Empty Elements

The last remaining DTD content model is the empty-content model. In this case, the elements you declare cannot hold any content (either PCDATA or other elements).

Declaring an element to be empty is easy; you just use the keyword EMPTY , like this:

 <!ELEMENT CREDIT_WARNING EMPTY> 

Now you can use this new element, <CREDIT_WARNING> , like this:

 <CREDIT_WARNING /> 

Note that although empty elements cannot contain any content, they can have attributes (such as the XHTML <img> element). We'll see how to add attributes to element declarations in the next chapter. Here's how I declare and put the <CREDIT_WARNING> element to work:

Listing ch03_06.xml

 <?xml version = "1.0" standalone="yes"?> <!DOCTYPE DOCUMENT [ <!ELEMENT DOCUMENT (CUSTOMER)*>  <!ELEMENT CUSTOMER (CREDIT_WARNING?,NAME,DATE,ORDERS)>  <!ELEMENT NAME (LAST_NAME,FIRST_NAME)> <!ELEMENT LAST_NAME (#PCDATA)> <!ELEMENT FIRST_NAME (#PCDATA)> <!ELEMENT DATE (#PCDATA)> <!ELEMENT ORDERS (ITEM)*> <!ELEMENT ITEM (PRODUCT, NUMBER, PRICE)> <!ELEMENT PRODUCT (#PCDATA)> <!ELEMENT NUMBER (#PCDATA)> <!ELEMENT PRICE (#PCDATA)>  <!ELEMENT CREDIT_WARNING EMPTY>  ]> <DOCUMENT>     <CUSTOMER>         <NAME>             <LAST_NAME>Smith</LAST_NAME>             <FIRST_NAME>Sam</FIRST_NAME>         </NAME>         <DATE>October 15, 2003</DATE>         <ORDERS>             <ITEM>                 <PRODUCT>Tomatoes</PRODUCT>                 <NUMBER>8</NUMBER>                 <PRICE>.25</PRICE>             </ITEM>             <ITEM>                 <PRODUCT>Oranges</PRODUCT>                 <NUMBER>24</NUMBER>                 <PRICE>.98</PRICE>             </ITEM>         </ORDERS>     </CUSTOMER>     <CUSTOMER>  <CREDIT_WARNING />  <NAME>             <LAST_NAME>Jones</LAST_NAME>             <FIRST_NAME>Polly</FIRST_NAME>         </NAME>         <DATE>October 20, 2003</DATE>         <ORDERS>             <ITEM>                 <PRODUCT>Bread</PRODUCT>                 <NUMBER>12</NUMBER>                 <PRICE>.95</PRICE>             </ITEM>             <ITEM>                 <PRODUCT>Apples</PRODUCT>                 <NUMBER>6</NUMBER>                 <PRICE>.50</PRICE>             </ITEM>         </ORDERS>     </CUSTOMER>     <CUSTOMER>         <NAME>             <LAST_NAME>Weber</LAST_NAME>             <FIRST_NAME>Bill</FIRST_NAME>         </NAME>         <DATE>October 25, 2003</DATE>         <ORDERS>             <ITEM>                 <PRODUCT>Asparagus</PRODUCT>                 <NUMBER>12</NUMBER>                 <PRICE>.95</PRICE>             </ITEM>             <ITEM>                 <PRODUCT>Lettuce</PRODUCT>                 <NUMBER>6</NUMBER>                 <PRICE>.50</PRICE>             </ITEM>         </ORDERS>     </CUSTOMER> </DOCUMENT>