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   RE: How to ??

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  • From: keshlam@us.ibm.com
  • To: xml-dev@ic.ac.uk
  • Date: Tue, 16 Mar 1999 22:23:16 -0500

In the DOM world, default attribute values should show up automatically as
attributes of the elements they apply to. The Attr.isSpecified() method can be
used to distinguish between a default and an explicitly entered attribute. Note
that even if an explicit value is the same as the default, it is considered
Specified. Note too that unless you use a validating parser, the whole concept
of default attributes is moot.

Complication: DOM Level 1 defined that behavior but did not define where the DTD
information to support it should be stored. There are many parts of the DTD
behavior that were deferred in Level 1, in the hope that schemas would shape up
quickly enough that Level 2 could easily support both DTDs and schemas...
unfortunately that doesn't seem to be happening. So for now, the DOM really
doesn't provide a good API for working with DTDs, and parsers and applications
have to either accept those limits or fall back on nonstandard interfaces that
may vanish in later versions of the code.

In the case of IBM's parser, both versions of the DOM (TXDocument and
DocumentImpl) can support default attributes. DocumentImpl is pretty much a
strict Level 1 DOM for now, and they may have decided not to attempt to set
default attributes as a result. The TXDocument implementation is somewhat
heavily loaded with non-DOM behaviors, and among those is a set of custom
classes that provide a bit more DTD support (but may bear no resemblance to how
DTDs are handled in future versions of the DOM).

Welcome to the bleeding edge. Wear your crash helmet.

Joe Kesselman  / IBM Research
Unless stated otherwise, all opinions are solely those of the author.

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