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XML Elements

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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>

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>1999-08-01</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

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:

Book Title: My First XML

Chapter 1: Introduction to XML

  • What is HTML
  • What is XML

Chapter 2: XML Syntax

  • Elements must have a closing tag
  • Elements must be properly nested

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>

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 ..)
  • 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. It could be a mess if your software tried to subtract name from first (first-name) or think that "name" is a property of the object "first" (first.name).

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).

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