The Extensible Markup Language, or XML, is a metalanguage for creating markup languages
used to describe structured data. XML is a self-describing language, composed of tags and val-
ues. It is often used to describe objects that are passed in messages between applications. An
example of a simple XML document is included in Listing 10.1.
L ISTING 10.1
<DESCRIPTION> Austin Powers: International Man of Mystery</DESCRIPTION>
The first line of this snippet describes a processing instruction that states that this XML docu-
ment is based on version 1 of the XML specification. Processing instructions begin with a less-
than sign and a question mark ( <? ) and end with a question mark and a greater than sign ( ?> ).
The rest of this document describes an ITEM object with four attributes: ID , DESCRIPTION ,
PRICE , and QUANTITY . Each of these attributes is contained in an open < TAG > and closed </ TAG >
pair. You should notice how the hierarchy of the object is described in a container-like fashion,
wherein the attributes of the ITEM are between the ITEM 's open and closing tags. This shows the
parent/child relationship of the ITEM object. All XML documents can be viewed as navigable
tree structures. Figure 10.1 shows the standard structure of an XML document.
F IGURE 10.1
The XML document tree structure.