Building a DTD in Java

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Building a DTD
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The next step in XML design is to build a DTD based on the rough instance we worked out for the previous example A DTD, in its simplest form, contains a declaration of every element you want to use in your XML instances Examining the XML instance we defined in the previous section, the elements are
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E-MAIL FROM TO SUBJECT BODY
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Each of these elements must be defined within the DTD using an "ELEMENT" keyword, such as:
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<!ELEMENT E-MAIL (FROM, TO, SUBJECT, BODY)> <!ELEMENT FROM (#CDATA)*> <!ELEMENT TO (#CDATA)*> <!ELEMENT SUBJECT (#CDATA)*> <!ELEMENT BODY (#CDATA)*>
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Each of the preceding lines declares an element that legally can be part of our XML instances Each declaration has two parts: the element name, such as FROM, and its content model, which defines what the element can contain Notice what's going on in the declaration for the E-MAIL element: We've defined its content model very rigidly It must contain a FROM element, followed by a TO element, followed by a SUBJECT element, followed by a BODY element Any other order is illegal under this content model The content models of the FROM, TO, SUBJECT, and BODY elements, however, are defined with the cryptic #CDATA, which stands for "character data" and really just means "character text" such as the text of this sentence This means that the content model of these items can be any character text, but other XML elements or entities are not allowed If you want to include elements or entities in your character data, you use the designation content model
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#PCDATA (which stands for parseable character data)
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XML Character Entities
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What happens if the text within your XML instance needs to include a character like "<" (the "less than" symbol) Because "<" has a special meaning in XML, if you just stick it in the middle of a sentence, you're going to get a big fat parsing error when you try to use this XML instance The answer is to use XML entity references in place of these characters Entity references start with an ampersand (&), followed by a code and then a semicolon For instance, the XML standard entity reference for < is < Of course, this makes & into a reserved symbol as well, requiring its own entity reference (&) XML defines a set of standard entity references for these and other special characters, but you can also define your own entity references For example, if your XML instances were music reviews about Prince albums released after 1993, you might want to define a new entity for that little squiggly ankh-thingy (&theartist;) Your application can then substitute a suitable graphic image when representing the article to a reader I've used this approach with Greek letters (for example, α for ) and other mathematical symbols with great success You define which character entities you want to be legal in your XML at the top of your DTD with an external reference like:
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<!ENTITY % HTMLlat1 PUBLIC "-//W3C//ENTITIES Latin1//EN//HTML" "HTMLlat1ent"> %HTMLlat1;
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This defines that all of the character entities in the "Latin 1" set are now part of your DTD These include £ ( ), ½ ( ) and Ö ( ) for documents about heavy metal bands) Full definitions of these predefined entity sets are available from the W3C site as part of the definition of HTML (wwww3org/MarkUp/) Entities can also be used to build a shorthand for a complex content model for use in a DTD For instance, if you wanted your e-mail subject and body to be able to contain
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characters, entities, and the elements <i> and <b> (for italic and bold), you might define an entity in your DTD like this:
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