5a_dtd.html

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<!ELEMENT title (#PCDATA)>
&lt;!ELEMENT item (#PCDATA | item)* ></b></pre>
</blockquote>
<p>The first line you added says that a slide consists of a <code>title</code> 
  followed by zero or more <code>item</code> elements. Nothing new there. The 
  next line says that a title consists entirely of <i>parsed character data</i> 
  (<code>PCDATA</code>). That's known as &quot;text&quot; in most parts of the 
  country, but in XML-speak it's called &quot;parsed character data&quot;. (That 
  distinguishes it from <code>CDATA</code> sections, which contain character data 
  that is not parsed.) The &quot;#&quot; that precedes <code>PCDATA</code> indicates 
  that what follows is a special word, rather than an element name. </p>
<p>The last line introduces the vertical bar (<code>|</code>), which indicates 
  an <i>or</i> condition. In this case, either <code>PCDATA</code> or an <code>item</code> 
  can occur. The asterisk at the end says that either one can occur zero or more 
  times in succession. The result of this specification is known as a <a href="../glossary.html#mixedContent">mixed-content 
  model</a>, because any number of <code>item</code> elements can be interspersed 
  with the text. Such models must always be defined with <code>#PCDATA</code> 
  specified first, some number of alternate items divided by vertical bars (<code>|</code>), 
  and an asterisk (<code>*</code>) at the end.</p>
<h3><a name="limitations"></a>Limitations of DTDs</h3>
<blockquote> 
  <p></p>
</blockquote>
<p>It would be nice if we could specify that an <code>item</code> contains either 
  text, or text followed by one or more list items. But that kind of specification 
  turns out to be hard to achieve in a DTD. For example, you might be tempted 
  to define an <code>item</code> like this: </p>
<blockquote> 
  <pre>&lt;!ELEMENT item (#PCDATA | (#PCDATA, item+)) ></pre>
</blockquote>
That would certainly be accurate, but as soon as the parser sees <code>#PCDATA</code> 
and the vertical bar, it requires the remaining definition to conform to the mixed-content 
model. This specification doesn't, so you get can error that says: <code>Illegal 
mixed content model for 'item'. Found &amp;#x28; ...</code>, where the hex character 
28 is the angle bracket the ends the definition. 
<p>Trying to double-define the item element doesn't work, either. A specification 
  like this:</p>
<blockquote> 
  <pre>&lt;!ELEMENT item (#PCDATA) &gt;
&lt;!ELEMENT item (#PCDATA, item+) ></pre>
</blockquote>
<p>produces a &quot;duplicate definition&quot; warning when the validating parser 
  runs. The second definition is, in fact, ignored. So it seems that defining 
  a mixed content model (which allows <code>item</code> elements to be interspersed 
  in text) is about as good as we can do. </p>
<p>In addition to the limitations of the mixed content model mentioned above, 
  there is no way to further qualify the kind of text that can occur where <code>PCDATA</code> 
  has been specified. Should it contain only numbers? Should be in a date format, 
  or possibly a monetary format? There is no way to say in the context of a DTD. 
</p>
<p>Finally, note that the DTD offers no sense of hierarchy. The definition for 
  the <code>title</code> element applies equally to a <code>slide</code> title 
  and to an <code>item</code> title. When we expand the DTD to allow HTML-style 
  markup in addition to plain text, it would make sense to restrict the size of 
  an <code>item</code> title compared to a <code>slide</code> title, for example. 
  But the only way to do that would be to give one of them a different name, such 
  as &quot;<code>item-title</code>&quot;. The bottom line is that the lack of 
  hierarchy in the DTD forces you to introduce a &quot;hyphenation hierarchy&quot; 
  (or its equivalent) in your namespace. All of these limitations are fundamental 
  motivations behind the development of schema-specification standards.</p>
<h3><a name="specialValues"></a>Special Element Values in the DTD</h3>
<p>Rather than specifying a parenthesized list of elements, the element definition 
  could use one of two special values: <code>ANY</code> or <code>EMPTY</code>. 
  The <code>ANY</code> specification says that the element may contain any other 
  defined element, or <code>PCDATA</code>. Such a specification is usually used 
  for the root element of a general-purpose XML document such as you might create 
  with a word processor. Textual elements could occur in any order in such a document, 
  so specifying <code>ANY</code> makes sense.</p>
<p>The <code>EMPTY</code> specification says that the element contains no contents. 
  So the DTD for email messages that let you &quot;flag&quot; the message with 
  <code>&lt;flag/&gt;</code> might have a line like this in the DTD:</p>
<blockquote> 
  <pre>&lt;!ELEMENT flag EMPTY&gt;</pre>
</blockquote>
<h3></h3>
<h3><a name="referencing"></a>Referencing the DTD</h3>
<p>In this case, the DTD definition is in a separate file from the XML document. 
  That means you have to reference it from the XML document, which makes the DTD 
  file part of the <a href="../glossary.html#externalSubset">external subset</a> 
  of the full Document Type Definition (DTD) for the XML file. As you'll see later 
  on, you can also include parts of the DTD within the document. Such definitions 
  constitute the <a href="../glossary.html#localSubset">local subset</a> of the 
  DTD.</p>
<blockquote> 
  <p><b>Note: </b>The XML written in this section is contained in <a href="samples/slideSample05.xml"><code>slideSample05.xml</code></a>.</p>
</blockquote>
<p>To reference the DTD file you just created, add the line highlighted below 
  to your <code>slideSample.xml</code> file:</p>
<blockquote> 
  <pre>&lt;!--  A SAMPLE set of slides  -->

