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XML Elements are extensible and they have
relationships.
XML Elements have simple naming rules.
XML Elements are Extensible
XML documents can be extended to carry more information.
Look at the following XML NOTE example:
<note>
<to>Tove</to>
<from>Jani</from>
<body>Don't forget me this weekend!</body>
</note>
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Let's imagine that we created an application that extracted
the <to>, <from>, and <body> elements from the XML document to
produce this output:
| MESSAGE
To: Tove
From: Jani
Don't forget me this weekend! |
Imagine that the author of the XML document added some extra
information to it:
<note>
<date>2002-08-01</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
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Should the application break or crash?
No. The application should still be able to find the <to>,
<from>, and <body> elements in the XML document and produce the
same output.
XML documents are Extensible.
XML Elements have Relationships
Elements are related as parents and children.
To understand XML terminology, you have to know how
relationships between XML elements are named, and how element
content is described.
Imagine that this is a description of a book:
| My First XML Introduction to XML
XML Syntax
- Elements must have a closing tag
- Elements must be properly nested
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Imagine that this XML document describes the book:
<book>
<title>My First XML</title>
<prod id="33-657" media="paper"></prod>
<chapter>Introduction to XML
<para>What is HTML</para>
<para>What is XML</para>
</chapter>
<chapter>XML Syntax
<para>Elements must have a closing tag</para>
<para>Elements must be properly nested</para>
</chapter>
</book>
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Book is the root element. Title, prod, and chapter are
child elements of book. Book is the parent element
of title, prod, and chapter. Title, prod, and chapter are
siblings (or sister elements) because they have the
same parent.
Elements have Content
Elements can have different content types.
An XML element is everything from (including) the
element's start tag to (including) the element's end tag.
An element can have element content, mixed
content, simple content, or empty content. An
element can also have attributes.
In the example above, book has element content,
because it contains other elements. Chapter has mixed content
because it contains both text and other elements. Para has
simple content (or text content) because it contains
only text. Prod has empty content, because it carries no
information.
In the example above only the prod element has attributes.
The attribute named id has the value "33-657". The
attribute named media has the value "paper".
Element Naming
XML elements must follow these naming rules:
- Names can contain letters, numbers, and other characters
- Names must not start with a number or punctuation
character
- Names must not start with the letters xml (or XML, or
Xml, etc)
- Names cannot contain spaces
Take care when you "invent" element names and follow these
simple rules:
Any name can be used, no words are reserved, but the idea is
to make names descriptive. Names with an underscore separator
are nice.
Examples: <first_name>, <last_name>.
Avoid "-" and "." in names. For example, if you name
something "first-name," it could be a mess if your software
tries to subtract name from first. Or if you name something "first.name,"
your software may think that "name" is a property of the object
"first."
Element names can be as long as you like, but don't
exaggerate. Names should be short and simple, like this:
<book_title> not like this: <the_title_of_the_book>.
XML documents often have a corresponding database, in which
fields exist corresponding to elements in the XML document. A
good practice is to use the naming rules of your database for
the elements in the XML documents.
Non-English letters like йтб are perfectly legal in XML
element names, but watch out for problems if your software
vendor doesn't support them.
The ":" should not be used in element names because it is
reserved to be used for something called namespaces (more
later). |
