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Unspecified #IMPLIED attributes in Java (was: Goals: XML ...)

  • From: Toby Speight <tms@a...>
  • To: "XML developers' list" <xml-dev@i...>
  • Date: 18 Dec 1997 16:32:39 +0000

xml implied
Peter> Peter Murray-Rust <URL:mailto:peter@u...>

> In article <3.0.1.16.19971218152122.29672990@p...>,
> Peter wrote:

Peter> Is it the question of the return value of a non-existent
Peter> attribute.  IOW what does
Peter>
Peter> <!ATTLIST FOO PLUGH CDATA #IMPLIED>
Peter>
Peter> <FOO BAR="baz" XYZZY="" BLORT="six      spaces"/>
Peter>
Peter> return for
Peter> String s = element.getAttval("PLUGH");// could be "", or null

David has answered the original question (what is isSpecified() for in the
Java simple API?), but I thought I'd mention that DSSSL's attribute-string
function returns #f for PLUGH; the Java equivalent of this is of course,
null.  I think this is the Right Thing to do; it's sometimes important to
tell the difference between <Fu bargh=""/> and <Fu/>.

The first case is often used to mean a known, empty value; the second
to mean "not known" or "not applicable".

Concrete example: I'm a rock climber, and I keep a record of all my
climbing in XML format.  Climbs are defined as

climbs.dtd>   <!ELEMENT climb (#PCDATA)>
climbs.dtd>   <!ATTLIST climb
climbs.dtd>      grade CDATA ""
climbs.dtd>      stars CDATA #IMPLIED
climbs.dtd>      style (l|2|al|s|tr|mt) #IMPLIED
climbs.dtd>      with CDATA #IMPLIED
climbs.dtd>      >

Note the "stars" attribute, which is used for a climb's star rating
(an indication of quality).  An instance looks like

climbs.xml>  <climb grade="VD" stars="" style="2"
climbs.xml>   with="&p-hkm;">Difficult Crack</climb>

Here, the lack of stars is explicit - it's not a high-quality climb.
Whereas

climbs.xml> <climb grade="D" with="&p-ejk;, &p-dmj;"
climbs.xml>  style="l">King's Chimney</climb>

is a climb in a part of Britain where the star system isn't used, and
so I omitted the attribute - even though it probably deserves a star
or two.

I would not want these two values confused!

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