the context of XML: nextElement
> >returns the name of the next attribute!  This is not going to be an
> >efficient way to get at all the attributes (which is a common
> >application need).  To get at all the attributes, I have first to create
> >an Enumeration (an unnecessary allocation). Then for each attribute
> >name: I have to make two non-final method calls (nextElement and
> >hasMoreElements); I then have a cast (which must be checked) from Object
> >to String; I then have to look the attribute up using getValue.  Compare
> >this to the simple interface I suggested:
> >
> >void startElement(String elementName, String[] attributeNames, String[]
> >attributeValues, int nAttributes)
> 
> I agree that the AttributeMap is too complicated. On the other hand, your
> alternate proposal seems questionable. Passing three parameters to the event
> handler may be simple, but this eliminates any abstraction, which makes it
> hard to extend the interface cleanly.

If SAX is supposed to be abstract and extensible, then it needs a
substantial rework.  Something like this would be much more extensible:

interface DocumentHandler {
  void startElement(StartElementEvent event);
  void endElement(EndElementEvent event);
  void characters(CharactersEvent event);
  //...
}

Simplicity was the main design goal of SAX.

Why do we get abstraction and extensibility for attributes but for
nothing else?

> Also, this makes iteration easy but
> finding attributes by name very hard.
> 
> An AttributeMap interface should be used, but:
> 
> 1) It should provide a standard iterator interface (this is the only
> reasonable way to iterate over a map).

This has all the inefficiencies that I listed for Enumeration. 
Requiring an object to be allocated on each start-tag is really not a
good idea (it makes a measurable difference to performance in Java).

Something like this:

interface AttributeList {
  int length();            // or maybe size
  String getValue(int i);  // or maybe valueAt
  String getName(int i);   // or maybe nameAt
  String get(String name);
}

would be significantly more efficient.

At the very least provide an isEmpty() so that I don't have to do the
allocation in the common case there are no attributes.

James


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