<b>&lt;!DOCTYPE slideshow SYSTEM "slideshow.dtd"></b>

&lt;slideshow 
</pre>
</blockquote>
<p><a name="root"></a>Again, the DTD tag starts with &quot;<code>&lt;!</code>&quot;. 
  In this case, the tag name, <code>DOCTYPE</code>, says that the document is 
  a <code>slideshow</code>, which means that the document consists of the <code>slideshow</code> 
  element and everything within it: </p>
<blockquote> 
  <pre>&lt;slideshow&gt;
  ...
&lt;/slideshow&gt;</pre>
</blockquote>
<p>This tag defines the <code>slideshow</code> element as the <a href="../glossary.html#root">root</a> 
  element for the document. An XML document must have exactly one root element. 
  This is where that element is specified. In other words, this tag identifies 
  the document <i>content</i> as a <code>slideshow</code>. </p>
<p><a name="DOCTYPE"></a>The <code>DOCTYPE</code> tag occurs after the XML declaration 
  and before the root element. The <code>SYSTEM</code> identifier specifies the 
  location of the DTD file. Since it does not start with a prefix like <code>http:/ 
  </code>or <code>file:/</code>, the path is relative to the location of the XML 
  document. Remember the <code>setDocumentLocator</code> method? The parser is 
  using that information to find the DTD file, just as your application would 
  to find a file relative to the XML document. A <code>PUBLIC</code> identifier 
  could also be used to specify the DTD file using a unique name -- but the parser 
  would have to be able to resolve it </p>
<p>The <code>DOCTYPE</code> specification could also contain DTD definitions within 
  the XML document, rather than referring to an external DTD file. Such definitions 
  would be contained in square brackets, like this:.</p>
<blockquote> 
  <pre>&lt;!DOCTYPE slideshow SYSTEM "slideshow1.dtd" <b>[
  ...<i>local subset definitions here</i>...
]></b></pre>
</blockquote>
<p>You'll take advantage of that facility later on to define some entities that 
  can be used in the document. </p>
<blockquote> 
  <p><b><a name="Resolver"></a>Note:</b> <br>
    If a public ID (<a href="../glossary.html#URN">URN</a>) is specified instead 
    of a system ID (<a href="../glossary.html#URL">URL</a>), then the parser has 
    to be able to resolve it to an actual address in order to use it. To do that, 
    the parser can be configured with a <a href="../../api/internal/com/sun/xml/parser/Resolver.html"><code>com.sun.xml.parser.Resolver</code></a> 
    using the parser's <code>setEntityResolver</code> method, and the URN can 
    be associated with a local URL using the resolver's <code>registerCatalogEntry</code> 
    method. </p>
</blockquote>
<p></p>
<blockquote> 
  <hr size=4>
</blockquote>
<p>
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